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Daily Mass Gains among Woodland Migrants at an Inland Stopover Site
Abstract
We investigated fat deposition in transient, nocturnal, long-distance migrants at a wooded stopover site that is not near an ecological barrier (e.g. desert, large water body). The changes in body mass of recaptured birds have traditionally been used as a measure of mass gains at stopover sites. This technique ignores the majority of transients, however, possibly hindering the ability to answer species-level questions regarding stopover mass gain. We compare an analysis of recaptures with a technique that considers all captures and their condition by time of day. Eleven woodland-associated migrant species were analyzed, as well as a resident species (Black-capped Chickadee, Parus atricapillus) for comparison. Based on recapture data alone, our study site appears to be primarily a location of mass loss, rather than one of fat deposition. Conversely, the examination of condition by time of day suggests that individuals of several species have net daily gains greatly exceeding those of recaptured individuals. During autumn, however, several species exhibited net daily losses. Although some of these losses may be related to molt, it seems unlikely that molt is the only contributing factor. Differences among species in mass gains at our site suggest that various fat-deposition patterns (and, thus, migration strategies) may occur among migrants that are not approaching ecological barriers.
... Because mass change of recaptured birds may not be representative of the entire migrant population (Winker et al. 1992) and in order to increase the number of species that we could assess for energetic condition, we also used a regression of energetic condition on time since sunrise for all birds captured (Winker 1995). We defined energetic condition as the residuals from individuals' mass controlling for body size (wing chord). ...
... Following Carlisle et al. (2005a), we generated residuals from a regression of mass on wing chord for each species and we then regressed these residuals against time since sunrise to examine rates of diurnal change in energetic condition for all species with at least 20 individuals captured, excluding recaptures. We compared mass gain estimates from recapture data and regression models to determine if the two methods were consistent in assessing whether migrants were gaining or losing mass (Winker et al. 1992). To examine possible differences among years, we compared annual variation in stopover duration and mass changes among recaptured birds for species with at least 5 recaptures in each year. ...
... This may suggest that stopover at this site during spring is not a successful strategy in these species and/or that the individuals stopping over were not representative of the population as a whole (e.g. Winker et al. 1992). However, many of the individuals in these species stopping over at Camas NWR did so during inclement spring weather, suggesting that stopover may have been more challenging during this time. ...
Report summarizes 2 years of data from spring and fall songbird migration banding at a national wildlife refuge (NWR) and point count surveys at the NWR and 2 nearby wildlife management areas (WMAs).
... Analyzing mass at first capture is a means of assessing daily mass change in individuals with short stopover times, making use of the much greater sample sizes of birds captured only a single time (Mueller and Berger 1966, Collins and Bradley 1971, King 1976, Winker et al. 1992. Assumptions are that birds arrive at the stopover site at or before dawn and that time of capture is independent of a bird's mass at dawn; therefore, a gain in average mass of birds trapped over the course of a day represents the average mass gain of all individuals in the area. ...
... Therefore, it is important to standardize mass across age/sex groups. Winker et al. (1992) did this by substituting ''condition index'' (mass ϫ 10,000/wing length 3 ) as the dependent variable in the simple regression model shown above: ...
... I conducted F-tests to determine whether hourly changes in mass estimated from the regressions were significantly different from the estimated value required for net gain in mass over a 24-h period. The latter threshold was an arbitrary value calculated on the assumption that mass gain continues at the average rate over all hours of daylight, and that overnight mass loss of a nonmigrating bird equals 4.5% of average body mass (Winker et al. 1992). The threshold values used in this test (based on local day length during Magnolia Warbler migration) were 0.026 g/h in spring and 0.031 g/h in fall. ...
Whether or not migrants gain mass at a stopover site is an index of site quality. Previous studies have examined mass gain of recaptured birds, and of short-term stopovers by regressing mass at first capture on hour of day. I developed an extension of the latter method using multiple regression to examine the effects on mass gain of hour of day, date, and year. I then used the method to compare the quality of three stopover sites at Long Point, Ontario, for Magnolia Warblers (Dendroica magnolia). At the peak of fall migration, warblers at all three sites gained sufficient mass for a net gain over 24 h, but they gained mass at only two of three sites during spring. Mass gain varied significantly over the course of the day, by date in the season, and among years. The earliest spring migrants lost mass at all sites, but rate of mass gain increased as the season progressed. Similar information for many more species and stopover sites might aid in habitat conservation for migrants.
... Previous studies have used mass changes In recaptured birds to quantify site quality (Cherry 1982, Moore andKerlinger 1987), However, the use of recapture data may be biased and has been criticized (Winker et al. 1992). Sample sizes for recapture analyses are generally small because only a limited number of transients are encountered more than once. ...
... An alternative approach for assessing changes in condition during stopover is to relate a measure of condition to time of capture during the day for all individuals encountered of a given species (Winker et al. 1992, Dunn 2000. The slope of the resulting regression line represents the average hourly change in condition for birds at the site. ...
... Estimates of net daily change in condition were calculated for each species by multiplying the slope of the regression line by an estimate of foraging time based on day length (12 h in autumn, 15 h in spring). Estimated losses in condition resulting from nocturnal metabolism were then subtracted (4.5% of mean condition; Winker et al. 1992). Note that actual nocturnal losses likely varied in relation to body size (Dunn 2001) and environmental conditions, and the fixed 4.5% value is intended only to provide an approximation of losses. ...
Assigning conservation priorities to areas used by birds during migration requires information on the relative quality of areas and habitats. The rate at which migratory birds replenish energy reserves during stopover may be used as an indicator of stopover-site quality. We estimated the rate of mass gain of 34 landbird species during stopover at a near-shore terrestrial site on the south shore of Lake Ontario in New York during 12 migration seasons from 1999 to 2004. The average rate of mass gain was estimated by relating a measure of condition to time of capture (hour after sunrise) with linear regression. Data from 25,385 captures were analyzed. Significantly positive rates of mass change were detected for 20 of 30 species during spring migration and 19 of 21 species during autumn migration. No significantly negative trends were detected in either season. Daily rates of mass gain across all species averaged 9.84% of average lean body weight during spring migration and 9.77% during autumn migration. Our regression estimates were significantly greater than estimates from traditional analyses that examine mass changes in recaptured birds. Analyses of mass changes in recaptured birds revealed a mean daily change of −0.68% of average lean mass in spring and 0.13% in autumn. Because of sampling biases inherent in recapture analyses, the regression approach is likely more accurate when the assumptions of the method are met. Similar studies in various habitats, landscapes, and regions are required to prioritize conservation efforts targeting migratory stages of the annual cycle.
Cambios de Peso Diarios de Aves Terrestres durante las Paradas Migratorias en la Costa sur del Lago Ontario
... Because mass change of recaptured birds may not be representative of the entire migrant population (Winker et al. 1992) and adequate sample sizes were available for relatively few species, we also used a regression of energetic condition on time since sunrise for all birds captured to include more species in the analysis. We generated equations for rates of diurnal change in energetic condition from a regression of mass on wing chord for all species with at least 40 individuals captured, excluding recaptures (Winker 1995). ...
... Due to the similarity of the results, we only present data from the regressions of residuals on time since sunrise. We compared mass gain estimates from recapture data and regression models to determine if the two methods were consistent in assessing whether migrants were gaining or losing mass (Winker et al. 1992). ...
... Diurnal changes in energetic condition. A number of studies have used changes in energetic condition among recaptured individuals to examine the suitability of a site, but recaptured individuals may not be representative of the migrant population as a whole (Winker et al. 1992). In particular, birds with less fat are more likely to remain at a stopover site than fatter birds (Moore and Kerlinger 1987, Loria and Moore 1990, Morris 1996, potentially biasing recapture data toward leaner birds. ...
The topography of western North America provides a complex landscape for landbird migrants, and stopover patterns in this region are poorly understood. We examined seven years of stopover data (1997–2003) from a montane area in southwestern Idaho to determine whether this area provides suitable stopover habitat. We compared the proportion of birds recaptured, stopover duration, and changes in energetic condition within and among species and between two mist-netting sites located in different habitats. The proportion of birds recaptured ranged from zero to over 20%, and fewer than 5% of individuals were recaptured in most species. Mean minimum stopover durations from recapture data ranged from 1 to 10 days; most species averaged less than 6 days. Stopover duration estimates from open-population models were comparable but generally greater than estimates from recapture data. As found in stopover studies from other regions, stopover metrics varied within and among species in Idaho. However, most migrants in this study exhibited an ability to gain mass, evidenced both by recapture data and by regression of energetic condition against time since sunrise. These data imply that montane habitats in Idaho are suitable stopover sites. It follows that these habitats might serve an important role for many landbird migrants during the period of late summer molt and autumn migration, a time when many lowland areas of the West, including some riparian systems, are especially arid. We suggest that including montane nonriparian habitats in future stopover ecology studies will allow for a more complete understanding of migrant habitat needs in the West.
Ecología de Aves Migrantes de Otoño Durante Períodos de Escala en el Piedemonte de Boise, Suroeste de Idaho
Resumen. La topografía del oeste de Norte América representa un paisaje complejo para las aves terrestres migratorias, y los patrones de escala migratoria en esta región son poco conocidos. En este estudio examinamos datos de escalas migratorias colectados a través de siete años (1997–2003) en un área montana del suroeste de Idaho para determinar si esta área provee hábitats de escala adecuados. Comparamos la proporción de aves recapturadas, la duración del período de escala y los cambios en la condición energética por especie y entre especies y entre dos sitios de captura con redes de niebla ubicados en hábitats diferentes. La proporción de aves recapturadas varió entre cero y más del 20%, y en la mayoría de las especies menos del 5% de los individuos fueron recapturados. La duración mínima promedio de los períodos de escala estimada a partir de datos de recapturas estuvo entre uno y 10 días, y la estancia promedio de la mayoría de las especies fue menor de seis días. Los estimados de la duración de los períodos de escala calculados con modelos de población abierta fueron comparables (pero generalmente mayores) a los estimados basados en datos de recaptura. Tal como se ha encontrado en estudios sobre escalas migratorias realizados en otras regiones, en Idaho las mediciones tomadas durante el período de escala variaron dentro de cada especie y entre especies. Sin embargo, la mayoría de las especies migrantes que estudiamos tuvieron la habilidad de incrementar su peso, lo que se evidenció por medio de los datos de recaptura y de análisis de regresión entre la condición energética y el tiempo transcurrido desde la salida del sol. Estos datos implican que los ambientes montanos de Idaho son lugares de escala migratoria adecuados. Por lo tanto, estos hábitats pueden ser importantes para muchas aves terrestres durante el período de la muda del final del verano y la migración de otoño, un momento durante el cual muchas áreas de tierras bajas del oeste, incluyendo sistemas riparios, son especialmente secas. Sugerimos que incluir ambientes montanos no riparios en estudios futuros de la ecología de los períodos de escala migratoria permitirá tener un entendimiento más completo de los requerimientos de hábitat de las aves migratorias en el oeste.
... In this study, we explored the degree to which regional geography, age, migration distance, and foraging guild explain observed variation in three different measures of a migrant's energetic state: fat content, size-corrected body mass, and mean daily mass change over time. While related, condition index and fuel loads can be applied differently to understanding broader migration strategies within the region, and diel patterns in mass gain is useful for assessing stopover site quality (Adams 2014, Bonter et al. 2007, Dunn 2002, Winker et al. 1992). We use the information summarized here to characterize four stopover sites in the Gulf of Maine and inform the prioritization of sites for conservation and management. ...
... While the presence of a particular species or a certain number of individuals at a stopover site might suggest important habitat for migrants, this may not always be the case (Winker et al. 1992). Usage of stopover sites may vary widely depending on status of migration, distance left to travel, individual molt condition, or weather, so some studies have used these diel patterns of mass gain to infer stopover site quality and classify good quality stopover sites within a functional category (Adams 2014, Bonter et al. 2007, Dunn 2001, 2002b, Winker et al. 1992. ...
... While the presence of a particular species or a certain number of individuals at a stopover site might suggest important habitat for migrants, this may not always be the case (Winker et al. 1992). Usage of stopover sites may vary widely depending on status of migration, distance left to travel, individual molt condition, or weather, so some studies have used these diel patterns of mass gain to infer stopover site quality and classify good quality stopover sites within a functional category (Adams 2014, Bonter et al. 2007, Dunn 2001, 2002b, Winker et al. 1992. As mentioned in the methods, we assumed all captures were of birds that arrived on site the day of capture. ...
In and around the Gulf of Maine, over 300 species of birds have been documented during migration, and tens of millions of songbirds may pass through the region on a single autumn night. Shorelines are widely documented as major migration corridors. There is ample evidence that coastal areas concentrate migrants and many species make overwater movements to and from breeding and wintering grounds. Data collected from radar, banding, and ceilometry studies in the northeast have provided us with evidence that birds migrate along the coast and make overwater movements across the Gulf of Maine and Bay of Fundy during both spring and fall. However, all of these studies were conducted at the far northern and southern regions of the Gulf and there is still little detailed information about bird migration within the Gulf itself. With more than 85% of the annual mortality for many songbirds occurring on migration (Sillett and Holmes 2002), improving our understanding of migration throughout this region is imperative for making conservation based management decisions. This is especially pertinent in the Gulf of Maine where climate and anthropogenic changes to the landscape continue to increase challenges facing migrants. Given the concentration of individuals in this region, local scale impacts on individuals could have population level consequences. My project contributes to improving our understanding of migration by identifying key stopover areas throughout the region, presenting a baseline description of species abundance and diversity distributions, and identifying origins and factors explaining patterns of occurrence for select species in the Gulf. Additionally, I characterize physiological condition of migrants across multiple sites to identify stopover site conservation priorities in the region. I also use physiological condition at a local scale to identify possible constraints on migrants’ ability to adapt to a changing Gulf of Maine.
... Myriad factors influence detectability in surveys of wildlife, including individual behaviors (e.g., trap shyness, Nichols et al. 1984), habitat , Ballard et al. 2003, Mallory et al. 2004, environmental conditions (Dunn and Hussell 1995, Dunn et al. 1997, Simons et al. 2007) and time of year (Royle et al. 2005). Often, mark-recapture methods are employed to estimate population size or migratory behaviors while estimating detection bias, but in the open populations created by migration such methods can introduce their own biases (Winker et al. 1992). Techniques have been used to deal with transients moving through a resident population but, to this point, work has focused on a way of removing them from analysis rather than quantifying them directly (Hines et al. 2003). ...
... Additionally, the subset of the migrating population that is recaptured tends to be individuals that are in poor condition on their first capture (Winker et al. 1992, Hochachka and Fiedler 2008, Ktitorov et al. 2008). Thus recaptures are not a random subset of the entire migratory population, making inferences about abundance using recapture rates biased. ...
... Seven songbird species common to the site were used in this study to understand the effects of songbird abundance across taxa with variable life history and migratory strategies. We use population-level changes in size-controlled body mass-a technique commonly used in other migration monitoring studies as an estimate of stopover habitat quality (Winker et al. 1992, Moore and Kerlinger 1987, Dunn 2002, Bonter et al. 2007)-to quantify daily stopover habitat quality over 10 years of migration monitoring. We examine the effect of estimated migrant density on mass gained per minute for birds around the monitoring station for the current day (the negative effects of co-incident interference competition and/or the positive effects of flocking facilitation) as well as the effect of migrant density over the previous days and weeks (the negative effects of resource depletion). ...
Across the world, researchers use migration banding stations to document bird migration and study the phenomenon. In this dissertation, I focus on ways of analyzing bird migration banding data and the utility migrating birds as indicators of ecosystem health that make these monitoring efforts more useful to answering ecological questions and managing migratory species. In Chapter 1, we provide background on hierarchical modeling and an overview of our findings. In Chapter 2, we developed and validated new methods to estimate daily changes in migratory population size while controlling for changes in detectability due to environmental conditions. In Chapter 3, this modeling technique was then employed to evaluate the continental-scale and the local-scale determinants of migratory population size for ten species of migrants using a Key Biscayne, FL site for migratory stopover in the fall. Species showed diverse relationships between abundance and local weather conditions. Wet conditions on the breeding grounds consistently increased migratory onset and dry conditions on the non-breeding grounds from the previous winter consistently reduced population size across all species. In Chapter 4, we looked at how daily changes in migrant density influenced the stopover behavior of seven songbirds at the Key Biscayne stopover site. Density-dependence had positive and negative effects on mass gain across species, the chance of that effect being negative increased with the average daily stopover population size of the species. Density-dependence was hypothesized to be a function of overall migrant abundance at the site, with only highly abundance species showing negative effects. Finally, in Chapter 5 migrating birds are used to tell us about contaminant exposure in their breeding and non-breeding environments. We found higher amount of mercury in fall than the spring and there was evidence that fall mercury exposure was altering migratory behavior. These patterns provide the first evidence that Hg exposure alters migratory physiology in songbirds. Overall, this dissertation suggests that migration monitoring is useful for both basic and applied research and provide a tool for understanding the complicated life cycles of migratory animals.
... We also performed linear regression with the semi-log species-area model for a single lake as it filled and dried during the study. Regression analyses were performed using Systat version 10 (Wilkinson 2000). ...
... Stopover sites are habitats that provide migrant birds a place to refuel and rest during their journeys. Inland stopover sites may not concentrate the large numbers of birds that wintering or breeding sites do, nevertheless, they are critical areas for the survival of these migrant birds (Winker et al. 1992). We know very little about the importance of the karst system as an inland stopover site. ...
We provide a review of how waterbirds utilize the temporary lakes of the karst area of Lagoa Santa in southeastern Brazil. The area is punctuated by dozens of shallow depressions of different sizes and shapes, which are located at different altitudes, sometimes fed by creeks and connected in some situations to the endokarst by sinks. With the onset of the wet season, in years with typical rainfall levels, these depressions become lakes, most of which disappear during the dry season. Species richness, abundance, and foraging guilds of waterbirds vary according to lake area, depth, and available food types. Common species appear as soon as a thin film of water accumulates in the shallow depressions, while more specialized or migratory birds respond to water level, food availability, and migratory movements. Birds may use different lakes on different occasions, and they may abandon drying lakes, and even the entire region when lake conditions change with the advance of the dry season. We describe the behavior of the assemblage of birds associated with the flooding and receding waters of lakes of different sizes. We update the species list of nonpasserine waterbirds for this karst ecosystem, call attention to the growing threats to it and its sensitive lakes, and describe our initiative to protect it.
... To avoid the effect of body mass changes in the diel cycles (Winker et al. 1992), the data from birds captured during the morning and afternoon were treated separately for comparison of captures and recaptures or juveniles and adults. Biometric data were not available for all birds for every capture, resulting in a variation in sample sizes for the various analyses. ...
... A possible explanation is that the birds do not need to reach high fat and body mass at Tömörd for their further migration. If a stopover site is not used for fat accumulation, birds of lower energetic condition may actually stay for a shorter duration in order to more quickly reach a refuelling site (Winker et al. 1992). In other studies, Eurasian blackcaps often remained for several days and put on up to 45% of lean body mass at stopover sites with berries, which are preferred by this species. ...
Stopover strategies of the lesser whitethroat Sylvia curruca during the post-breeding period were studied
at Tömörd, western Hungary, between 1998 and 2017. Capture data of males and females were pooled
for both juveniles and adults. Recapture rate, stopover length and fat deposition patterns were analysed.
During stopover, juveniles and adults did not differ significantly either in stopover length, total change in
body mass and fat score or in proportion and rate of body mass change. All recaptures showed an overall
significant positive correlation between mass deposition rate (g/day) and departure body mass. According
to the results, the lesser whitethroats used a time-minimisation migration strategy in autumn. The benefit
of this strategy might be that it favoured early arrival in overwintering areas, prior to its competitors. The
individuals arriving earlier at wintering sites might obtain higher-quality territories and achieve higher
winter survival. This might be particularly important for populations of long-distance migrants such as the
lesser whitethroat. The majority of lesser whitethroats used the site at Tömörd as a stopover area and only
a part of them used it specifically as a site to fatten up.
... Many studies have looked at recaptures of birds later the same day, or on subsequent days, to estimate mass gain during stopover (Mueller and Berger 1966, Moore and Kerlinger 1987, Yong and Moore 1997. However, this approach can be hampered by small sample sizes, and mass change in recaptured birds may not be representative of the rest of the migratory population (Mueller and Berger 1966, Winker et al. 1992, Dunn 2000. An alternative approach is to analyze changes in mass at first capture for different birds over the course of the day (King 1976, Winker et al. 1992, Dunn 2000. ...
... However, this approach can be hampered by small sample sizes, and mass change in recaptured birds may not be representative of the rest of the migratory population (Mueller and Berger 1966, Winker et al. 1992, Dunn 2000. An alternative approach is to analyze changes in mass at first capture for different birds over the course of the day (King 1976, Winker et al. 1992, Dunn 2000. This approach assumes that birds arrive at the stopover site at or before dawn on the date of capture, that an individual's capture time is independent of its mass at dawn, and that birds are foraging at the site during the day. ...
Age-related differences in stopover ecology of migrant songbirds are poorly understood. We compared body mass, fat scores, and rates of mass gain of adults and immatures of 52 species of birds during autumn migration stopover at Long Point, Ontario, Canada, on the north shore of Lake Erie. Mean body mass of adults was greater than that of immatures in the majority of species with a detectable difference, but the average difference across species was only 1%. Fat scores were also higher for adults in many species, suggesting that mass differences were due to differences in condition rather than body size. Mean rate of mass gain, estimated from changes in body mass of first captures over the course of the day, did not differ significantly between adults and immatures of most species. However, the power to detect differences was low. Averaged across species (n = 117 903 birds), the estimated rate of mass gain for adults was 10% higher than that for immatures, but with 95% confidence limits ranging from 12% lower to 32% higher. The observed differences in body mass could be produced by a small difference in rate of mass gain. Small differences in body mass and rate of mass gain between immatures and adults could indicate that young passerines rapidly develop similar foraging skills to those of adults, or that young birds are not particularly disadvantaged at Long Point either because of good food supply, or because there is little need to accumulate large amounts of fat in the early stages of migration.
Diferencias Dadas por la Edad en la Masa Corporal y la Tasa de Aumento de Masa de Aves Paserinas durante Escalas Migratorias Otoñales
Resumen. La ecología de las aves canoras migratorias de diferentes edades en sus sitios de descanso es poco conocida. Comparamos la masa corporal, los niveles de grasa y la tasa de aumento de masa de adultos e inmaduros de 52 especies de aves durante escalas migratorias otoñales en la costa norte del Lago Erie, Long Point, Ontario, Canadá. La masa corporal de los adultos fue superior a la de los inmaduros en la mayoría de las especies, pero en promedio esta diferencia fue sólo del 1% para todas las especies. Los niveles de grasa también fueron mayores en adultos de muchas especies, lo que sugiere que las diferencias en masa se debieron a diferencias en la condición física y no al tamaño corporal de las aves. La tasa media de aumento de masa, estimada a partir de cambios en la masa corporal de las primeras capturas en el curso del día, no difirió significativamente entre adultos e inmaduros para la mayoría de las especies, pero el poder de la prueba estadística para detectar diferencias fue bajo. Promediada para todas las especies (n = 117 903 aves), la tasa estimada de aumento de masa para los adultos fue superior en un 10% a la de los inmaduros, pero con intervalos de confianza del 95% fluctuando entre un mínimo de 12% y 32%. Las diferencias observadas en masa corporal pudieron ser producidas por una pequeña diferencia en las tasas de aumento de masa. Las pequeñas diferencias en la masa corporal y la tasa de aumento de masa entre inmaduros y adultos podrían indicar que las aves jóvenes desarrollan habilidades de forrajeo similares a las de los adultos rápidamente o que las aves jóvenes no están particularmente en desventaja en Long Point ya sea porque el alimento es abundante o porque no es necesario acumular grandes cantidades de grasa en las primeras etapas de la migración.
... body mass and improve body condition (Winker et al. 1992, Morris et al. 1996, sometimes showing an average daily gain of about 1.5% of body mass (Otahal 1995, Yong andFinch 1997), others may undergo a decline in energetic condition during stopover (Winker et al. 1992, Parrish 1997. ...
... body mass and improve body condition (Winker et al. 1992, Morris et al. 1996, sometimes showing an average daily gain of about 1.5% of body mass (Otahal 1995, Yong andFinch 1997), others may undergo a decline in energetic condition during stopover (Winker et al. 1992, Parrish 1997. ...
Philopatry to stopover site and changes in body condition of migrating Reed Warblers Acrocephalus scirpaceus were studied in Bet Shean Valley, Israel, where warblers were netted throughout the year. Although the majority of birds were seen only once, the proportion of transients seen twice or more in different years is comparable to the figure for summer residents returning between years, indicating a high degree of philopatry among transients. Transients get heavier with longer duration of stay, up to about 15 days, after which body mass increase appears to level off at about 3 g. Change in body condition, taken to be body mass divided by wing length, also was noted, albeit of less significance. The mean date of arrival in the autumn of birds in their first year was about 20 days later than that of older birds. Reed Warblers use their time effectively to replenish their body mass and improve their condition before starting the dangerous crossing of the Sahara Desert.
... An alternative method is to regress body mass on first capture against the time of day; if migrants at a site are gaining mass, then the slope should be positive. Increases in body mass of the population over the day are assumed to represent the daily gain of all individuals (e.g., King 1976;Winker et al. 1992;Dunn and Moore 2000;Bonter et al. 2007). We use this second approach to examine the nightly mass change of bats using a 3-year data set. ...
... To estimate rates of nightly mass gain, we regressed fat mass and lean mass against the time of night bats were captured (minutes since sunset). For a discussion of the theory and assumptions of this analysis see Winker et al. (1992). We determined the relative deposition of fat mass and lean mass by regressing each on body mass. ...
Migrating animals must acquire sufficient fuel to sustain migratory movement, but how time is allocated to achieve this can vary greatly. The fuel strategies used by migrating bats are not well understood and have not been investigated during the spring when insectivorous bats face low food abundance. Migrating silver-haired bats (Lasionycteris noctivagans (Le Conte, 1831)) were captured at a stopover site in Long Point, Ontario, Canada, in April and May of 2012–2014. We followed the movements of 40 bats outfitted with radio transmitters using an automated telemetry array and examined the effects of ambient temperature, fat stores, and sex on stopover duration. As seen previously in autumn, most bats departed the evening following capture, but one-third of bats used multiday stopovers. Extended stopover was associated with lower ambient temperature. There was no effect of sex or fat at capture on stopover departure probability. Bats captured closer to dawn had greater fat mass and lean mass than those captured early in the night, a trend indicative of fuel deposition at this site. This is the first study to provide evidence that bats use stopover habitat for refuelling.
... We estimated mass change rates based on the relationship between body mass and time of capture (expressed as hours since sunrise; Winker et al. 1992, Horton and Morris 2012, Ware et al. 2015. We tested this relationship in each species by first using general linear models (GLMs), with body mass as the dependent variable, time of capture and age as independent variables, and an interaction term between time of capture and age to identify potential age differences in mass change rates (Jones et al. 2002). ...
... We estimated minimum length of stay among recaptured birds by subtracting the date of first capture from the date of last capture while recognizing that the minority of individuals recaptured may not have been representative of all birds (Winker et al. 1992), and that initial and final captures may not have occurred on the date of arrival and departure, respectively (Cherry 1982, Yong andFinch 2002). We report this information with the intent to provide only a general indication of whether birds were quickly departing the study site, perhaps in search of more natural, less disturbed habitat elsewhere, or remaining onsite for a more extended stopover. ...
Former landfills have long been recognized as a potential source of early successional habitat for wildlife, but their use by migrating grassland and shrubland songbirds has yet to be studied. We estimated mass change rates of 5 grassland and shrubland songbird species during autumn stopovers at a reclaimed landfill in New Jersey to assess the quality of a former landfill as a stopover habitat. We also examined minimum length of stay, age ratios, and age differences in body mass and fat scores. Regressions of capture time and body mass were statistically significant and indicated gains of 0.8–1.2% of average body mass per hour in Savannah Sparrows (Passerculus sandwichensis), Lincoln's Sparrows (Melospiza lincolnii), and Palm Warblers (Setophaga palmarum), but coefficients of determination were weak (<0.06). White-crowned Sparrows (Zonotrichia leucophrys leucophrys) and Indigo Buntings (Passerina cyanea) did not gain significant mass. Minimum length of stay based on recaptures ranged from an average of 4.7 d in Savannah Sparrows to 10.1 d in Indigo Buntings. Adults did not have higher mass gain rates, body mass, or fat scores than immature birds in any species, with the exception of adult Savannah Sparrows being heavier than immatures in 1 year. The age ratio was significantly skewed toward immatures in all species except the Indigo Bunting, in which the opposite pattern occurred. Food availability at our site may have been poor, limiting the ability of birds to gain mass, or possibly time is not as important to these species at this stage of their migration as energy minimization and predator avoidance are. Considering the low temporal pressure and slow pace of autumn migration relative to spring, these autumn migrants might be using the landfill for rest, energy maintenance, and predator avoidance more so than rapid and substantial fuel deposition. The independence of mass change rate and energetic condition from age suggests that any potential age differences in dominance or foraging ability in these species do not affect their ability to refuel during stopover.
... To avoid the effect of body mass changes in the diel cycles (Winker et al. 1992), only data from birds captured during the morning period were used. The individual capture dates were not standardized to the mean arrival date for the given year (Yosef & Wineman 2010). ...
... The greatest rate of change in mass was observed in a male Blackcap that gained 37% of its lean body mass in 33 days at Tömörd. If a stopover site is not used for fat accumulation, birds of lower energetic condition may actually stay for a shorter duration in order to more quickly reach a refuelling site (Winker et al. 1992). ...
Stopover strategies of the Eurasian Blackcap (Sylvia atricapilla) during the post-fledging period were studied at Tömörd, western Hungary, between 1998 and 2015. Capture data of juveniles and adults were pooled for both sexes, and recapture rate, stopover length and fat deposition patterns were analyzed. During stopover, males and females did not differ significantly in stopover length, total change in body mass, proportion or rate of body mass change, or change in fat score. Among all recaptures, an overall significant positive correlation was recognized between mass deposition rate (g/day) and departure body mass. According to our results, both males and females use a time-minimization migration strategy in autumn. The benefit of this strategy might be that it favors early arrival in the overwintering areas, before competitors. The individuals arriving earlier at wintering sites might obtain higher-quality territories and achieve higher winter survival. This might be particularly important in populations that are increasing, which is the case for the Blackcap. Our data support the idea that most Blackcaps were using the study site at Tömörd as a stopover area, but only a small number of them were using it as a site specifically to fatten up.
... Likewise, the Common Yellowthroat (Geothlypis trichas) can increase between 4.4-7.2%, and the Chestnut-sided Warbler (Setophaga pensylvanica) can extraordinarily increase its mass from 13.8-16.1 % before departing from stopover site (Winker et al. 1992). In the Gray Catbird (Dumetella carolinensis), some individuals arrive on the breeding grounds with lower hematocrit levels, which has been associated with dehydration and lower than average body condition , and thus supports the hypothesis that successful arrival depends on the bird's condition (Morton 1994). ...
... In general, migratory birds can suffer a 26% reduction in mass during migration (Butler et al. 1998, Whittow 2000. The Least Flycatcher (Empidonax minimus), for example, is similar in size to the Lesser Elaenia, loses 0.30 g per day after arriving from migration, and over time, can gain 6-8% (0.05 g) of its mass per day while in residence at stop-over sites (Winker et al. 1992). Here, we show that body mass of the Lesser Elaenia decreases little during the reproductive period after arrival but increases again before departure to the non-breeding grounds. ...
Migration and reproduction are energetically expensive processes that migratory animal species must confront during their life-cycle. The relationship between hematocrit and mass in birds highlight their ability to invest energy in activities such as reproduction and migration and face costs linked to all subphases. We tested three hypotheses to evaluate the relationship between hematocrit and mass for an intratropical migrant bird, the Lesser Elaenia (Elaenia chiriquensis): (1) In the arrival phase, we expected that there would be a decrease in mass accompanied by a drop in hematocrit levels in a negative linear relationship; (2) during the reproduction phase we expected, at most, a weak relationship and a decrease in mass and hematocrit, due to the physiological complexity of this phase; (3) on the departure phase, we expected an increase in mass complemented by an increase in hematocrit as a positive linear relationship. We found that the Lesser Elaenia has average mass and hematocrit levels in the arrival phase similar to those in the other phases, thus not showing the expected trend as predicted above. On the other hand, during the reproduction phase, as expected, there was no significant relationship between mass and hematocrit levels, whereas a strong relationship between these variables was detected during the departure phase. Overall, the Lesser Elaenia arrives in good condition in their breeding sites, then it undergoes some physiological stress during the breeding period, but later is able to rapidly recover optimal physiological conditions upon departure to wintering grounds. © 2015, Sociedade Brasileira de Ornitologia. All rights reserved.
... However, naturally occurring molt-breeding overlap has been found in some high-latitude populations. For example, some birds begin molting while still caring for fledglings (Rimmer 1988, Hemborg et al. 1998, Flockhart 2010, and overlap of molt and migration has been documented in several passerine migrants (Cannell et al. 1983, Rimmer 1988, Winker et al. 1992, Jenni and Winkler 1994, Yuri and Rohwer 1997, Norris et al. 2004, Flockhart 2010. The cost of molting during migration is not well documented, but in Barn Swallows (Hirundo rustica), there are reduced energy stores for birds molting compared with those that are not molting during migration (Rubolini et al. 2002). ...
... Some individuals of all 17 species in this study overlapped energetically demanding events in interior Alaska. Indeed, the frequency and intensity of overlaps occurring among individuals at our study site has not been found in previous studies documenting molt-fattening or molt-migration overlaps in passerines (Cannell et al. 1983, Rimmer 1988, Winker et al. 1992, Jenni and Winkler 1994, Lindström et al. 1994, Yuri and Rohwer 1997. Only Flockhart (2010), in a study of five species of wood warblers (Parulidae) in northern Alberta, Canada (55u 229) has shown high levels of moltmigration overlap. ...
Temporal constraints on migratory birds to molt, store fat, and migrate in autumn are probably most severe in populations breeding at high latitudes. We examined whether high-latitude time constraints were related to the overlap of these energetically demanding events in migratory passerine species. We also examined how much overlap of molt and fattening occurs within individuals. Data were collected on molt intensity and subcutaneous fat during autumn migration from 1992 to 2004 in Fairbanks, Alaska, (64° 50' N 147° 50' W). Among 17 migrant species, we found a negative relationship between length of breeding ground occupancy (number of days between median spring and autumn passage, our measure of time constraints) and the amounts of molt-migration overlap. There was also a positive relationship between molt-fat overlap and distance to wintering range among these 17 species. No individual completely overlapped the peak levels of both molt intensity and fat storage observed within a species, but several individuals approached this theoretical maximum in four species. Molt-fat overlap was highest in an individual Yellow Warbler (Setophaga petechia) that achieved 70% of the maximum possible overlap of peak fat storage and peak molt intensity for that species. These findings indicate that high-latitude passerines can overlap energetically demanding events during the annual cycle but that there is considerable variation among species in how they juggle time and energy constraints. Our data provide strong support for a conceptual model that passerine migrants breeding at high latitudes use strategies that reduce the time required to complete breeding season activities. In doing so, many of these birds appear to push energetic limits by overlapping molt, migration, and fattening to a degree not previously documented.
... We used the following body condition indexes (BCI): [14] ICC weight (g) wing length (mm). = (1) [15] ICC weight (g) 100 wing length (mm). = × (2) [16] ICC weight (g) wing length tail length tarsus length . ...
... Higher tolerance to human presence by STRDEC [29] compared to ZENMAC makes the species more abundant all year round as they have foodTable 2. Body condition index of Eurasian Collared-Doves and mourning doves during two seasons spring-summer and fall-winter in the municipality of Durango, Mexico. Body condition index: 1 [13], 2 [14], 3 [15] ; same letter shows no statistical difference; Tukey's test (p < 0.05); M: male; F: females; SS: springsummer season; FW: fall-winter season. ...
In order to incorporate the knowledge of two species of wild doves and compare the results of both, we assessed morphometrics (body weight, bill, tarsus, tail, wing, head and culmen length) and body condition index (using weigh, wing, tail and tarsus length) of Eurasian Collared-Dove Streptopelia decaocto and Mourning Dove Zenaida macroura (Aves: Columbidae) based on 40 specimens of each species collected during fall-winter 2013 and spring-summer 2014 in the municipality of Durango, Durango, Mexico. We found that body condition index was higher during fall-winter for both species. We also found size dimorphism. Males of Eurasian Collared-Dove were larger than females in head length (p = 0.002) and tail length (p = 0.05) but smaller in culmen length. Female Mourning Doves had a higher body condition index compared to males (p = 0.02) during both seasons. As expected, Eurasian Collared-Dove was larger in all measurements than Mourning Dove (p > 0.05) but tail length (p = 0.12).
... Migrant landbirds using high-quality stopover habitat will deposit fat, causing them to gain mass over the progression of the morning (Dunn 2002, Bonter et al. 2007) as they prepare for a subsequent night's migration (Moore andKerlinger 1987, Moore 2018). Although estimating the direction and rate of mass change of individuals captured over the course of a morning is a commonly used metric to evaluate both the energetics at a stopover site (Winker et al. 1992, Newton 2008, Moore 2018) and the quality of that site (Dunn 2002, Smith andHatch 2017), few studies have looked for changes in the energetics of site use by examining long-term changes in rates and direction of mass change as a consequence of climate change (VanTol et al. 2021). ...
Although there is abundant evidence that migrant landbirds have modified their migratory timing in response to climate change, few studies have looked for evidence of long-term changes in site use or function, while even fewer studies have looked for differential effects on demographic groups within a species. Here, we analyze 18 years of daily weather data and 17 years of Gray Catbird (Dumetella carolinensis) and Common Yellowthroat (Geothlypis trichas) capture data to look for evidence of long-term changes in temperature and precipitation as well as arrival timing by species, sex, and age during spring migration in northeastern Pennsylvania, USA. We also determined whether there was evidence of protandry in Gray Catbirds, a sexually monochromatic species. Additionally, we investigated changes in site use, as indicated by long-term change in capture rates or rates of mass gain by age or sex in both species. Although average daily temperatures did not change, we found long-term changes in the amount and probability of precipitation during the spring migratory period (April–May). We also found that both species advanced their arrival timing (Gray Catbirds ~6.6 d/decade, Common Yellowthroats ~2.8 d/decade) and that advances in arrival timing varied by sex or age in both species. We found no evidence of protandry in Gray Catbirds. Further, we found evidence that site functionality changed for both species, as demonstrated by sex-related differences in yearly mass gain for birds using the study site. Understanding the phenological response of migratory species to climate change requires consideration of climate change effects across multiple temporal and geographic scales, and, as our results suggest, consideration of differential effects of climate change by demographic groups within species.
... First, we regressed body mass or scaled body mass with time of capture (expressed as hours since sunrise) to obtain an hourly rate of mass change from the b coefficient. This method assumes that change in the mass of birds captured over the course of a day reflects the average rate of mass gain for the entire population on stopover (Winker et al. 1992, Dunn 2001, McCabe et al. 2019. We tested the relationship for each species by using a general linear model with body mass or scaled body mass as the dependent variable, time of capture as the independent variable, and year as a covariate. ...
Barrier islands along the northern Gulf coast provide critical stopover habitats for NearcticNeotropical migratory songbirds using a spring trans-Gulf migration strategy. Islands bordering Apalachicola Bay, Florida, project southward into the Gulf of Mexico providing an opportunity for first landfall. In 2013 and 2014, we studied spring stopover ecology on St. George Island, Florida. We provide a detailed description of the arrival body condition and fuel deposition rates of 8 species of migratory birds. We used 3 approaches to measure fuel deposition rate: (1) estimated an hourly rate of mass change by regressing body mass or scaled body mass by time of day, (2) estimated a daily rate of mass change by examining body mass of recaptured individuals, and (3) for 4 species, measured plasma triglyceride concentrations as a proxy of individual refueling rate. The majority of migratory birds in the eastern Gulf of Mexico arrived in poor body condition with depleted fat and muscle stores. We found no significant changes in body mass or scaled body mass as the day progressed, except for Indigo Bunting (Passerina cyanea), which gained mass throughout the day. Recapture rate was low on St. George Island (<5%), but most recaptured individuals tended to lose mass. Using plasma triglyceride concentration, we found that refueling rate for Gray Catbird (Dumetella carolinensis) declined throughout the day. We found no relationship between body condition and refueling rate in Gray Catbird or Catharus thrushes. Our results suggests that long-distance migratory birds arriving on St. George Island after a nonstop migratory flight may be physiologically challenged due to poor body condition. Our results provide the first quantitative assessment of songbird refueling performance on Florida panhandle barrier islands and reinforce the importance of Gulf of Mexico barrier islands as critical stopover sites for long-distance migrants in poor body condition.
... This second method, which we refer to as the CUBED RATIO index, divides the linear feature by the measurement of the linear feature raised to a power of 3. This is multiplied by 10,000 to avoid errors with small numbers (Winker et al. 1992). Third, a more recent method is to use the residual of body mass regressed by the linear feature (Peig and Green 2009). ...
Vanausdall RA, Dinsmore SJ. 2021. Stopover ecology of the least sandpiper (Calidris minutilla) in Iowa: implications for reservoir management. Lake Reserv Manage. XX:XXX–XXX.
Multipurpose reservoirs can be used to manage habitat for shorebirds during migration, an energetically costly event that may influence demographics and population numbers. This is particularly true for shorebirds that migrate through the interior of the United States, which has lost much of its wetland habitat. Shorebirds use aquatic systems during stopover periods throughout migration to rest and refuel. Understanding the factors that influence the probability of a shorebird remaining at a stopover area can inform decisions regarding reservoir management. We examined the influence of habitat and environmental covariates and body condition on the daily local residency probability of migrant least sandpipers (Calidris minutilla) at Saylorville Lake, a reservoir in central Iowa. We monitored 189 least sandpipers in fall 2016, 2017, 2019, and 2020 using radiotelemetry. Using a nest survival model, our most competitive model included significant negative effects of water level (β = −8.35, 85% CI = −11.18, −5.52) and body condition (β = −4.91, 85% CI = −6.82, −3.01) on residency probability. We calculated a mean daily local residency probability of 0.78 (95% CI 0.56, 1.00) and used this value to calculate a minimum stopover duration of 3.96 d (95% CI = 3.45, 4.46). Our findings indicate that the timing of water level management at Saylorville Lake can impact the residency probability of the least sandpiper. Drawing down water levels just prior to fall migration and maintaining water levels at or below the conservation pool level (254.8 m) can provide habitat for this species and likely other shorebirds.
... Since a very small percentage of birds are recaptured within a stopover visit, it is difficult to assess individual weight gain directly. However, assessing whether individuals captured throughout a morning become heavier as the morning progresses has been used regularly and successfully as a good measure of energetics at stopover sites (Mueller and Berger 1966, Winker et al. 1992, Morris et al. 2013. ...
As the climate changes, mismatches in phenology between predator and prey species may become more common. For example, many songbirds rely on short peaks in insect and fruit production at stopover sites during migration. Previous research indicates that migratory songbirds are able to modify their departure and arrival dates to some extent despite their reliance on more stable cues such as photoperiod and more stable endogenous factors. However, insect and plant phenology may shift more rapidly with changing climate, reducing foraging opportunities along migratory routes. To quantify changes in songbird arrival patterns at stopover sites during fall migration we analyzed forty years of banding data in nine passerine species commonly captured at banding stations in southwestern Michigan. Weather data revealed that the region has warmed by nearly 2° (C) over this timeframe. For each species, we assessed annual trends in arrival date and temperature at arrival. To determine whether arrival trends impacted stopover site function we also quantified trends in site use and morning mass gain. Arrival dates advanced significantly in three species, and were delayed significantly in three other species. However, air temperature at arrival increased significantly over time for all nine study species. Over the same time period, site use and the pattern of morning mass gain remained stable or increased for all species. Despite the changing climate and the resulting increase in temperature at arrival for migrants, our data indicate that these stopover locations continue to function as a refueling sites. Nonetheless, we must be wary of thresholds and ecological mismatches that may occur if warming trends continue.
... For Common Yellowthroats, we also included a sex by minutes since sunrise interaction term to look for sex-related differences in mass gain. If birds gained mass then we expected a positive relationship between mass at first capture and the time elapsed since sunrise (Winker et al. 1992, Dunn 2000, Smith et al. 2007a). To look for age effects on arrival timing we also used mixed models with capture day as the response variable, year and site as random effects, and age as an explanatory variable. ...
... The base model for each endpoint tested for the effect of year and species as categorical effects, combined with time of day and THg tissue concentration as continuous predictors. This structure allowed us to measure change over time of day in physical condition, which provided a population-level estimate of refueling rate (Winker et al. 1992). The base model for body mass differed from the others in that we included wing chord length as a predictor to control for differences in body size within species. ...
Many migratory songbirds are at high risk of methylmercury (MeHg) exposure due to their trophic position and foraging in and around wetland habitats. Methylmercury has the potential to alter migratory behaviors and physiology via neurological impairment or reduced flight performance and can be remobilized from songbird muscle tissue during migration, increasing the risk of acute MeHg exposure. To document MeHg exposure and its relationship with physical condition in migratory songbirds, we sampled passerine blood and feathers at a migration stopover site on Key Biscayne, FL during fall and spring from 2009 to 2012. We found evidence that spring blood total mercury (THg) concentrations decreased throughout the day and that fall feather THg concentrations changed over the migratory season. Total mercury exposure was marginally correlated with migratory fat stores and related to changes in pectoral muscle thickness by time of day. These patterns suggest that environmentally relevant levels of THg are related to, and may be influencing, the physical condition of free-living migrating songbirds. Further research and monitoring during the migratory period will be important to elucidate exposure risk across multiple species and assess the potential for effects during this complex period of the annual cycle.
... Plasma metabolite analysis offers unique insight into the dynamics of physiological state of wild birds, and should be considered a standard tool for the avian biologist. Future research should be directed at (1) demonstrating in the field that plasma metabolite levels correlate with the rate of mass gain estimated independently (Winker et al. 1992, Gannes 1999, Dunn 2000, and (2) the effects of diet nutrient composition on metabolite to mass change relationships. ...
Plasma lipid metabolites may be useful indicators of mass changes in migratory birds. To test utility of plasma metabolites in field studies, we examined effects of several extrinsic (bleed time, time of day, location) and intrinsic (body mass, sex, age, migratory state) factors on plasma concentrations of triglycerides (TRIG), glycerol (GLYC), and B-OH-butyrate (BUTY) in free-living Western Sandpipers (Calidris mauri). TRIG and GLYC decreased rapidly following capture (2–20 min), whereas BUTY did not change. GLYC and BUTY were negatively correlated to body mass. TRIG was positively correlated to body mass in migrant females, but not consistently in migrant males, or in females captured on the wintering grounds. Taking into account other sources of variation, the two measures of lipid utilization (GLYC and BUTY) varied little through the year. TRIG showed the greatest potential for use in field studies. TRIG was lowest during winter, when birds were leanest, and highest during spring and fall migration, when sandpipers were gaining mass rapidly at stopovers. TRIG differed between sandpipers refuelling a two stopover sites separated by 35 km, demonstrating that populations of birds can have characteristic lipid metabolite profiles that may reflect local differences in fattening rate.
... Lower body mass in spring and summer may be attributed to physiological stress during the breeding season or adaptive reduction in wing-loading to ease the labour of feeding nestlings (Freed 1981;Nagy et al. 2007). Other temporal factors that affect body mass include diurnal variation and migration (Winker et al. 1992;Cresswell 1998;Schaub and Jenni 2000). ...
Avian body mass reflects a trade-off between risk of starvation and predation, and may vary with ambient temperature, age, and time of day. Seasonal variability in body mass is a common occurrence in northern temperate regions, including adaptive fattening. Previous evidence suggests that seasonal variability is less pronounced in tree-feeding bird species, as their food sources during winter are less limited and variable compared to ground-foraging species. We determined fat scores of tree-feeding Black-capped Chickadees (Poecile atricapillus) captured year-round between 2004 and 2015 (n = 4248) in southern Quebec, to test the relative strength of possible drivers of variability in chickadee body mass, including time, date, and year of capture, age, and temperature. First, we demonstrated that scaled mass index (SMI) was the body condition index, out of four possible indices tested, which most strongly correlated with fat scores measured in the field. We used SMI subsequently as our estimator of body condition to avoid observer effects associated with fat scores. Similar to other studies, time of capture significantly affected SMI, in which birds captured later were heavier, indicating that chickadees experience overnight weight loss and subsequent weight gain from foraging throughout the day. SMI was constant from April to November, then peaked in late winter, but was not influenced by daily temperature after accounting for month and year. SMI was not significantly affected by age. We concluded that adaptive fattening is an evolutionary response to risk of starvation in winter, rather than a proximal response to immediate ambient temperature.
... Daily refueling rate was measured as daily mass change over time among individuals at their first capture, while controlling for the effects of passage day, bird density, and year. Diel patterns in mass gain have been used by others to assess stopover site quality (Winker 1992, Dunn 2002, Bonter et al. 2007, Adams 2014, and they rest on the assumption that increases in the mass of birds during first captures over the course of a banding day (of a given species) are at least partially a result of the average rate of mass gain for the entire population of individuals on stopover. Further, this approach provides a much larger sample size for assessing the mean population refueling rate than using only the subset of individuals captured multiple times. ...
The period of migration can pose significant energetic challenges as birds attempt to reach their destinations. Suitable stopover habitat is, therefore, important to the success of migrating individuals, especially as they move along major migration corridors and geographic features, like coastlines. In this study, we used metrics of individual body condition (i.e. fat score, size-corrected body mass, and refueling rate) of fall migrants as they moved across the Gulf of Maine region, a complex coastal landscape. We investigated the extent to which these body condition indices varied by stopover site geography (island vs. mainland) and how these spatial patterns varied with species-specific characteristics such as migratory distance, foraging guild, and age. Geography was an important factor explaining variation in all 3 condition indices, and age explained additional variance in 2 of the 3. In general, individuals captured on islands exhibited significantly lower energetic condition than individuals on the mainland, and this pattern held true across all migratory strategies and foraging guilds. Immature individuals had, on average, lower energy reserves, with less fat and lower size-corrected mass than adults among all stopover sites. We also found that at all sites, size-corrected body mass, on average, significantly increased over the capture day, providing evidence that both island and mainland sites provided energetically beneficial stopover habitat. Our finding that birds offshore are in lower body condition at capture than those along the coast is suggestive that birds use mainland sites for longer stopover bouts than island sites, or that poorer condition birds reorient to land during over-water movements disproportionately use off-shore islands as their initial landing area. Decreases in either island or mainland stopover site availability or quality may affect individual fitness, with population-level consequences, but through different ways.
... Fat score was recorded on a 0-5 scale following Helms and Drury (1960): 0 ¼ no visible fat, 1 ¼ trace fat visible in furcular region, 2 ¼ furcular region filling but concave, 3 ¼ furcular region filled with fat, 4 ¼ furcular region bulging and fat visible on abdomen, and 5 ¼ furcular region bulging and abdomen mounded. We calculated the condition index (CI), an estimate of fat storage adjusted for overall body size, for each bird as CI ¼ mass (g)/wing chord (mm)*100 (Winker et al. 1992). Birds were immediately released following measurement, banding, and blood draw. ...
... We limited our analyses to first captures only to create a data set containing mostly migrating individuals, with local breeders making up only a small percentage of captures (Dunn 2002, Jones et al. 2002, Smith et al. 2007). We used mixed-models to estimate mass change (e.g., Smith et al. 2007) before and after beetle infestation-if birds gained mass, then we expected a positive relationship between mass at first capture and the time elapsed since daylight (Winker et al. 1992, Dunn 2000, Smith et al. 2007). We ran models in IBM SPSS Version 21 (IBM Corp. 2012), with capture year, date, and mins since daylight considered fixed effects. ...
Over the last two centuries, a large and increasing number of non-native phytophagous insect species have become established in North America. In addition to the direct effects these insects have on their new host plants, indirect effects such as changes in community composition, community structure, and resource abundance have been reported. We investigated the indirect effects of viburnum leaf beetle (Pyrrhalta viburni), a Eurasian native, on a landbird community using shrubland habitat during spring migration. We compared avian community composition and bird mass before and after viburnum leaf beetles invaded our site. Before invasion, southern arrowwoods (Viburnum dentatum) were a prominent native component of the vegetation community. Not only was the avian community more diverse prior to infestation, our estimates from this period also suggest seven of nine species examined gained mass, and most did so at high rates. The avian community was less diverse after beetle infestation, and capture rates for seven of 69 species dropped significantly while capture rate of one species increased significantly. Finally, after infestation we found that no bird species showed evidence of mass gain. Given the decline of early successional habitats in eastern North America and the significance of early successional habitats to both birds that breed in these habitats as well as forest breeding birds during nonbreeding phases of the annual cycle, degradation of these habitats by invasive insects may have a larger effect than previously realized.
... To assess this, we conducted a model II reduced major axis regression (R package lmodel2; Legendre 2013) of duration of stay on size-ad-justed mass at capture. We controlled for differences due to structural bird size to better reflect an individual's fat load using the equation from Winker et al. (1992): ...
The objective of this study was to determine the implications of extreme tidal events on duration of stay and behavior of Semipalmated Sandpipers (Calidris pusilla) during migratory stopover in Cobequid Bay, Nova Scotia. This area is part of the Bay of Fundy and experiences the largest tidal range in the world. Radiotelemetry was used to monitor duration of stay of 30 adult and seven juvenile Semipalmated Sandpipers. Adults arriving in Cobequid Bay early in the migration period experienced a greater number of extreme high tides (> 15 m) that submerged preferred roost sites, and stayed on average 8.1 days longer than those that arrived later. Juvenile duration of stay was not significantly different from adults. When tides exceeded 15 m, Semipalmated Sandpipers engaged in over ocean flocking at high tide; however, this behavior was not observed when high tides were 13.6 m or less. These extra flights led to higher energy expenditure by early migrants, requiring an estimated 3.8 to 4.3-day increase in length of stay to reach the same mass as later migrants. In the future, predicted sea level rise could increase the frequency of extreme tidal amplitudes and result in greater energetic costs during Semipalmated Sandpiper stopover.
... We considered stopover efficiency the ability of a bird to increase body condition during a day (e.g. Marra, Hobson & Holmes, 1998;Dunn, 2001;Carlisle et al., 2005) and therefore measured stopover efficiency as the slope of a regression of BCI against minutes after sunrise (Winker, Warner & Weisbrod, 1992;Dunn, 2001;Carlisle et al., 2005;Bonter et al., 2007). Data from recaptures were not included in analyses. ...
Several past studies have demonstrated the effects of anthropogenic noise on populations of animals. Yet, differing effects of noise by age and subsequent changes in the age structure of populations are poorly understood. We experimentally tested the effects of traffic noise alone on the age structure of a community of migrating birds at a fall stopover site in south-western Idaho using an array of speakers – creating a phantom road – that replicated the sound of a roadway without other confounding aspects of roads. Both hatch-year and adult birds were negatively affected by noise – having lower capture rates, lower body condition and lower stopover efficiency along the phantom road when the noise was on compared to control conditions. However, hatch-year birds responded more strongly which lead to a significant shift in the ratio of hatch-year to adult birds under noisy conditions. Our previous work using the phantom road demonstrated that traffic noise can degrade the quality of a stopover site by affecting the ability of migrating birds to gain body condition. Here, we demonstrate differences between age classes such that although noise degrades habitat for both hatch-year and adult migrants, there are still differences in responses to noise between age groups. Despite alternative explanations of our results such as changes in behavior affecting capture likelihood, evidence suggests that younger birds avoided the phantom road more than adult birds perhaps because of different tradeoffs between foraging and predation risk and differing strategies of site selection during migration.
... Half of the Neotropical-Nearctic migratory birds we examined during our study (four out of eight species), improved their body condition as the day progressed, including Cerulean and Canada Warbler, Moore and Wang 1991;Winker et al. 1992;Dunn 2002;Jones et al. 2002), and daily increase in body condition is well established theoretically (e.g. McNamara et al. 1994); empirically (e.g. ...
Despite the widely accepted idea that shaded plantations are valuable habitats for Neotropical migrants in disturbed landscapes, little empirical evidence is available in relation to the quality of this habitat for Neotropical-Nearctic migratory birds in the Andes. We evaluated the suitability of shaded plantations for overwintering Neotropical-Nearctic migratory birds by examining diurnal and seasonal variation in body condition of migrants in these agroforests in the Andes. During October to April 2008–2009 and 2009–2010, we mist-netted eight species of Neotropical-Nearctic migrants in shaded plantations in the Colombian Andes. Body condition improved throughout the day for Cerulean Warbler (Setophaga cerulea), Blackburnian Warbler (Setophaga fusca), Tennessee Warbler (Leiothlypis peregrina), and especially Canada Warbler (Cardellina canadensis). Similarly, body condition improved across the season for Tennessee Warbler, Rose-breasted Grosbeak (Pheucticus ludovicianus), and Summer Tanager (Piranga rubra). Our results provided additional evidence that several common Neotropical migrants, including species of conservation concern such as Cerulean Warbler and Canada Warbler, may improve their body condition in shaded plantations. However, about half of the species assessed in this study did not show any significant improvement in their body condition (e.g. Red-eyed Vireo). Furthermore, on average, species were in poor condition (i.e. body condition scores were negative) in these coffee farms. Thus, the extent to which this agricultural system represents high quality habitat should be looked with caution. Overall, our data contribute novel information on daily patterns of body condition improvement for several common migratory birds in Andean shaded plantations.
... Wing chord was considered a random effect (i.e., a covariate that might account for variation in mass gain that could not be experimentally controlled). If birds gain mass while using the site, then there should be a positive relationship between first capture mass and the time elapsed since daylight (Dunn, 2000;Winker et al., 1992). Model residuals were screened for agreement with assumptions using techniques available in SPSS. ...
Early successional habitats are declining in eastern North America while at the same time remaining habitats are being invaded by a suite of nonnative shrub species. While the significance of these transitional habitats to breeding birds is well known, increasing evidence suggests they are important during the postfledging/premigratory and migratory periods, not only for shrub-nesting species but also for many species that breed in late-successional habitats. Additionally a number of studies suggest exotic species have the potential to alter habitat quality, in turn affecting the fitness of migratory landbirds. The purpose of this study was to evaluate fitness correlates associated with migrant use of shrubland habitat dominated by nonnative honeysuckle ";Lonicera spp." in order to gauge habitat quality for spring migrants using an inland stopover site in northeastern Pennsylvania. We used estimates of mass change as our fitness indicator, with positive mass change indicating quality habitat. Our results suggest most birds gain mass while using honeysuckle-dominated habitat and many species, including species that characteristically breed in forested habitats, accrue fitness advantages from using shrubland habitat during spring stopover in northeastern Pennsylvania. However, we emphasize the need to examine the cumulative effects of exotic vegetation through multiple stages of the avian annual cycle to better understand the fitness consequences of nonnative vegetation on migratory landbirds.
... From the total number of indices for calculating body condition two indices using variables collected during the sampling period were chosen. The first was calculated as BCI1 = weight (g)/wing length (mm) [10], and the second as BCI2 = ((weight (g)/Winglength 3 (mm)) * 10,000 [19]. ...
... Among the cowbirds studied by Heinz and Johnson, those that were heavier before dieldrin treatment took significantly longer to stop feeding. Migrant songbirds often arrive at stop over sites in reduced body condition, and continue to lose weight before locating food (Winker et al. 1992, Scott et al. 1994, Schaub and Jenni 2001. In that situation, a bird would be at risk of an anorexic effect if exposed to dieldrin or another anorexic agent at lower concentrations than would normally cause death. ...
... Date and wing length terms account for some of the variation in mass related to differential migration of age and sex groups by date, and the higher-order date terms model nonlinear changes across the season. Hour of capture was included because there is often significant increase in mass over the course of a day (Winker et al. 1992, Morris et al. 1996 press). In this model, b,, the regression coefficient for T, represents the amount of mass lost per hour between capture and weighing, providing an estimate of mass loss during holding time without ever having to weigh a bird twice. ...
Mass loss between capture and weighing was estimated from multiple regression analysis of nearly 183,000 weights of 48 species of small birds banded during migration. In effect, the analysis compared mass of birds weighed immediately after capture to mass of birds captured at the same time but not weighed until later. No individual had to be recaptured or weighed more than once. Significant mass loss occurred in 36 of the 48 species, at a median rate of 1.18% of lean body mass/h (1.41% in the first hour); rates considerably lower than from direct measures involving repeated weighing of the same individuals. Excretion and water loss comprise most of the decline in mass, but banders should take extra steps to minimize holding time in arid regions, in hot weather, or when feeding conditions are poor.
... Each bird was classified as adult (ASY) or second year (SY) based on plumage characteristics [43]. Fuel stores were measured from fat scores, assessed on a scale from 0-5 [44], and body weight. Body weight was measured using an index of size-adjusted mass that adjusts body mass for size using the volumetric conversion of wing chord (size-adjusted mass = mass/(wing chord) 3 *10,000; [10,45,46]. ...
Twenty-eight species of migratory shorebirds rely on the coastlines of the northern Gulf of Mexico (NGOM) to
fuel migrations to near-arctic breeding grounds. Shorebird species vary in their migration ecology: some species
use a “jump” strategy, migrating long distances without stopping, while others use “skip” and “hop” strategies,
stopping to refuel at shorter intervals along their journey. We compared stopover duration, body condition (fat
scores and size-adjusted mass), and refueling rates (plasma metabolite concentrations), in three Calidrid
sandpiper species (Calidris pusilla, C. mauri, and C. alpina) that differ in migration strategy after leaving the NGOM during spring. Results indicate that, while birds refueled at similar rates, C. alpina, an intermediate distance jump migrant, reached higher fuel stores before departing on migration than the hop and skip migrants, C. pusilla and C. mauri. C. alpina also spent more time on the NGOM than the other two species. Results suggest that NGOM habitats may be particularly important for migration success in C. alpina. This knowledge will help us predict the potential population level consequences of habitat loss due to global change on NGOM shorebird populations and develop conservation plans to mitigate these impacts.
... BCI is a size-adjusted metric of body mass calculated as mass (g)/natural wing chord (mm). Small changes in BCI represent large differences in condition (21). During migration, high body condition signifies birds with the energy stores needed for long migratory flights (15). ...
Significance
Using landscape-scale traffic noise playbacks to create a “phantom road,” we find that noise, apart from other factors present near roads, degrades the value of habitat for migrating songbirds. We found that nearly one third of the bird community avoided the phantom road. For some bird species that remained despite noise exposure, body condition and stopover efficiency (ability to gain body condition over time) decreased compared with control conditions. These findings have broad implications for the conservation of migratory birds and perhaps for other wildlife, because factors driving foraging behavior are similar across animals. For wildlife that remains in loud areas, noise pollution represents an invisible source of habitat degradation.
... podría explicar que esta especie insectívora (e.g., DeGraaf y Rappole, 1995), encuentre mayor disponibilidad de recursos en los parches de bosque, seleccionándolos frente a cafetales. La importancia de la grasa, que fue la única variable que estuvo afectada por el hábitat, se debe a que actúa como reserva de energía durante la migración (e.g., Rappole et al., 1993;Rappole, 1995), es movilizada rápidamente en momentos de necesidad (Winker et al., 1992;Carrascal et al., 1998) y permite avanzar más rápidamente (Newton, 2006). La presencia de individuos con más grasa en bosque puede deberse a que fueron mejores competidores y llegaron primero a este hábitat (Newton, 2006), o a que acumularon más grasa (Newton, 2004) en él. ...
Most passerines use fat to fuel migration and pause at stopover sites to rest or refuel. Moreover, during spring migration, en route to breeding grounds, passerines may deposit “excess” fat as either insurance against unpredictable environmental conditions or in anticipation of breeding. We analyzed the energetic condition of Hermit Thrush (Catharus guttatus), Swainson's Thrush (C. ustulatus), and Veery (C. fuscescens) during spring and autumn migration at the Braddock Bay Bird Observatory (Rochester, New York, USA). We used path analysis to determine if the “spring fatter” or insurance hypotheses could help explain some of the variation in energetic condition in Catharus thrushes by designing and analyzing biologically plausible models of the potential effects of season, capture date, hour captured, and age on energetic condition during stopover. While path models differed among species and seasons, capture (or arrival) date was the strongest predictor of energetic condition; contrary to the insurance hypothesis condition increased with date during both seasons for all species. Hour of capture predicted much less variation in condition but was consistently positive (when significant). In long-distance migrants (i.e., Swainson's Thrush and Veery), less experienced or young migrants exhibited better condition than adults regardless of arriving later, which was revealed by including a direct path between age and condition and an indirect path mediated via capture date to control for potential differences in arrival timing related to age. Despite being closely related, we found only a few patterns in common among Swainson's Thrush, Hermit Thrush, and Veery. We suspect differences in phenology, flight, morphology, and migratory strategy may play a significant role in the differences among these species.
La mayoría de las aves paserinas utilizan la grasa para impulsar la migración y los sitios de escala para descansar o recargarse. Por ejemplo, durante la migración de primavera, camino a las zonas de reproducción, las aves paserinas pueden formar depósitos de grasa ya sea como seguro contra condiciones ambientales impredecibles o en anticipación a la reproducción. Analizamos la condición energética de los zorzales Catharus guttatus, C. ustulatus, y C. fuscescens durante las migraciones de primavera y de otoño en Braddock Bay Bird Observatory (Rochester, New York, USA). Utilizamos análisis de trayectorias para determinar si la hipotésis de “más gordo en primavera” o la hipótesis de aseguramiento podrían explicar parte de la variación en la condición energética de zorzales Catharus al diseñar y analizar modelos biológicamente posibles de efectos potenciales de estacionalidad, fecha de captura, hora de captura y edad en la condición energética durante su escala. Aunque los modelos de trayectoria difirieron entre especies y estaciones, la fecha de captura (o llegada) fue el predictor más importante de condición energética y esta condición se incrementó a lo largo de ambas estaciones para todas las especies. La hora de captura predijo mucha menos variación pero fue consistentemente positiva (cuando fue significativa). En aves migratorias de larga distancia (como los zorzales C. ustulatus y C. fuscescens), las aves migratorias con menos experiencia o más jóvenes mostraban mejor condición que los adultos aunque llegaran más tarde, lo que fue revelado al incluir una trayectoria directa entre la edad y la condición y una trayectoria indirecta mediada por la fecha de captura para controlar por diferencias potenciales en el tiempo de llegada relacionadas a la edad. A pesar de su relación cercana, encontramos solo unos pocos patrones relacionados entre los zorzales C. guttatus, C. ustulatus, y C. fuscescens. Sospechamos que diferencias en fenología, vuelo, morfología y estrategia migratoria pueden jugar un papel importante en las diferencias entre estas especies.
Palabras clave: análisis de confirmación de trayectoria, aves canoras, ecología de escala, hipótesis de aseguramiento, hipótesis de “más gordo en primavera”, migración, modelado de ecuaciones estructurales.
Semipalmated sandpipers are Arctic breeding shorebirds that migrate to South America during the non-breeding season. Little work has been done to understand the daily movements, foraging habits and metabolic state of this species on stationary nonbreeding grounds. Our work was conducted at the Banco dos Cajuais Western Hemisphere Shorebird Reserve Network (WHSRN) site in Northeast Brazil. We captured semipalmated sandpipers in February and March 2019 and 2020 and attached nanotags to monitor their daily movements. Blood samples were taken to measure plasma triglycerides (an index of fattening rate). We also conducted behavioral observations on foraging birds. Using tracking data we determined that most semipalmated sandpipers appeared to use sunrise/sunset as an indicator for movement between salina and tidal flat habitats, and a smaller portion used tidal height. We found birds spent similar amounts of time foraging on tidal flats and in salinas, though different foraging modes were used. Plasma triglyceride measures suggest semi-palmated sandpipers had not started preparing to migrate when sampled. We successfully tracked semipalmated sandpipers to North America during northward migration in 2020, detecting eight within the United States. Tracking results suggest many stopped elsewhere in South America to fuel for migration , though some may have fueled at the Banco dos Cajuais. By demonstrating substantial use of both natural and altered habitats in the region by migrant semipalmated sandpipers, these data highlight the need for broader conservation measures throughout coastal regions of South America.
We documented prebasic flight feather molt of passerines captured in fall 2013 and 2015 at McGill Bird Observatory (MBO) in Montreal, Quebec. We recorded active molt of flight feathers (remiges) in 11 species that do not breed on site. Flight feather molt was frequent among Swainson's Thrush (Catharus ustulatus; 64% of adults), Tennessee Warbler (Oreothlypis peregrina; 57%), Nashville Warbler (Leiothlypis ruficapilla; 67%), and Yellow-rumped Warbler (Setophaga coronata; 44%), and was observed less frequently in other species. The minimum stopover length of molting individuals was on average 8 times longer than that of non-molting individuals of the same species. Among Swainson's Thrushes and Yellow-rumped Warblers, far more females were undergoing molt than males, whereas for Tennessee Warblers molt was slightly more frequent among males. Frequency of molt was similar between years for most species but not Yellow-rumped Warbler, with 59% of adults captured in 2013 molting compared to none in 2015. We also observed molting site fidelity with multiyear returns of Tennessee and Nashville warblers. The use of separate breeding and molting sites is not well understood among eastern North American species, and with recent studies highlighting the importance of molt locations in western North America, we demonstrate the value in additional study of the use of discrete molt locations in the East.
Recent work in Michigan's eastern Upper Peninsula suggests that terrestrial areas bordering northern Lake Huron provide important stopover habitat for spring migrating landbirds, principally because of the presence of emergent aquatic midges (Diptera: Chironomidae). Migrants were concentrated in lakeshore habitats abundant with midges during spring migration. American Redstarts (Setophaga ruticilla) and Black-throated Green Warblers (Dendroica virens) foraged and used habitat differently, depending on their distance from the lakeshore. Here, we describe results of an integrative study in which we sampled resources, quantified American Redstart foraging behavior, and estimated mass change in American Redstarts and five other common migrant landbird species to evaluate the importance of adult midges as an early season resource for spring migrants. Resource sampling and American Redstart foraging behavior suggested that more food was available in shoreline habitats than inland during spring migration. Furthermore, migrants gained mass in shoreline habitat during stopover, which supports the argument that nearshore areas provide important stopover habitat for spring migrants. Finally, resource sampling, mass change estimates, and American Redstart foraging behavior suggested that midges and spiders (Araneae: Arachnidae) provided an important early season resource for migrating landbirds. Evidence suggests that midges were responsible for elevated spider abundance at the shoreline and that birds foraged on both midges and spiders. Midges appear to play an important role in providing high-quality stopover habitat for landbirds migrating through Michigan's eastern Upper Peninsula.
Hábitat de las Paradas Migratorias a lo Largo de la Costa Norte del Lago Huron, Michigan: Insectos Acuáticos Emergentes como Recurso Alimenticio para las Aves Terrestres Migratorias Durante la Primavera
Concentrations of plasma metabolites, including triglyceride, glycerol, and B-OH butyrate, can be used to determine refueling performance of migratory birds. We tested the effects of diet type and diet lipid content on the predictive relationships between metabolites and body mass changes in three species of passerines in captivity. In addition, we tested whether measurement of plasma phospholipids improved predictions of mass change. Rate of mass change was positively related to triglycerides in a captive frugivore (Cedar Waxwing; Bombycilla cedrorum) and a granivore (White-crowned Sparrow; Zonotrichia leucophrys). B-OH butyrate was negatively and nonlinearly related to mass change in Cedar Waxwings and White-crowned Sparrows. Glycerol was negatively related to mass change in Cedar Waxwings but not in White-crowned Sparrows. Mass change was positively related to triglycerides and negatively related to B-OH butyrate in Yellow-rumped Warblers (Dendroica coronata). There was no effect of diet lipid content (9% vs. 28% dry mass) on these predictive relationships. Plasma phospholipids were positively related to mass change in White-crowned Sparrows, but negatively related to mass change in Yellow-rumped Warblers and not related to mass change in Cedar Waxwings. Measurement of phospholipids did not improve prediction of mass change by metabolite profiles. Nevertheless, plasma metabolite profiles are predictive of short- term mass changes in captive birds regardless of diet type or diet lipid content, and can provide valuable information on refueling performance in studies of stopover ecology.
Efectos de la Dieta sobre las Predicciones de los Cambios en el Peso Corporal de Aves por medio de Metabolitos Plasmáticos
We studied fat stores in passerine migrants at a high-latitude site in Fairbanks, Alaska (64°50'N, 147°50'W). We examined fat-deposition strategies during the final (spring) and initial (autumn) stages of long-distance migration, 1992–1998, to (1) improve understanding of geographic fat-deposition patterns by adding a high-latitude perspective; (2) determine whether there are age-related differences in fat-deposition strategies in autumn; and (3) test the “spring fatter” hypothesis of seasonal fat-deposition, which suggests that migrants should carry more fat in spring when they near their breeding areas than in autumn when they depart. Our analyses examined factors affecting daily fat scores during migration and compared between-season differences in fat stores among a total of 18,685 individuals of 16 migrant species. In autumn, adults had higher visible subcutaneous fat scores than immatures in 11 of 16 species. However, in all but two species, those differences were attributable to the effects of overnight low temperature, day length, and time of day, rather than age, probably because of later departures by adults. Fat scores were higher in autumn than in spring in 6 of 16 species, and body-condition indices were higher in autumn in 5 of 16 species. Only one species showed higher fat scores in spring, but that difference was not reflected in a seasonal comparison of body- condition indices. No species arrived with high fat loads in spring, and generally low fat levels in autumn suggest that high-latitude passerine migrants in North America are paying most of the energetic costs of long-distance migration with resources obtained en route to their wintering grounds. Among passerine migrants near these high-latitude breeding grounds, seasonal fat-deposition strategies appear to be responding to energetic needs at the level of daily maintenance, rather than to hypothesized insurance needs in spring or to the forthcoming needs of a long- distance migration in autumn.
One-quarter to one-third of the migrant bird species of the world are forest-dependent during one or more phases of their life cycle (Rappole, 1995) (Table 13.1). This figure alone should be a cause for some apprehension given the rate at which the world’s forests are being altered (World Resources Institute, 1992). However, until quite recently, the connection between forest alteration and migratory bird conservation was not recognized as an issue by many students of migrant ecology because of the apparent flexibility of migrant species in terms of habitat use (Morse, 1971; Karr, 1976). Migrant needs in terms of specific habitat requirements are still a subject of debate (Petit et al., 1993; Rappole and McDonald, 1994) but population declines recorded for 109 species of Nearctic migrants to the Neotropics (DeGraaf and Rappole, 1995) have forced the problem into a different context. In the absence of obvious alternative explanations for most migrant declines, the question of the effects of habitat loss in general and forest loss specifically is no longer strictly academic; for some species, it has become a critical conservation issue (Rappole et al., 1994).
Increased knowledge of North American landbird populations and concern for their status (e.g., Robbins et al., 1989; Askins et al., 1990; Askins, 1993) has led to greater interest in methods of monitoring population change in nongame birds. Without monitoring we are unable to document long-term change, to determine whether short-term fluctuation is within a normal range, or to evaluate the effectiveness of management.
We estimated fat load, length of stopover, and rate of mass change for six Neotropical migrant landbird species at a site along the northern coast of the Gulf of Mexico. Swainson's Thrushes (Catharus ustulatus), Gray Catbirds (Dumetella carolinensis), White-eyed Vireos (Vireo griseus), Red-eyed Vireos (V. olivaceus), Magnolia Warblers (Dendroica magnolia), and American Redstarts (Setophaga ruticilla) were captured on Fort Morgan Peninsula in coastal Alabama during fall migration, 1990 to 1992. In Swainson's Thrushes, White-eyed Vireos, and American Redstarts, adults carried significantly higher fat loads than young birds, whereas no age-related differences in fat loads were evident in the other species. The likelihood of staying beyond the day of capture and the rate of change in body mass did not differ between age classes. One consequence of differences in fat load is reflected in flight range in relation to the Gulf of Mexico. On average, adult Swainson's Thrushes, White-eyed Vireos, American Redstarts, and both age classes of Magnolia Warblers carried sufficient energy stores to complete a trans-Gulf flight, whereas young Swainson's Thrushes, White-eyed Vireos, American Redstarts and both age classes of Gray Catbirds carried insufficient stores for such a flight, based on flight-performance simulations.
In this chapter I review the methods of estimating the fuel deposition rate (FDR) of stopover migrants. These methods are based either on body mass change in the birds captured at least twice, or on body mass relationship with the time of day in single captures, or on analysis of metabolites in blood plasma. There is no perfect method; every approach has its benefits and pitfalls. The empirical values of the FDR reported in the literature are reviewed, and various factors that influence the FDR during migratory stopovers are discussed. Very often the FDR does not remain constant throughout stopover: it is low or even negative during 1–2 days after arrival but subsequently increases. It may also drop again towards the end of stopover. The ecological and behavioural causes and implications are discussed.
We investigated the relationship between migratory restlessness and stored energy reserves in two species of landbird migrants at Braddock Bay, Lake Ontario during spring 1999 and spring 2000. There was no significant difference in the amount of nighttime locomotor activity between lean and fat Swainson's Thrushes (Catharus ustulatus) or White-throated Sparrows (Zonotrichia albicollis) held overnight in activity cages. We found no significant relationships between nocturnal activity level and energetic condition (mass/wing chord) or date of capture. In addition, there were no significant differences in nocturnal activity levels between age groups in Swainson's Thrushes. These results suggest that the migratory restlessness of spring migrants stopping at this site is not strongly influenced by their energetic condition upon arrival, regardless of age or date of passage. Birds nearing the end of their spring migratory route that are very close to their breeding grounds may show decreased levels of migratory restlessness and short stopover durations, independent of energy reserves. When stopping at northerly sites, spring migrants may therefore display distinct differences in stopover behavior and requirements compared to sites used earlier along the spring migratory route.
Garden Warblers appear to be equipped with a migratory strategy which matches the problems arising from migration very well. They are equipped with innate endogenous programmes which determine the seasonal events for migration (for reviews see Berthold 1984, Gwinner 1986). They exhibit strong habitat preferences in resting areas which are likely to be endogenously controlled and which allow the specimens to be well accommodated in the various migratory stopoverareas (Bairlein 1981). They possess physiological and behavioural adaptations in their nutrition which guarantee optimal fattening up prior to migratory flights. Garden Warblers apparently avoid the risk of dehydration during long nonstop flights across the Sahara by crossing the desert in stages with regular stopovers at suitable sites rather than in one long hop.
Length of stopover and rate of weight gain (fat deposition) were studied in several species of passerine birds that stopped in southwestern Louisiana along the northern coast of the Gulf of Mexico after a trans-Gulf flight. Fatdepleted birds were more common among the birds that arrived at our study site in southwest Louisiana, though variability characterized our samples. Migrants that landed after encountering opposing winds or rain over the northern Gulf of Mexico were, on average, fatter than migrants that landed when weather was favorable for continued migration. Some of the variation in the energetic condition of arrivals may be explained by the location where migrants initiated crossings. Our simulation of flight over the Gulf of Mexico showed that with following winds a warbler can cross the Gulf of Mexico from Yucatan with fat reserves to spare, and stronger tailwinds make flights from as far south as Honduras energetically permissible. The length of stay after a trans-Gulf flight was related to the extent of fat-depletion upon arrival: lean birds stayed longer than fat migrants. Migrants stopped over for 1–7 days and replenished energy reserves at rates that varied from 0.19 g/d for Hooded Warblers (Wilsonia citrina) to 0.87 g/d for Ovenbirds (Seiurus aurocapillus). Within each species, most individuals gained weight at a rapid rate, though a few individuals lost or maintained weight during their stay.
Trans-Saharan insectivorous passerine night migrants stopping-over at a small oasis in Sinai were weighed as frequently as possible, throughout the entire length of fall and spring migration passages. Due to the small size of the oasis and the intensive trapping effort, most stopping-over birds were captured and weighed throughout a sizeable portion of their stopover period. Weights at first capture were either similar in both seasons, or greater in fall than in spring. In some species fall migrants that stayed one day were heavier than those staying longer. In other species, and for all species in spring, weights of birds that stayed one day did not differ from those staying longer. In most species the weight of birds that stayed for more than one day did not vary significantly between arrival and departure, and cases of weight gains were commoner in fall than in spring. Trends of weight changes of an individual during stopover were usually inconsistent, but the longer it stayed, more it gained (in fall), or less it lost (in spring). It is proposed that stopping-over birds do not always resume their migration only after their fat reserves have been replenished, but that their decision to take off, or the reappearance of the migration impulse, are also controlled by a time program incorporated into their endogenous migration scheme, which constantly updates the time left for sampling and refuelling. It seems that in spring less time is allotted for the whole migration program, hence the time constraint overrides then all other tactical considerations, such as the state of fat reserves, and the weather.
Field metabolic rates (FMR) measured using doubly labeled water, of 23 species of eutherian mammals, 13 species of marsupial mammals, and 25 species of birds were summarized and analyzed allometrically. FMR is strongly correlated with body mass in each of these groups. FMR scales differently than does basal or standard metabolic rate in eutherians (FMR slope=0.81) and marsupials (0.58), but not in birds (0.64 overall, but 0.75 in passerines and 0.75 in all other birds). Medium-sized (240-550 g) eutherians, marsupials, and birds have similar FMRs; these are approx 17 times as high as FMRs of like-sized ectothermic vertebrates such as iguanid lizards. For endothermic vertebrates, the energy cost of surviving in nature is enormous compared with that for ectotherms. Within eutherians, marsupials or birds, FMR scales differently for the following subgroups: rodents, passerine birds, herbivorous eutherians, herbivorous marsupials, desert eutherians, desert birds, and seabirds. Equations are given for use in predicting daily and annual FMR and food requirement of a species of terrestrial vertebrate, given its body mass.-from Author
Since linear measurements are unlikely to be directly proportional to mass, it is necessary to raise linear measurements by an exponent when using them as covariates in comparisons of mass, or when using them to control for the effect of size on mass. For Purple Sandpipers wintering in Britain exponents range from 2.54–2.95 (wing-length) to 0.82–0.85 (bill-length). These values cannot be used for other populations of Purple Sandpiper since the relationships between linear measurements and mass are different
Formulae for calculating the lean weight and fat reserves of shorebirds from their total body weight and wing-length and/or bill-length are given for six species wintering in Britain. General formulae for shorebirds in various seasons and areas are also given. The application of these, and other published formulae, for estimating lean weight is discussed. Formulae derived from single species should be used whenever possible. A better estimate of lean weight is obtained from formulae using both winglength and bill-length than from formulae using only one of these. The general formulae can be used when a single-species formula is not available.
A population of 400–600 Acrocephalus orientalis wintering in a Phragmites habitat at 3°N in West Malaysia was studied during four northern hemisphere winters, by means of systematic mist‐netting. Data from other study‐areas, other habitats and other winters are also used.
Intensive mist‐netting appears to have made birds move over longer distances than they did in the absence of disturbance, and to have led to the emigration of marked birds from the study‐area. Trapping also affected feeding behaviour, resulting in weight‐loss; repeated trapping may have increased mortality.
Males and females could be separated by means of wing‐length in fresh plumage. Females were largely confined to Phragmites; males were more numerous on the edge of reed‐beds and in scrub vegetation. Males suffered greater feather‐wear than females.
As measured by the trapping rate, birds were uniformly distributed throughout the Phragmites habitat, at the same density in different winters. Undisturbed birds used a “home‐range” of 1–4 ha, overlapping with 15–50 other individuals. Disturbed birds overlapped with 100–200 others.
Individual birds returned to exactly the same “home‐range” in successive winters. After correcting for the effects of disturbance and incomplete sampling, the proportion of adults ringed in one winter which returned in the next is estimated as 65% in each of two study‐areas. This is a minimum estimate of the annual survival rate for adults.
Mean total body‐weights were at a minimum in midwinter (November‐February). Fat‐free weights were also lower in midwinter than in autumn and spring.
Body‐moult was observed in March and April. Moult of the flight‐feathers takes place between July and September, on the breeding grounds or slightly to the south.
Females departed on spring migration between 10 and 25 May; males some 11–14 days earlier. Adults arrived in autumn between 8 September and 7 October; males and females often came in in separate “waves”. Females were absent for only about 127 days, about the minimum required for migration, breeding and moult.
Dates of migration match those of the more northern breeding populations. Spring departure is later than dates of passage recorded in south China; hence birds of this population appear to make long nights.
On average, birds departing in spring carried about 9 g of fat, roughly 40% of total fat‐free body‐weight. This is about half the energy reserve required for the entire journey. Dates of passage in central China are consistent with a hypothesis that they make the journey (4,500‐5,000 km) in two “hops”.
A few birds which remained light until very late in the spring showed a significantly lower return rate in the next year.
Most birds arriving in autumn appear to have carried 1–2 g of fat, but some were at or below the normal fat‐free weight. Many birds appear to have lost weight soon after arrival.
Returning ringed adults were amongst the very first birds trapped in September. Individual birds appear to have migrated on very similar dates in different years: many of the dates of trapping differed by 2 days or less in successive years.
Trapping rates reached a peak in early October and then declined rapidly, reaching the midwinter level by 21 October. The decline coincided with the differential disappearance of juvenile birds. However, birds collected at this time had adequate fat reserves, and the disappearance appears to have preceded the period of food‐shortage. It is suggested that the loss of juvenile birds resulted from behavioural interactions favouring the more dominant individuals, as has been described for several temperate zone residents.
The first few weeks in the wintering area may thus be the critical period of mortality during the year. Because birds from different breeding areas are expected to be mixed in the winter‐quarters, and vice versa, local mortality factors in winter may affect a number of breeding populations.
High adult survival rates have been recorded in several other birds which breed in the temperate zones and winter in the tropics. In general their breeding success appears to be high, so the first‐year mortality must be high.
The closely related A. arundinaceus , which winters in Africa, differs from A. orientalis in size, wing‐shape, timing of spring migration and timing of moult. These differences can be interpreted as adaptations to different environmental (primarily climatic) factors experienced during migration and on the breeding grounds.
The segregation of males and females into different habitats probably reduces inter‐sexual competition in winter, but this is not necessarily its primary function. Males collected in the evening in Phragmites had smaller fat reserves than females, suggesting that the females are better adapted to this habitat. The large size of the males is probably maintained in part by sexual selection in the breeding season. On the other hand, the size of females and their habitat is probably limited by the specialisation of their nest. These factors would suffice to explain the sexual dimorphism in size and habitat.
Populations of avian transients were studied at a stopover area in southern Texas during four consecutive migration seasons, fall, 1973-spring, 1975. We captured individuals by mist net for banding and fat level determination. Concurrently we made observations on behavior of free-flying birds.
We worked intensively with a single species, the Northern Waterthrush (Seiurus noveboracensis) while gathering weight fat and behavioral data on other species as well. Many of the patterns of weight change and behavior seen in the Northern Waterthrush were observed in other common passerines which occurred as transients on our study site.
Results showed that birds in Zugstimmung and Zugdisposition differ, not only physiologically but behaviorally as well. Individuals of normally non-gregarious species that are in Zugstimmung are gregarious and stay in an area for only a short period. Their habitat needs are broad since these birds are not dependent on the food resources of the area in which they stop while in this physiological state. In contrast, normally non-gregarious migrants in Zugdisposition are hyperphagic and aggressively territorial in defense of resources and may stay at the same site for several days. Their habitat needs are quite specific since they must increase food intake by as much as 40% to build up fat reserves. Not all individuals in Zugdisposition are able to find territories at the same time. Those birds unable to claim territories either continue to migrate or stay in an area as floaters, continually attempting to obtain territories. Weather conditions probably act as a third variable that must be balanced by the individual in a complex optimization strategy with physiological state and success in competition.
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Body weight and fat of birds passing across the middle Asia deserts in spring
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