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The migration pattern of a monogamous shorebird challenges existing hypotheses explaining the evolution of differential migration
Abstract
Differential migration by sex, where one sex migrates further than the other, occurs in many bird species. How this pattern evolves is however little understood. The first aim of this study was to investigate the extent of differential migration in the common ringed plover Charadrius hiaticula, breeding in southeast Sweden, and test three main hypotheses (the social dominance, body size and arrival time hypothesis) regarding the evolution of differential migration. Geolocators were used to derive spatiotemporal data and morphometrics were collected from the studied population. Males migrated 800 km farther compared to females, were marginally larger and there was no statistical difference in spring arrival between the sexes. In contrast to other studies none of the previously proposed hypotheses could account for the observed pattern. An additional finding was that both sexes arrive up to 1.5 months before egg laying, but males initiate territorial behaviour upon arrival. Based on these observations we suggest that males have a higher energetic demand, and challenges to meet those, early in the breeding season. Therefore we hypothesise that males arrive to the breeding site with residual fuel reserves accumulated at the wintering site to cover at least parts of these demands .Based on this hypothesis we present a simple model to explain the longer migration by males. The model is contingent on a trade-off between site specific fuelling rates (which we assume to increase with decreasing latitude), cost of the extra migration distance and predation risk during fuelling. This framework may be applicable to other cases of differential migration, especially in temperate breeding species which exhibit long pre-egg laying periods.
... The migration of temperate populations of common ringed plovers (hereafter "ringed plover") offers an interesting system to investigate how short distance migratory populations divide the migration between flights and stopovers in accordance to habitat distribution. These populations, which breed in northern Europe (including the Baltic Sea), mainly winter along the European Atlantic coast, and the migration distance varies between short movements of a few kilometers (mainly British and Irish populations) up to ~ 4000 km in Scandinavian populations [19,20]. During the non-breeding period ringed plovers can be found along nearly the whole western-European seaboard and the British Isles, which during migration also includes (sub-) Artic breeding populations on their way to or from Africa [21]. ...
... Individuals of the studied population have been shown to spread out across the entire documented winter range used by temperate breeding ringed plovers, thus exhibiting a marked difference in individual migration distances varying between ~ 1300 and ~ 3000 km [19]. These distances are all within the range of documented non-stop flights of more long-distance migratory populations of this species [26,27]. ...
... Because of the relatively uniform distribution of potential stopover sites along the European coasts and that the ringed plover is a food generalist [23], we hypothesize that ringed plovers breeding in southern Sweden will make several flights during autumn migration, where number of flights is proportional to migration distance. In spring however, ringed plovers migrate relatively early (February to early March) [19,28], with progressively harsher weather conditions and thus potentially declining fueling rates from south to north. Therefore, during spring migration we hypothesize that ringed plovers will make use of more and shorter flights when compared with autumn, given that a similar route is taken in both seasons. ...
Background
The migratory process in birds consists of alternating periods of flight and fueling. Individuals of some populations make few flights and long stopovers, while others make multiple flights between short stopovers. Shorebirds are known for executing marathon flights (jumps), but most populations studied are long distance migrants, often crossing major barriers and thus forced to make long-haul flights. The sub-division of migration in short/medium distance migratory populations, where the total migration distance is shorter than documented non-stop flight capacity and where routes offer more homogenous stopover landscape, is little explored.
Methods
Here we combine data based on conventional light level geolocators and miniaturized multi sensor loggers, comprising acceleration and light sensors, to characterize the migratory routes and migration process for a short/medium distance (~ 1300 to 3000 km) migratory population of common ringed plover (Charadrius hiaticula) breeding in southern Sweden. We were specifically interested in the variation in number and duration (total and individual) of flights/stopovers between seasons and in relation to migration distance.
Results
Most stopovers were located along the European Atlantic coast. On average 4.5 flights were made during autumn migration irrespective of migration distance, but in spring the number of flights increased with distance. The equal number of flights in autumn was explained by that most individuals migrating farther performed one longer flight (all but one lasting > 20 h), likely including crossing of the Bay of Biscay. Median duration of single flights was 8.7 h in autumn and 5.5 h in spring, and median stopover duration was ~ 1 day in both seasons. There was a positive relationship between total flight duration and migration distance, but total flight duration was 36% lower in spring compared to autumn.
Conclusions
Our results suggest that when suitable stopovers are abundant common ringed plovers prefer making shorter flights even if longer flights are within the capacity of the species. This behaviour is predicted under both time and energy minimizing strategies, although the variable flight distances suggest a policy of time selected migration. Even if populations using several stopovers seem to be more resilient for environmental change along the route, these results are informative for conservation efforts and for predicting responses to future environmental change.
... The pattern also seems to be nested within itself, so that populations breeding on the British Isles (representing the southwestern limit of the European breeding range) are mainly sedentary and breeding populations towards the north "leap-frog" their southern counterparts in subsequent order during autumn migration (Salomonsen 1955). Tracking studies of two European populations have revealed that ringed plovers from southern Sweden utilize wintering sites along the whole European Atlantic coast, including Morocco (Hedh and Hedenström 2020), and ringed plovers from northern Norway winter along the coast of West Africa (Lislevand et al. 2017). Also, tracked populations breeding in Arctic Canada and sub-Arctic Russia have been shown to winter in West and East Africa, respectively (Tomkovich et al. 2017;Léandri-Breton et al. 2019). ...
... Here we use light-level geolocator data from two populations of ringed plovers breeding in Arctic (henceforth the 'northern breeding population', NBP) and temperate Sweden (the 'southern breeding population', SBP), and citizen science data on arrival dates for the corresponding regions over 15 years, to investigate how migration distance and variations of the annual cycles within a species affect migratory behaviours. First, we ask whether the NBP winters primarily in West Africa, which based on previous published information on the SBP wintering location (Hedh and Hedenström 2020) should generate a clear leap-frog migration pattern. If so, we first predict that regional arrival dates corresponding to the breeding location for the SBP exhibit larger between year variation compared to the NBP. ...
... After visually inspecting the data, we excluded all positions between 5 days prior and 20 days after the autumn equinox and between 20 days prior and 5 days after the spring equinox. Given the general migration period of the ringed plover, positioning during movement periods may be affected in the autumn for birds breeding in Abisko (7 out of 12) and during spring for birds breeding at Ottenby (19 out of 23) (Lislevand et al. 2017;Hedh and Hedenström 2020). ...
A common migratory pattern in birds is that northerly breeding populations migrate to more southerly non-breeding sites compared to southerly breeding populations ( leap-frog migration ). Not only do populations experience differences in migration distances, but also different environmental conditions, which may vary spatiotemporally within their annual cycles, creating distinctive selective pressures and migratory strategies. Information about such adaptations is important to understand migratory drivers and evolution of migration patterns. We use light-level geolocators and citizen science data on regional spring arrivals to compare two populations of common ringed plover Charadrius hiaticula breeding at different latitudes. We (1) describe and characterize the annual cycles and (2) test predictions regarding speed and timing of migration. The northern breeding population (NBP) wintered in Africa and the southern (SBP) mainly in Europe. The annual cycles were shifted temporally so that the NBP was always later in all stages. The SBP spent more than twice as long time in the breeding area, but there was no difference in winter. The NBP spent more time on migration in general. Spring migration speed was lower in the SBP compared to autumn speed of both populations, and there was no difference in autumn and spring speed in the NBP. We also found a larger variation in spring arrival times across years in the SBP. This suggests that a complex interaction of population specific timing and variation of breeding onset, length of breeding season, and proximity to the breeding area shape the annual cycle and migratory strategies.
Significance statement
Migration distance, climate, and the resulting composition of the annual cycle are expected to influence migration strategies and timing in birds. Testing theories regarding migration behaviours are challenging, and intraspecific comparisons over the full annual cycle are still rare. Here we compare the spatiotemporal distributions of two latitudinally separated populations of common ringed plovers using light-level geolocators. We found that there was a larger long-term variation in first arrival dates and that migration speed was slower only in spring in a temperate, short-distance migratory population, compared to an Arctic, long-distance migratory population. This suggests that a complex interaction of population specific timing and variation of breeding onset, length of breeding season and proximity to the breeding area shape the annual cycle and migratory behaviours.
... There may even be differences among species within the same genus. For example, Piping Plover (Charadrius melodus) exhibit differential latitudinal segregation with females wintering further south than males (Stucker et al. 2010), while in Common Ringed Plover (Charadrius hiaticula), males winter further south than females (Hedh and Hedenström 2020). Additionally, the "arrival time hypothesis" indicates that birds arriving earlier on the breeding grounds (e.g. ...
The Lesser Yellowlegs (Tringa flavipes) is a migratory shorebird species that has experienced a precipitous population decline. The factors governing this decline are complex and may correspond to habitat traits and migratory dynamics. Recent advancements in GPS telemetry have allowed for a precise description of migratory patterns to interpret the spatial and temporal distributions of migratory bird species compared to prior approaches that used band recoveries, surveys, and morphological measurements. Understanding the similarities and differences in distributions among and within disparate populations of birds is critical for identifying the potential exposure to threats that influence a species’ productivity and survival. Detailed distribution data provides the foundation for the development and implementation of targeted conservation applications for declining species, such as the Lesser Yellowlegs.
In 2018, 2019, and 2020, project cooperators and I deployed 110 PinPoint GPS Argos satellite tags on adult Lesser Yellowlegs at six sites spanning the boreal biome of Alaska and Canada. The Lesser Yellowlegs is a Neotropical migrant shorebird that breeds in the boreal forest and spends the winter in Central and South America and the Caribbean. Upon summarizing the locations received, I found that geographically disparate populations followed different routes during autumn migration, but experienced weak migratory connectivity, or high population mixing, at wintering locations. Differentiation in migratory timing, distances, and strategies were also variable among sexes and breeding populations. Further, I described the primary stopover, staging, and wintering sites and determined that the Prairie Pothole region and the Gulf Coast region were the primary stopover sites during autumn and spring migration, whereas northeastern Argentina was the primary wintering area. Within each of those regions, the highest proportion of Lesser Yellowlegs detections were in wetland habitats.
Lastly, I modeled the probability of Lesser Yellowlegs occurring within Caribbean and northeastern South American countries where shorebirds are harvested for sport and subsistence. I found that geographically disparate populations were differentially exposed to shorebird harvest. Populations originating from eastern Canada had the highest probability of occurrence and longest duration of stay within harvest zones from mid-August through October, while populations originating from Alaska had an exposure probability of nearly zero throughout the autumn.
The Lesser Yellowlegs has experienced a precipitous population decline of ~63% since the 1970s. Within the next decade, it is predicted that an additional 50% of the current population size will be lost if science-driven conservation actions are not practiced. By using real-time location data to identify annual migration patterns and the probabilities of harvest exposure among disparate populations of Lesser Yellowlegs, my thesis provides the knowledge for tailoring conservation priorities and actions for specific geographic regions or subpopulations that are at high risk (e.g. populations originating in eastern Canada). Focusing conservation efforts to areas where scientifically rigorous analyses illustrate serious concern is an effective approach to ensure the perseverance of a steeply declining shorebird.
... We also proposed a multifactor hypothesis that added the cost of migration. Recent research by others on differential migration continues to sift the degree to which these explanations for differential migration apply to other species (Gow and Wiebe 2014;MacDonald et al. 2016;Deakin et al. 2019) and to generate new hypotheses, such as thermal tolerance (Alonso et al. 2009) and differential energy requirements (Hedh and Hedenstrom 2019). ...
Biologists who publish in The American Naturalist are drawn to its unifying mission of covering research in the fields of ecology, evolution, behavior, and integrative biology. Presented here is one scientist's attempt to straddle these fields by focusing on a single organism. It is also an account of how time spent in the field stimulates a naturalist to wonder "why did that animal just do that?" and how research is guided by chance and intention interacting with the scientific literature and the people one meets along the way. With respect to the science, the examples come from bird migration, hormones and their connection to phenotypic integration, sexual and natural selection, and urban ecology. They also come from research on the impact of environmental change on the timing of reproduction and the potential for allochrony in migratory species to influence population divergence.
Latitudinal sexual segregation during the nonbreeding season, in which males tend to winter farther north than females, is known to occur in many temperate-zone bird species. This pattern, however, has rarely been reported in species wintering in tropical regions. We investigated latitudinal sexual segregation in 45 Nearctic-Neotropical migratory bird species that winter in Mexico, using specimen records from 35 natural-history museums. We found significant latitudinal segregation in nine species: Yellow-bellied Sapsucker (Sphyrapicus varius), Swainson’s Thrush (Catharus ustulatus), Orange-crowned Warbler (Vermivora celata), Yellow Warbler (Dendroica petechia; aestiva group), Yellow-rumped Warbler (Dendroica coronata; coronata group), Black-and-white Warbler (Mniotilta varia), Ovenbird (Seiurus aurocapilla), Wilson’s Warbler (Wilsonia pusilla), and Indigo Bunting (Passerina cyanea). In most cases, males predominated in the northern part of species’ winter ranges and females in the south, but the trend was reversed in Indigo Buntings. Discovering the frequency of latitudinal sexual segregation in Neotropical migratory birds will influence understanding of avian ecology and migration systems.
Evidencia de la Segregación Latitudinal Sexual en Aves Migratorias durante el Invierno en México
In migratory birds, early arrival on breeding sites is typically associated with greater breeding success, but the mechanisms driving these benefits are rarely known. One mechanism through which greater breeding success among early arrivers can potentially be achieved is the increased time available for replacement clutches following nest loss. However, the contribution of replacement clutches to breeding success will depend on seasonal variation in nest survival rates, and the consequences for juvenile recruitment of hatching at different times in the season. In particular, lower recruitment rates of late‐hatched chicks could offset the benefits to early arrivers of being able to lay replacement clutches, which would reduce the likelihood of replacement clutch opportunities influencing selection on migratory timings. Using a simulation model of time‐constrained capacity for replacement clutches, paramaterized with empirically‐derived estimates from avian migratory systems, we show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers, even when later renesting attempts contribute fewer recruits to the population. However, these relationships vary depending on the seasonal pattern of nest survival. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although replacement clutches can mitigate this effect. The time benefits of early arrival facilitating replacement clutches following nest loss may therefore be an important but overlooked source of selection on migratory timings. Empirical measures of seasonal variation in nest survival, renesting, and juvenile recruitment rates are therefore needed in order to identify the costs and benefits associated with individual migration phenology, the selection pressures influencing migratory timings, and the implications for ongoing shifts in migration and breeding phenology. We use simulation models, paramaterized with empirically derived estimates from avian migratory systems, to show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although replacement clutches can mitigate this effect. Current advances in timing of migration and breeding in many species means that the relative contribution of replacement clutches to productivity and recruitment may also be increasing and contributing to population changes.
Variation in migratory behavior is the result of different individual strategies and fluctuations in individual performances. A first step toward understanding these differences in migratory behavior among individuals is, therefore, to assess the relative contributions of inter- and intra-individual differences to this variation. We did this using light-level geolocators deployed on the breeding grounds to follow continental black-tailed godwits (Limosa limosa limosa) throughout their south- and northward migrations over multiple years. Based on repeated tracks from 36 individuals, we found two general patterns in godwit migratory behavior: First, migratory timing in black-tailed godwits varies mostly because individual godwits migrate at different times of the year. Second, individuals also exhibit considerable variation in timing within their respective migratory windows. Although the absolute amount of inter-individual variation in timing decreased over the course of northward migration, individual godwits still arrived at their breeding grounds across a span of more than 5 weeks. These differences in migratory timing among individuals are larger than those currently observed in other migratory bird species and suggest that the selective forces that limit the variation in migratory timing in other species are relaxed or absent in godwits. Furthermore, we could not attribute these individual differences to the sex or wintering location of an individual. We suggest that different developmental trajectories enabled by developmental plasticity likely result in these generally consistent, life-long annual routines. To investigate this possibility and to gain an understanding of the different selection pressures that could be acting during migration and throughout a godwit's life, future studies should track juvenile godwits and other migratory birds from birth to adulthood while also manipulating their spatiotemporal environment during development.
In many taxa, the most common form of sex-biased migration timing is protandry-the earlier arrival of males at breeding areas. Here we test this concept across the annual cycle of long-distance migratory birds. Using more than 350 migration tracks of small-bodied trans-Saharan migrants, we quantify differences in male and female migration schedules and test for proximate determinants of sex-specific timing. In autumn, males started migration about 2 days earlier, but this difference did not carry over to arrival at the non-breeding sites. In spring, males on average departed from the African non-breeding sites about 3 days earlier and reached breeding sites ca 4 days ahead of females. A cross-species comparison revealed large variation in the level of protandry and protogyny across the annual cycle. While we found tight links between individual timing of departure and arrival within each migration season, only for males the timing of spring migration was linked to the timing of previous autumn migration. In conclusion, our results demonstrate that protandry is not exclusively a reproductive strategy but rather occurs year-round and the two main proximate determinants for the magnitude of sex-biased arrival times in autumn and spring are sex-specific differences in departure timing and migration duration.
The uneven distribution of biodiversity on Earth is one of the most general and puzzling patterns in ecology. Many hypotheses have been proposed to explain it, based on evolutionary processes or on constraints related to geography and energy. However, previous studies investigating these hypotheses have been largely descriptive due to the logistical difficulties of conducting controlled experiments on such large geographical scales. Here, we use bird migration-the seasonal redistribution of approximately 15% of bird species across the world-as a natural experiment for testing the species-energy relationship, the hypothesis that animal diversity is driven by energetic constraints. We develop a mechanistic model of bird distributions across the world, and across seasons, based on simple ecological and energetic principles. Using this model, we show that bird species distributions optimize the balance between energy acquisition and energy expenditure while taking into account competition with other species. These findings support, and provide a mechanistic explanation for, the species-energy relationship. The findings also provide a general explanation of migration as a mechanism that allows birds to optimize their energy budget in the face of seasonality and competition. Finally, our mechanistic model provides a tool for predicting how ecosystems will respond to global anthropogenic change.
Migration is a widespread phenomenon in the animal kingdom. On the basis of the considerable variation that exists between and within species, and even within populations, we may be able to infer the (age‐ and sex‐specific) ecological trade‐offs and constraints moulding migration systems from assessments of fitness associated with migration and wintering in different areas.
During three consecutive breeding seasons, we compared the reproductive performance (timing of breeding, breeding success, chick body condition and post‐fledging survival) of Eurasian spoonbills P latalea leucorodia that breed at a single breeding site in The Netherlands, but migrate different distances ( c . 4,500 vs. 2,000 km, either or not crossing the Sahara) to and from wintering areas in southern Europe and West Africa. Using mark – recapture analysis, we further investigated whether survival until adulthood (recruitment probability) of chicks hatched between 2006 and 2010 was related to their hatch date and body condition.
Long‐distance migrants bred later, particularly the males, and raised chicks of poorer body condition than short‐distance migrants. Hatch dates strongly advanced with increasing age in short‐distance migrants, but hardly advanced in long‐distance migrants, causing the difference in timing of breeding between long‐ and short‐distance migrants to be more pronounced among older birds.
Breeding success and chick body condition decreased over the season, and chicks that fledged late in the season or in poor condition were less likely to survive until adulthood. As a result, long‐distance migrants—particularly the males and older birds—likely recruit fewer offspring into the breeding population than short‐distance migrants. This inference is important for predicting the population‐level consequences of changes in winter habitat suitability throughout the wintering range.
Assuming that the long‐distance migrants—being the birds that occupy the traditional wintering areas—are not the poorer quality birds, and that the observed age‐dependent patterns in timing of breeding are driven by within‐individual effects and not by selective disappearance, our results suggest that the strategy of long‐distance migration, involving the crossing of the Sahara to winter in West Africa, incurred a cost by reducing reproductive output, albeit a cost paid only later in life.
Migrating long distances requires time and energy, and may interact with an indi-vidual's performance during breeding. These seasonal interactions in migratory animals are best described in populations with disjunct nonbreeding distributions. The black-tailed godwit (Limosa limosa limosa), which breeds in agricultural grasslands in Western Europe, has such a disjunct nonbreeding distribution: The majority spend the nonbreeding season in West Africa, while a growing number winters north of the Sahara on the Iberian Peninsula. To test whether crossing the Sahara has an effect on breeding season phenology and reproductive parameters, we examined differences in the timing of arrival, breeding habitat quality, lay date, egg volume, and daily nest survival among godwits (154 females and 157 males), individually marked in a breeding area in the Netherlands for which wintering destination was known on the basis of resightings. We also examined whether individual repeatability in arrival date differed between birds wintering north or south of the Sahara. Contrary to expectation, godwits wintering south of the Sahara arrived two days earlier and initiated their clutch six days earlier than godwits wintering north of the Sahara. Arrival date was equally repeatable for both groups, and egg volume larger in birds wintering north of the Sahara. Despite these differences, we found no association between wintering location and the quality of breeding habitat or nest survival. This suggests that the crossing of an important ecological barrier and doubling of the migration distance, twice a year, do not have clear negative reproductive consequences for some long-distance migrants. K E Y W O R D S carryover effect, limosa limosa, migration, phenology, repeatability, wintering strategies
In this study we compare autumn migration strategies in two subspecies of the Common Ringed Plover Charadrius hiaticula in the southern Baltic Sea. This species exhibits a leap-frog migration pattern, whereby northerly breeding populations (C. h. tundrae; henceforth tundrae) migrate past the whole range of the southern population (C. h. hiaticula; henceforth hiaticula). Hiaticula migrates the shortest distance and is hypothesised to minimize energy spent on migration, whereas tundrae is hypothesized to minimize time, because a longer migration imposes time constraints upon other stages of a migrant's life history, such as moult and breeding. We use biometric data collected at Ottenby Bird Observatory, southern Öland, Sweden, between 1946-2012, to test whether each subspecies demonstrates characteristics associated with either an energy- or time-minimized migration. We used the decline in wing and total-head length over the season to distinguish the subspecies. Hiaticula migrated earlier in the season (July-mid August) compared to tundrae (late July-late September). Also the relative timing of age groups between the two subspecies differed. Juvenile and adult hiaticula migrated synchronized in time, whereas tundrae had two main periods of passage for adults (earlier) and juveniles (later). The timing fits that of other studies and gives complementary information about the passage in Europe. Juvenile tundrae showed a positive trend of observed fuel loads as the season progressed, whereas the other groups did not. Daily fuelling rates within the same season were low compared to other wader species that use Ottenby as a stopover and no difference between subspecies was found. However, tundrae stopped over for a shorter time compared with hiaticula. There was no difference in average migration speed between the subspecies, although tundrae had a higher maximum speed. There was large variation in yearly numbers between age classes in the two subspecies. In hiaticula the yearly average was 8 adults and 37 juveniles. The ratio of juvenile to adult tundrae on the other hand was almost 1:1, indicating equal use of Ottenby as stop over site in autumn. No trends in observed fuel loads over the season in hiaticula are consistent with an energy minimizing migration. As for tundrae, a shorter stopover time compared to hiaticula and a positive relationship between observed fuel load and time, are consistent with time minimizing migration. However, there are many uncertainties and more studies of migratory behaviours at several sites along the migration route are needed to understand the differences in migration strategies in this species.
The behavioural rhythms of organisms are thought to be under strong selection, influenced by the rhythmicity of the environment. Such behavioural rhythms are well studied in isolated individuals under laboratory conditions, but free-living individuals have to temporally synchronize their activities with those of others, including potential mates, competitors, prey and predators. Individuals can temporally segregate their daily activities (for example, prey avoiding predators, subordinates avoiding dominants) or synchronize their activities (for example, group foraging, communal defence, pairs reproducing or caring for offspring). The behavioural rhythms that emerge from such social synchronization and the underlying evolutionary and ecological drivers that shape them remain poorly understood. Here we investigate these rhythms in the context of biparental care, a particularly sensitive phase of social synchronization12 where pair members potentially compromise their individual rhythms. Using data from 729 nests of 91 populations of 32 biparentally incubating shorebird species, where parents synchronize to achieve continuous coverage of developing eggs, we report remarkable within- and between-species diversity in incubation rhythms. Between species, the median length of one parent’s incubation bout varied from 1–19 h, whereas period length—the time in which a parent’s probability to incubate cycles once between its highest and lowest value—varied from 6–43 h. The length of incubation bouts was unrelated to variables reflecting energetic demands, but species relying on crypsis (the ability to avoid detection by other animals) had longer incubation bouts than those that are readily visible or who actively protect their nest against predators. Rhythms entrainable to the 24-h light–dark cycle were less prevalent at high latitudes and absent in 18 species. Our results indicate that even under similar environmental conditions and despite 24-h environmental cues, social synchronization can generate far more diverse behavioural rhythms than expected from studies of individuals in captivity. The risk of predation, not the risk of starvation, may be a key factor underlying the diversity in these rhythms.
For vertebrates, annual cycles are organized into a series of breeding and non-breeding periods that vary in duration and location but are inextricably linked biologically. Here, we show that our understanding of the fundamental ecology of four vertebrate classes has been limited by a severe breeding season research bias and that studies of individual and population-level responses to natural and anthropogenic change would benefit from a full annual cycle perspective. Recent emergence of new analytical and technological tools for studying individual and population-level animal movement could help reverse this bias. To improve understanding of species biology and reverse the population declines of many vertebrate species, a concerted effort to move beyond single season research is vital.
© 2015 The Author(s).
Capsule Little Ringed Plovers breeding in South Sweden migrate towards the southeast in the autumn, via the Middle East, to winter in Saharan and sub-Saharan locations or in India, while the spring migration is more directly towards the north.Aims To study the migration routes and wintering area of Little Ringed Plovers (Charadrius dubius) breeding in South Sweden, and to investigate the migration strategy and speed for this little studied shorebird.Methods We use light-level geolocators to track the year-round movements of Little Ringed Plovers breeding in South Sweden.Results Autumn migration proceeded towards the southeast, in three birds via lengthy stopovers in the Middle East, followed by movements towards the west and southwest to final winter destinations in Africa, while one male made a long stopover in northwestern Iran before migrating to India. The birds wintering in Africa probably stayed at freshwater locations in the Sahara or just south or north of the Sahara. Spring migration was more directly back to the breeding area. Overall migration speeds were similar during autumn and spring migration at about 189 and 209km/day, respectively. The migration was carried out mainly as many short flights between stopovers. In particular, autumn migration was longer than the direct distance between breeding and wintering sites.Conclusions This study shows that the geolocator method can successfully be used with relatively small (40g) shorebirds. We found that a local population of Little Ringed Plover may have widely differing wintering sites (low connectivity), from sub-Saharan Africa to the Indian subcontinent. The migration strategy of the Little Ringed Plover, with multiple short flights, deviates from that of many other long-distance migrating shorebirds that, instead, make one or a few long flights.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
1. Determining global position by light measurements (‘geolocation’) has revolutionised the methods used to
trackmigratory birds throughout their annual cycle.
2. To date, there is no standard way of analysing geolocator data, making communication of analyses cumbersome
and hampering the reproducibility of results.
3. We have, therefore, developed the R package GeoLight, which provides basic functions for all steps of determining global positioning and a new approach in analysingmovement pattern.
4. Here, we briefly introduce and discuss the major functions of this package using example movement data of European hoopoe (Upupa epops).
Reviews hypotheses to account for differential migration, ie. where all individuals of a population migrate but distance travelled varies according to sex and/or age, using data from study of dark-eyed junco Junco h. hyemalis. Single-factor hypotheses involve ideas on body size, dominance and arrival time, but a multifactor hypothesis is here proposed, and the idea of a migration threshold mooted, ie. birds move when the advantage of remaining at one site are just outweighed by the advantages of leaving it.-P.J.Jarvis
We compared a number of univariate and multivariate measures of body size used commonly in ornithological research, including eight multivariate measures (from prin- cipal components analyses), plus skull length, ulna length, tibiotarsus length, wing length, and weight. Analyses are based on 26 measurements on three randomly selected male and three randomly selected female Savannah Sparrows (Passerculus sandwichensis) from each of 53 different geographic localities throughout the species' range. Six of the eight principal components analyses provided essentially the same information about body size. Analyses based on the variance-covariance matrix of raw or log-transformed data provided first axes that varied most from the other multivariate estimates of size. Among the univariate measures, ulna length, wing length, and body weight contributed information that diverged from the multivariate measures of overall size. Weight better represents general size (i.e. PC I) than wing length, but because of variation in reproductive condition, weight is a far better measure in males than in females. Wing length is not a representative measure of body size. Inasmuch as each principal components analysis provides information about body size on PC I, we encourage researchers to choose among the various approaches according to analytical ob- jectives rather than methodological simplicity or general utility. Received 29 November 1988, accepted 18 May 1989. ORNITHOLOGISTS are frequently faced with the challenge of measuring body size in birds. A measure of overall size is required to test hy- potheses predicting patterns of geographic variation (e.g. Bergmann's or Allen's rules; James
Three hypotheses have been proposed to explain sexual differences in wintering latitude for different bird species: (1) intersexual behavioral dominance leads the subordinate sex to migrate farther to avoid competition; (2) intrasexual selection favors those individuals of one sex that arrive earlier and thus selects for wintering closer to the breeding ground; and (3) sexual differences in physiological tolerance allow the larger sex to survive harsher climates. Using sex, age, date, and location data from specimens collected south of the breeding range in the western hemisphere, I tested predictions of these hypotheses for two scolopacid shorebirds showing reverse sexual size dimorphism, the red phalarope, Phalaropus fulicarius, and the sanderling, Calidris alba.Neither red phalaropes nor adult sanderlings showed any sexual difference in wintering latitude. First-winter male sanderlings tended to winter farther south than first-winter females. Combined with comparative data from other species of shorebirds and passerines, these results are consistent only with hypothesis 2.
Dark-eyed Juncos (./unto hyemalis) residing in central and eastern United States during winter exhibit clinal variation in sex ratio: the percentage of females increases with decreasing latitude from a low of approximately 20% in northern states to a high of about 75% in Texas (Ketterson and Nolan 1976). Similar cases of geographic variation in winter sex ratio have been described in other avian species (Nice 1937, Lack 1944, King et al. 1965, and references cited in Ketterson and Nolan 1976). Since energetic and other costs of migration presumably increase with distance traveled, it is interesting to inquire about compensating benefits that may accrue to the average female in her more southerly wintering grounds. One obvious possibility is a lessening of intersexual competition during the season when the sexes are not required to reside together and food is likely to be in short supply (Selander 1966, Balph 19'75, Ketterson and Nolan 1976). Recent studies indicate that among captive flocks, male juncos tend to dominate females (Balph 1977) and dominance improves survivorship (Baker and Fox 1978). If food has been limiting and males have tended over evolutionary time to dominate available resources, this may account for the longer migrations of females (Ketterson and Nolan 1976, Gauthreaux 1978). The purpose of the present paper is to corroborate male dominance in free- ranging flocks and to report on other determinants of dominance.
The nonbreeding distribution ofWestern Sandpipers (Calidris mauri)was documented
using 19 data sets from 13 sites along the Pacific and Atlantic coasts of the Americas.
Western Sandpipers showed latitudinal segregation with regard to sex and age. Females
wintered farther south than males. A ‘‘U’’ shaped pattern was found with respect to age,
with juveniles occurring at higher proportions at both the northern and southern ends of the
range. Distribution of sexes might be affected by differences in bill length and a latitudinal
trend in depth distribution of prey. For age class distribution, two different life-history tactics
of juveniles might exist that are related to the higher cost of feather wear for juveniles
compared to adults. Most juveniles complete three long-distance migrations on one set of
flight feathers whereas adults complete two. Juveniles may winter either far north, thereby
reducing feather wear induced by ultraviolet light, migration, or both, or far south and spend
the summer on the nonbreeding area
The question of how birds migrate over enormous distances with apparently minimal guidance continues to excite both professional and amateur ornithologists. Nearly ten years have elapsed since Peter Berthold, a leading researcher in the field, wrote the first edition of this highly readable and fascinating book. During that time the field has advanced by strides, so that this new edition has been extensively revised, expanded, and updated. No other book exists that brings together the vast amount of information that is available on the subject of bird migration, so that the book will be an inspiration to birdwatchers, naturalists, and ornithologists alike.
Protandry, the earlier arrival of males than of females on breeding areas, occurs in many taxa, including many migratory birds. Numerous hypotheses have been generated to explain protandry. Using bird-banding records, I show that protogyny, the earlier migration of females, frequently occurs in the autumn, though it is less universal and less dramatic than spring protandry. In one species, it occurs in both hatch-year and adult birds. When (1) spring and autumn, (2) departures and arrivals, and (3) breeding and wintering ground conditions are considered, hypotheses generated only to explain spring protandry can be more thoroughly evaluated. Using that approach, the most parsimonious explanation of differential migration between the sexes explains earlier male arrival in spring and later male departure in autumn through either (1) indirect selection operating on intrasexual male competition for territories or (2) direct selection operating on intersexual relations requiring males to be present on breeding territories when females are present. In autumn-protogynous species, males may ”play chicken,” balancing the benefits of remaining longer than females and protecting territories for subsequent years against the costs of remaining in the north under deteriorating conditions and delaying the acquisition of a good winter territory.
Protogynie et migration automnale: Est-ce que les mâles ”jouent les dégonflés”?
Parental behavior during incubation is an important aspect of the breeding system, which varies greatly among shorebirds. There are, however, few studies of incubation sex roles in shorebirds during darkness. In Charadrius species, males are believed to perform most of the incubation during the night. In this study of night- and daytime incubation sex roles in the Common Ringed Plover (Charadrius hiaticula), males tended to do more of the nighttime incubation (58%) than did females; during the day the roles tended to be reversed (males 45%), but the differences were not statistically significant. The reasons why mates of the Common Ringed Plover seem to share nocturnal incubation more equally than do other Charadrius species are not clear but may involve differences in food levels and day length between areas.
Papel de los Sexos durante la Incubación en Charadrius hiaticula
Resumen. El comportamiento parental durante la incubación es un aspecto importante del sistema reproductivo, el cual varía considerablemente entre las aves playeras. Sin embargo, los estudios sobre el papel de los sexos en la incubación nocturna en las aves playeras son escasos. En las especies de Charadrius, se cree que los machos realizan la mayor parte de la incubación nocturna. En este estudio, sobre el papel de los sexos durante los periodos de incubación nocturnos y diurnos en Charadrius hiaticula, los machos tendieron a realizar una mayor parte de la incubación nocturna (58%) que las hembras; durante el día los papeles tendieron a revertirse (machos 45%), pero las diferencias no fueron significativas estadísticamente. Las razones por las cuales las parejas de C. hiaticula parecen compartir más equitativamente la incubación nocturna que otras especies de Charadrius no están claras pero pueden estar relacionadas con diferencias en los niveles de alimento y duración del día entre áreas.
In order to test hypotheses explaining intraspecific differences in migration patterns of birds, a four-year field study of migrant raptors in southernmost Sweden was undertaken. The age and sex ratios among autumn migrants departing from the province of Scania at Falsterbo were compared to the corresponding ratios among birds of the same species that stayed in Scania the following winter. In most species (Red Kite [Milvus milvus], White-tailed Eagle [Haliaeetus albicilla], Northern Harrier [Circus cyaneus], Goshawk [Accipiter gentilis], Sparrowhawk [A. nisus], Common Buzzard [Buteo buteo] and Eurasian Kestrel [Falco tinnunculus]), the proportion of adults was distinctly higher among wintering birds than among the migrants. A marked exception was the Rough-legged Hawk (B. lagopus), which had significantly more juveniles among the wintering birds. In most species where the sexes could be separated more females were found among the wintering birds than among the migrants (Northern Harrier, Goshawk, Sparrowhawk and Rough-legged Hawk). The Eurasian Kestrel, however, showed the reverse pattern with a higher proportion of males among wintering birds. The results support the social-dominance hypothesis, where the dominant adults and the larger sex (in raptors the female) generally winter furthest north. That juveniles are driven away from the breeding territories is suggested by a significantly higher proportion of juvenile Common Buzzards wintering in marginal coastal regions as compared to the inland portion of Scania. The results are also in general agreement with Bergmann's rule. The higher proportion of male Eurasian Kestrels among wintering birds is most likely explained by the importance of early arrival at the breeding territory.
During winter, ground-feeding bird species may occasionally be forced to fast when weather conditions (e.g., snowstorms, icing) make food temporarily inaccessible. To describe the response of White-crowned Sparrows to fasting, we (1) investigated variations of body weight and temperature in fasting and fed birds confined in a cold room for up to 64 h, and (2) periodically measured oxygen consumption and locomotor activity of fasting and fed birds through a 39-h night/day/night cycle while simulating natural conditions of temperature and daylength. Our data indicate that the unusual White-crowned Sparrow might survive 3 nights and 2 days without food at air temperatures near freezing, but typical individuals could not be expected to survive longer than 1 day and 2 nights. Males lose weight at a greater rate than females, but are able to endure fasting longer because they are initially heavier. This suggests that males may be more resistant to winter storms and food deprivation, and that this attribute may have contributed to the evolution of geographic variation in the sex ratio in this species. Physiological and metabolic responses to fasting included a nighttime decline in body temperature (ca. 3 C) that was greater than that observed in fed birds, and a 21% decline in oxygen consumption on the second night of the fasting cycle when compared to the first night. The drop in body temperature would be sufficient, in terms of the van't Hoff effect, to account for the decline of metabolic rate, but visual inspection of the fasting birds at night suggested that adjustments in ptiloerection and posture may also play a role in effecting the decline. During the day, locomotor activity was 1.5-10 times greater in fasted than fed birds. Oxygen consumption, however, did not differ, indicating (as one possibility) that fasting birds may be able to utilize heat generated by exercise to meet some of the cost of thermoregulation. The short-term adjustment to starvation under natural conditions may thus include a moderately reduced energy requirement and a partial diversion of energy expenditure from thermoregulation to foraging.
In most mammals, both sexes display different survival patterns, often involving faster senescence in males. Being under intense sexual competition to secure mating opportunities, males of polygynous species allocate resources to costly behaviors and conspicuous sexual traits, which might explain these observed differences in longevity and senescence patterns. However, comparative studies performed to date have led to conflicting results. We aimed to resolve this problem by first reviewing case studies of the relationship between the strength of sexual selection and age-specific survival metrics. Then, we performed a comprehensive comparative analysis to test whether such relationships occur among species of captive ruminants. We found that the strength of sexual selection negatively influenced the onset of actuarial senescence in males, with males senescing earlier in polygynous than in monogamous species, which led to reduced male longevity in polygynous species. Moreover, males of territorial species senesced earlier but slower, and have a shorter longevity than males of species displaying other mating tactics. We detected little influence of the strength of sexual selection on the rate of actuarial senescence. Our findings demonstrate that the onset of actuarial senescence, rather than its rate, is a side effect of physiological mechanisms linked to sexual selection, and potentially accounts for observed differences in longevity. This article is protected by copyright. All rights reserved.
How can geckoes walk on the ceiling and basilisk lizards run over water? What are the aerodynamic effects that enable small insects to fly? What are the relative merits of squids' jet-propelled swimming and fishes' tail-powered swimming? Why do horses change gait as they increase speed? What determines our own vertical leap? Recent technical advances have greatly increased researchers' ability to answer these questions with certainty and in detail. This text provides an up-to-date overview of how animals run, walk, jump, crawl, swim, soar, hover, and fly. Excluding only the tiny creatures that use cilia, it covers all animals that power their movements with muscle--from roundworms to whales, clams to elephants, and gnats to albatrosses. The introduction sets out the general rules governing all modes of animal locomotion and considers the performance criteria--such as speed, endurance, and economy--that have shaped their selection. It introduces energetics and optimality as basic principles. The text then tackles each of the major modes by which animals move on land, in water, and through air. It explains the mechanisms involved and the physical and biological forces shaping those mechanisms, paying particular attention to energy costs. Focusing on general principles but extensively discussing a wide variety of individual cases, this is a superb synthesis of current knowledge about animal locomotion. It will be enormously useful to advanced undergraduates, graduate students, and a range of professional biologists, physicists, and engineers.
“Optimization is the process of minimizing costs or maximizing benefits, or obtaining the best possible compromise between the two. Evolution by natural selection is a process of optimization” (R. McNeill Alexander 1982).
(1) In this paper I report on how long-term behaviour patterns of grey plovers in the non-breeding season are established in the first year of life. (2) The behaviour of colour-marked individuals was observed at Teesmouth, an estuary in north-east England. I measured the wing lengths of captured birds, as an estimate of body size. (3) Each individual was consistent from its first year onwards in (a) being territorial or feeding non-territorially, and (b) in the timing of seasonal movements to and from the estuary. (4) The lifetime territorial behaviour and timing of seasonal movements of individuals were established during competition especially with adults. (5) Longer-winged juveniles were more likely to acquire territories and more likely to stay at Teesmouth all winter. (6) Two displaced juveniles have been recorded, in the same autumn, up to 900 km further south, in France.
At Lake Kvismaren, S-central Sweden, numbers of breeding birds increased over 1984-1990. About 55% of the breeding birds returned to the study site between years and 80% returned to the same breeding marsh. Birds that had bred successfully were more likely to return to their breeding marsh. Between years, 18% of females and 6% of males returned to breed in their former territories and males tended to move further than females. Less than 15% of the females with their previous year's mate alive, re-established the pair bond. The order of territory settlement was similar between years. Both males and females settled earlier in yeart+1 in territories that were occupied early in yeart, than in territories occupied late or in territories that had been unoccupied. Males and females that arrived relatively earlier than they did the year before occupied a territory of higher rank. Such males increased their reproductive success significantly; females did not. Males became mated with more females and had higher reproductive success in the attractive territories. Reproductive success of females did not differ between attractive and less attractive territories. The adaptive significance of returning to their breeding marsh (neighbourhood fidelity) is apparently that the birds can use knowledge about the quality of several territories, gathered in yeart, and settle in the best territory available at the time of their arrival in yeart+1. -from Authors
1. With the present interest in analysing and experimenting upon migration, it is important to keep in mind that it is a product of natural selection, but the danger of the evolutionary approach is that it is speculative. 2. It is probably advantageous for migrant birds to leave their breeding grounds because they could not find enough food there in winter (though they usually leave before food is short), and to return there because they can find more food for their young there than in crowded winter quarters. 3. Correspondingly, the size of the world population of a migrant species is probably determined by winter supplies and its breeding dispersion (range, habitat etc.) by where it can find most food for its young. 4. The critical factor is not the absolute quantity of food, but the amount available in relation to actual or potential competition. Indirect evidence for this is provided by the existence of competitive exclusion among related species on the wintering grounds. 5. The advantages of, respectively, migration and winter residence are roughly equal in partial migrants, but they differ in different places, different years in the same place, and different sexes and age-groups. 6. The historical factor in migration was formerly over-stressed, but it has had an influence on directions taken because it is hard to evolve a successful hereditary change in direction. 7. Movements into or out of England occur almost every day throughout the winter, hard-weather movements south or west because food becomes scarce, return movements in each mild spell presumably owing to intense competition in the crowded milder areas. 8. Irruptive movements, like migration, can have been evolved through natural selection. /// 1. В настоящее время возрос интерес к экспериментальному исследованию перелетов птиц. При этом важно помнить, что перелеты продукт естественного отбора. Опасность эволюционных подходов к этой проблеме состоит в том, что они могут быть спекулятивны. 2. Приспособление птиц к сезонным перелетам дает им ряд преимуществ в связи с тем, что птицы покидоют зимой места обитания, где они не могут достать пищу (хотя они улетают до того, как пища исчезает), и возвпащаются обратно, т.к. здесь легче прокормить своих птенцов, чем в густо населенных местах зимовки. 3. Размеры мировых популяций перелетных птиц определяются их зимней численностью, а также характером распределения при перелетах (дальность перелета, выбор летней резиденции и т.п.). 4. Критическим фактором является не абсолютное количество имеющейся пищи, а доступная пища в соотношении с потенциальной или существующей конкуренцией. Косвенным доказательством этого является элиминация родственных видов птиц в местах зимовки в результате конкуренции. 5. Преимущества зимних и летних резиденций у мигрантов грубо равны, но они различаются в разных местах и в разные годы, а также в одних и тех же местах, но для птиц разного пола и возпаста. 6. Исторический фактор перелетов птиц раньше переоценивался, но он имеет существенное значение в том отношении, что направление пролетного пути у птиц наследственно закрепляется. 7. Ежедневно в течение всей зимы регистрируются пролеты птиц на территорию Англии и обратно. Отмечены также пролеты на юг и на запад в поисках пищи при ухудшении погоды и обратные пролеты, очевидно в результате более сильной конкуренции в заселенных местах. 8. Местные непериобические перемещения птиц, как и перелеты, могут рассматриваться как результат естественного огбора.
We examined 824 museum skins and frozen specimens of Snowy Owls Nyctea scandiaca to determine the winter distribution of age-sex classes in North America. Although there was nearly total overlap in the distributions of the four age-sex classes, the proportions of each class varied dramatically and consistently along a north-south axis. On average, immature males wintered farthest south, while adult females wintered farthest north. The large proportion of adult specimens from the Central Zone indicates that the Great Plains is a regular wintering area for owls, while areas to the east and west are subject to irruptive movements composed primarily of first-year birds. Social dominance may explain the observed differences in winter distribution.
The sex ratio of Dark-eyed Juncos wintering in the eastern and central United States and Canada varies clinally along a latitudinal gradient. The percentage of @V @V among both museum skins and live-caught birds is @?70% in the south, 20% in the north. When abundance according to latitude is also considered, an average @V appears to winter farther south than an average @M and hence probably tends to migrate farther. Latitude alone is an excellent predictor of sex ratio (r^2 = 85%), and latitude plus 13 other measures of climate explain virtually all the variation (r^2 = 96.6%). Extreme measures of climate, as compared to mean measures, are equally predictive. Principal component analysis indicates that snowfall, temperature, and latitude are the most important climatic variables associated with sex ratio. Because @M @M average larger than @V @V and are concentrated northward, mean wing length increases with latitude and is significantly correlated with climatic measures that vary with latitude. Further, larger birds within each sex may select higher altitudes as wintering sites. Sex ratio does not vary measurably with date in wintering populations. Among possible explanations for clinal variation in sex ratio are sex-associated differences in (1) advantages of early arrival on the breeding or wintering grounds, (2) impacts of inter- and intrasexual competition, and (3) effects of low temperature and intermittent food availability. Comparison of @M @M and @V @V with respect to potential fasting endurance, a size-related metabolic parameter, indicates that at 0 degrees C an average @M should be able to fast 4% longer (1.6 h) than an average @V at standard metabolic rates. An extremely heavy @M might endured fasting up to 29% (10.7 h) longer than a very light @V. These differences may confer greater survival ability upon the @M at latitudes where snow cover can often preclude feeding.
The full magnitude and complexity of bird migration have not been possible to grasp until after the revolutionary discoveries made with the aid of, e.g., ringing, systematic field observations and radar. These have opened up opportunities for a more formalized theoretical construction work during the recent decades. The introduction of theoretical concepts and tools from flight mechanics into the field of bird flight and migration took place during the 1960s and 1970s and paved the way for the use of optimization analysis to evaluate adaptive aspects of flight behaviour, fuel deposition and responses to wind drift by migrating birds. The approach has been expanded, i.e. through the use of stochastic dynamic programming, to analyse the expected disposition of the migratory journey (with respect to Eight and stopover arrangement) under different ecological conditions and the adaptive temporal structure of the annual cycle of a migratory bird. Theoretical considerations also play an important role in analysing the orientation of migratory birds, their population ecology, and patterns of differential migration. Theoretical developments in these areas are reviewed. In the long run evolutionary and mechanistic theories must meet and join to provide a full understanding. While optimization models standing up to critical tests may help to identify primary forces of balancing selection and constraints, mechanistic theories are needed to tell us how the inherent biological algorithms operate within the sensory, neural and physiological systems to control behaviour and design in an adaptive way.
Production of successive clutches within the same breeding season has received less attention than many other aspects of avian reproduction. Waders are of particular interest because in these birds, multiple clutches are associated with at least three different breeding systems: double-clutching (uniparental care), monogamous double-brooding (biparental care) and polyandry (uni- or biparental care). Data from eight species and twelve breeding populations suggest that early second clutches, and thus brood overlap, are associated with parental role division and uniparental care, whereas species or populations with biparental care tend to have long intervals between successive clutches. We suggest that ecological factors influencing the relative timing of the second clutch will have consequences for the parental care system. In particular, conditions that favour early laying of the second clutch (large brood overlap) are likely to lead to parental role division, as found in double-clutching species. Factors determining the timing of second clutches are discussed, as are possibilities for testing these ideas.
Previously published data pooled over entire winters showed clinal variation in the winter sex ratio of Junco h. hyemalis. Females were less numerous than males in the north and progressively more numerous southward. In this study, regression coefficients calculated from the published data, with latitude and certain climatic measures as independent variables, accurately predicted the winter sex ratio of locations not previously sampled. Annual variation in sex ratio was nonsignificant. The pattern of distribution was established by the end of fall migration, before differential mortality could have accounted for it. Temperature rather than latitude, snowfall, or probable distance from the nearest breeding grounds correlated best with percentage of females in winter populations. It is suggested that climate has acted primarily indirectly to produce the sex-associated difference in winter distribution. Under this hypothesis, as climate becomes harsher, the impact of male dom- inance over females becomes more severe, resulting in increasing withdrawal by females from locations settled by males and thus in clinal variation in sex ratio. Received 15 December 1978, accepted 27 February 1979. WE earlier reported that the sex ratio of Dark-eyed Junco (Junco hyemalis) pop- ulations wintering in the eastern United States varies dinally, with the percentage of females increasing progressively toward the south (Ketterson and Nolan 1976). Latitude and a set of climatic variables accounted for more than 93% of the variation in sex ratio found in the 1976 data set. That sample consisted of 616 museum specimens and 473 live-caught birds, the former group collected over many winters, the latter banded by us during four winters. Only cases from the period December through February were considered, and these were combined for analysis. Climatic variables were based on weather averaged over 50 yr (U.S. Weather Bureau 1932). Thus the pattern of geographic variation that we described was based on long-term sampling of winter populations and weather. This approach left a number of inter- esting questions unanswered, and we now state these and present our recent efforts to answer them.
SYNOPSIS. Current evolutionary theory predicts that energy expenditure will be adjusted in contest situations to the value of the disputed resource and the relative probability of winningit. Estimates of energy expended in contest situations support this prediction. I report on theenergetic costs of display relative to other contest costs to individual fitness (e.g., risk ofpredation, losses in feeding time, injury and mortality) in territorial disputes of the spider Agelenopsis aperta. Cost estimates obtained in terms of decrements to milligrams wet-weight of future egg production resulting from single contests indicatethat actual energy expenditure inthese territorial disputes represent insignificant costs. Thesecosts are, in fact, 5-6 orders of magnitude smaller than the costs associated with injury, potential predation and even loss infood as a result of time spent in the interactions. Review of the literature indicates that in most instances, energy expenditure may be correlated with some other factor upon which selection is acting (e.g., short contests wherepredation risk is high, variation in levels of escalation exhibited). Two exceptions include the tremendous losses of workers to reproductives in ant, termite, and bee colony territorial disputes and the production of specializedagonistic organsexhibited by some corals, sea anemones and corallimopharians when encountering neighbors within"territorial" boundaries.
The relative fitness of individuals across a population can shape distributions and drive population growth rates. Migratory species often winter over large geographic ranges, and individuals in different locations experience very different environmental conditions, including different migration costs, which can potentially create fitness inequalities. Here we used energetics models to quantify the trade-offs experienced by a migratory shorebird species at locations throughout the nonbreeding range, and the associated consequences for migratory performance, survival, and breeding habitat quality. Individuals experiencing more favorable winter conditions had higher survival rates, arrived on the breeding grounds earlier, and occupied better quality breeding areas, even when migration costs are substantially higher, than individuals from locations where the energy balance on the wintering grounds was less favorable. The energy costs and benefits of occupying different winter locations can therefore create fitness inequalities which can shape the distribution and population-wide demography of migratory species.
For many migratory bird species, the latitudinal range of the winter distribution spans thousands of kilometres, thus encompassing considerable variation in individual migration distances. Pressure to winter near breeding areas is thought to be a strong driver of the evolution of migration patterns, as individuals undertaking a shorter migration are generally considered to benefit from earlier arrival on the breeding grounds. However, the influence of migration distance on timing of arrival is difficult to quantify because of the large scales over which individuals must be tracked. Using a unique dataset of individually-marked Icelandic black-tailed godwits Limosa limosa islandica tracked throughout the migratory range by a network of hundreds of volunteer observers, we quantify the consequences of migrating different distances for the use of stop-over sites and timing of arrival in Iceland. Modelling of potential flight distances and tracking of individuals from across the winter range shows that individuals wintering further from the breeding grounds must undertake a stop-over during spring migration. However, despite travelling twice the distance and undertaking a stop-over, individuals wintering furthest from the breeding grounds are able to overtake their conspecifics on spring migration and arrive earlier in Iceland. Wintering further from the breeding grounds can therefore be advantageous in migratory species, even when this requires the use of stop-over sites which lengthen the migratory journey. As early arrival on breeding sites confers advantages for breeding success, the capacity of longer distance migrants to overtake conspecifics is likely to influence the fitness consequences of individual migration strategies. Variation in the quality of wintering and stopover sites throughout the range can therefore outweigh the benefits of wintering close to the breeding grounds, and may be a primary driver of the evolution of specific migration routes and patterns.