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Review of seabird demographic rates and density dependence
Abstract and Figures
This report presents individual species accounts for a selection of British seabirds, sea ducks, divers and grebes. Each account gathers the most up to date published estimates on the following demographic parameters: age-specific survival, age-specific productivity, age of recruitment, incidence of missed breeding, and natal and adult breeding dispersal. Particular attention has been given to regional variation in demographic rates, indicating the extent to which estimates may be applied to other less-well studied colonies. Where possible, the intrinsic and extrinsic factors that influence demographic rates are also detailed.
The reported rates should enable population models that assess the impacts of offshore wind farms to be developed as reliably and realistically as possible. Where sufficient data could not be gathered using UK examples, data from colonies outside of the UK have been presented, or a proxy species has been identified. The evidence for density-dependent regulation of seabird demographic rates is also reviewed using examples from the UK, as well as non-UK studies on similar species.
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... These values reflect the demographic profile of Manx shearwaters breeding on Skomer Island that exhibit elevated rates of fecundity and breeding propensity . Like many seabirds, Manx shearwaters are largely unobservable during the first years of life and estimates of juvenile survival (i.e., during the first year post fledging j,t ) are lacking (Horswill & Robinson, 2015). Juvenile survival of seabirds is typically lower than that of adults (Horswill & Robinson, 2015) and additive across age classes (Cam et al., 2005;Horswill et al., 2014). ...
... Like many seabirds, Manx shearwaters are largely unobservable during the first years of life and estimates of juvenile survival (i.e., during the first year post fledging j,t ) are lacking (Horswill & Robinson, 2015). Juvenile survival of seabirds is typically lower than that of adults (Horswill & Robinson, 2015) and additive across age classes (Cam et al., 2005;Horswill et al., 2014). Therefore, to estimate juvenile survival for each year and colony, we applied a theoretical 50% scalar to the imputed time series of adult survival. ...
... Furthermore, to standardise the analysis, we apply a constant rate of fecundity and breeding propensity to both colonies. Typically, fecundity is highly temporally (Horswill & Robinson, 2015) and spatially variable in seabirds. Environmental stochasticity can also increase the demographic impact of demographic rates that typically have a low influence on population growth, such as fecundity in long-lived species (Gaillard et al., 1998). ...
Understanding the processes that drive interpopulation differences in demography and population dynamics is central to metapopulation ecology. In colonial species, populations are limited by local resource availability. However, individuals from larger colonies will travel greater distances to overcome density‐dependent competition. Consequently, these individuals may also experience greater carry‐over effects and interpopulation differences in demography. To test this prediction, we use mark‐recapture data collected over four decades from two breeding colonies of a seabird, the Manx shearwater ( Puffinus puffinus ), that exhibit strong spatial overlap throughout the annual cycle but differ in population size and maximum foraging distances. We quantify interpopulation differences and synchrony in rates of survival and assess whether local mean wind speeds act to strengthen or disrupt synchrony. In addition, we examine whether the imputed interpopulation differences in survival can generate population‐level consequences. The colony where individuals travel further during the breeding season had slightly lower and more variable rates of survival, indicative of individuals experiencing greater carry‐over effects. Fluctuations in survival were highly synchronous between the colonies, but neither synchronous, nor asynchronous, variation could be strongly attributed to fluctuations in local mean wind speeds. Finally, we demonstrate that the imputed interpopulation differences in rates of survival could lead to considerable differences in population growth. We hypothesise that the observed interpopulation differences in rates of adult survival reflect carry‐over effects associated with foraging distances during the breeding season. More broadly, our results highlight that breeding season processes can be important for understanding interpopulation differences in the demographic rates and population dynamics of long‐lived species, such as seabirds.
... Currently, most population models are also based on the assumption that individual breeding colonies are closed systems, partly due to a lack of empirical data on dispersal. However, dispersal of individuals between colonies, particularly natal dispersal, can be substantial in many seabird species (Horswill and Robinson, 2015), which emphasises the need for meta-population models to account for this connectivity (Inchausti and Weimerskirch, 2002;Jeglinski et al., 2023). ...
... Unknown to what extent dispersal rates and distances differ between regions. Age of first breeding/ recruitment Moderate Rate is relatively well quantified; however, the estimates are based on relatively old data and from a small number of colonies (Horswill and Robinson, 2015). Regional variation is not well understood, especially in relation to variation in colony size and regulatory mechanisms as mediated by density dependence. ...
... Moderate Adult survival rate estimates are available for several colonies across the Kittiwake's range, however few are recent and regional variation is not well represented or understood (Horswill and Robinson, 2015). ...
Species face a multitude of stressors due to human activities, especially in marine environments. Seabirds are among the most threatened group of birds globally. A key challenge in their conservation is quantifying the impact of multiple interacting stressors on populations effectively, especially for species that undertake large-scale movements. We use the relatively well-studied Black-legged Kittiwake Rissa tridactyla as a case study to highlight knowledge gaps in demographic rates and how key stressors act on populations across different parts of their biogeographic range. From this starting point we provide a strategic approach to identify and prioritise data collection and research efforts from species and regions where data are currently lacking. Obtaining accurate and precise empirical data on demographic rates and movement will increase the pre-dictive accuracy, and realism, of population models, and confidence in how populations will respond to multiple stressors over the life and annual cycle, facilitating better management decisions.
... Arctic Jaeger Based on related species Furness et al., 2013Horswill & Robinson, 2015Wade et al., 2016 Arctic Loon Based on related species Furness et al., 2013Horswill & Robinson, 2015Wade et al., 2016 Arctic ...
... Arctic Jaeger Based on related species Furness et al., 2013Horswill & Robinson, 2015Wade et al., 2016 Arctic Loon Based on related species Furness et al., 2013Horswill & Robinson, 2015Wade et al., 2016 Arctic ...
... We Horswill and Robinson (2015). Data for other species were taken from the literature. ...
With the projected increases in shipping activity and hydrocarbon extraction globally, there is an increased risk of negative ecological impacts from oil pollution on the marine environment, including seabirds. Oil Vulnerability Indices (OVIs) are a common approach to assess seabird species vulnerability to oil pollution and to identify where species are most at risk, typically across regional spatial scales and for a relatively limited number of species. This approach generally requires comprehensive data on at-sea distributions and densities; however, for many regions, these data are limited. We present a simplified OVI to assess seabird species vulnerability to oil pollution. To create the spatial component of the OVI, we used a predictive foraging radius approach, using existing colony size and foraging range data, to project at-sea distributions of seabird populations during the breeding season. We demonstrate this approach over a large spatial scale, the eastern North Atlantic, which includes areas where seabird at-sea data are lacking. Our results reveal areas off west Greenland, Iceland, and Norway where seabirds are most vulnerable to oil pollution during the breeding season, largely driven by large colonies of auks (Alcidae). We also identify locations along the coast of mainland Norway, Iceland, and Scotland, where seabirds are particularly at risk to oil pollution associated with major shipping routes. Identifying areas where species are most at risk can help inform where, and which, measures should be put in place to mitigate the impacts of oil pollution, such as protecting and avoiding high risk areas, for example, through adopting dynamic Areas to be Avoided (ATBAs). Our simplified OVI combined with the predictive foraging radius approach can be adapted to other regions globally that lack seabird-at-sea distribution data, to other marine wildlife, and to assess risk from hydrocarbon extraction and other anthropogenic threats, including fishing activities and offshore renewable developments.
... Several reviews of appropriate PVA model structure and parameter specification for marine birds considering OWED have been conducted Freeman et al., 2014;Horswill and Robinson, 2015;Maclean et al., 2007;Potiek et al., 2022;Searle et al., 2020;Trinder and Furness, 2015). These reviews identified varying methods depending upon the focus of the study, life history characteristics of the focal species, and/or availability of data at focal colonies. ...
Offshore wind energy development (OWED) is rapidly expanding globally and has the potential to contribute significantly to renewable energy portfolios. However, development of infrastructure in the marine environment presents risks to wildlife. Marine birds in particular have life history traits that amplify population impacts from displacement and collision with offshore wind infrastructure. Here, we present a broadly applicable framework to assess and mitigate the impacts of OWED on marine birds. We outline existing techniques to quantify impact via monitoring and modeling (e.g., collision risk models, population viability analysis), and present a robust mitigation framework to avoid, minimize, or compensate for OWED impacts. Our framework addresses impacts within the context of multiple stressors across multiple wind energy developments. We also present technological and methodological approaches that can improve impact estimation and mitigation. We highlight compensatory mitigation as a tool that can be incorporated into regulatory frameworks to mitigate impacts that cannot be avoided or minimized via siting decisions or alterations to OWED infrastructure or operation. Our framework is 2 intended as a globally-relevant approach for assessing and mitigating OWED impacts on marine birds that may be adapted to existing regulatory frameworks in regions with existing or planned OWED.
... In Bayesian statistics, prior distributions can be used to summarise our understanding of how the world works to obtain meaningful inference from small and fragmented datasets (Hobbs and Hooten 2015). Like many seabirds, kittiwakes are largely unobservable during the first years of life, and population-specific estimates of juvenile survival rates are limited (Horswill and Robinson 2015). Similar to other species of seabird (Horswill et al. 2014), previous studies on age-specific survival in kittiwakes report that probabilities are similar during the first two years following fledging and vary additively with time across age-classes (Link et al. 2002, Cam et al. 2005, Aubry et al. 2009, Desprez et al. 2011. ...
The source–sink paradigm predicts that populations in poorer‐quality habitats (‘sinks') persist due to continued immigration from more‐productive areas (‘sources'). However, this categorisation of populations assumes that habitat quality is fixed through time. Globally, we are in an era of wide‐spread habitat degradation, and consequently there is a pressing need to examine dispersal dynamics in relation to local population change. We used an integrated population model to quantify immigration dynamics in a long‐lived colonial seabird, the black‐legged kittiwake Rissa tridactyla, that is classified as globally ‘Vulnerable'. We then used a transient life table response experiment to evaluate the contribution of temporal variation in vital rates, immigration rates and population structure to realised population growth. Finally, we used a simulation analysis to examine the importance of immigration to population dynamics. We show that the contribution of immigration changed as the population declined. This study demonstrates that immigration is unlikely to maintain vulnerable sink populations indefinitely, emphasising the need for temporal analyses of dispersal to identify shifts that may have dramatic consequences for population viability.
... As observed in other seabird species, we assumed that the survival probability of birds in their first year at sea was lower than for birds older than 1 year (Horswill & Robinson, 2015;VanderWerf & Young, 2016). We allowed probabilities of survival, return and detection on Gough Island, and breeding success, to vary from ...
Invasive species are one of the greatest drivers of biodiversity loss worldwide, and the eradication of invasive species from islands is a highly efficient management strategy. Because eradication operations require large financial investments, uncertainty over the magnitude of impacts of both invasive species and their removal can impede the willingness of decision makers to invest in eradication. Such uncertainty is prevalent for long‐lived species that display an inherent lag between life stages affected by invasive species and those used for population status assessments.
Albatrosses are amongst the longest‐living bird species and are threatened on land by invasive species and at sea by industrial fisheries. As in many seabird species, usually only a segment of the population (breeding adults) is used for status assessments, making it difficult to assess albatross population trends and the potential benefit of conservation action, such as the management of predatory invasive species.
We used population monitoring and mark‐recapture data to estimate the past population trajectory of the critically endangered Tristan albatross Diomedea dabbenena by accounting for unobservable birds at sea in an integrated population model. We then projected the future population trajectory of Tristan albatrosses for scenarios with or without predation by invasive house mice Mus musculus on their main breeding site, Gough Island.
The adult breeding population remained stable between 2004 and 2021, but breeding success was low (31%) and our model indicated that the total population (including unobservable immature birds) decreased from a median estimate of 9,795 to 7,752 birds. Eradicating invasive mice leading to a two‐fold increase in breeding success would result in a 1.8–7.6 times higher albatross population by 2050 (median estimate 10,352 individuals) than without this intervention.
Low reproductive output for long‐lived species may lead to a cryptic population decrease, which can be obscured from readily available counts of breeding pairs by changes in the population structure. Mouse eradication is necessary to halt the ongoing population decrease of the Tristan albatross, even if this decrease is not yet apparent in the breeding population size.
... Like many seabirds, kittiwakes are largely unobservable during the first years of life. Therefore, colony-specific survival estimates for juveniles (i.e., during the first year post fledging ϕ j,t ) and immatures (i.e., from 1 to 2 years ϕ i,t ) are limited (Horswill & Robinson, 2015). Available estimates for black-legged kittiwakes breeding in France report fledgling survival rates to be 75% of adults, immature survival rates (i.e., from age 1 to 2 years) to be 87.5% of adults, and survival rates to be comparable to adults from age 2 onwards (Cam et al., 2005). ...
Population viability analyses (PVA) are now routinely used during the consenting process for offshore wind energy developments to assess potential impacts to vulnerable species, such as seabirds. These models are typically based on mean vital rates, such as survival and fecundity, with some level of environmental stochasticity (i.e., temporal variation). However, many species of seabird are experiencing population decline due to temporal (i.e., directional) trends in their vital rates. We assess the prevalence of temporal trends in rates of fecundity for a sentinel species of seabird, the black‐legged kittiwake Rissa tridactyla, and examine how accounting for these relationships affects the predictive accuracy of PVA, as well as the projected population response to an extrinsic threat. We found that temporal trends in kittiwake rates of fecundity are widespread, and that including these trends in PVA assessments dramatically influences the projected rate of population decline. We advocate that model validation become a prerequisite step in seabird PVA assessments to identify potential biases influencing the projected population response. We also argue that environmental factors driving current population dynamics need to be incorporated in PVA impact assessments as potential “worst‐case” scenarios. These findings have immediate application for improving and reducing uncertainty in impact assessments conducted as part of the consenting process for offshore wind energy developments.
Chick diet of Common Guillemots Uria aalge was studied by direct observation at Sumburgh Head, Shetland during 2007-09. Lesser Sandeels Ammodytes marinus comprised 55% of prey items, the remainder being Gadidae, including Whiting Merlangius merlangus and Saithe Pollachius virens, and a few Snake Pipefish Entelurus aequoreus. This contrasts with 1990-91 when chick were fed 80% sandeels, the remainder mostly being unidentified Gadidae. When this shift from a diet dominated by sandeels took place is unknown, but a similar shift in chick diet at Fair Isle (40 km southwest of Sumburgh Head) seems to have occurred some time during 2000-02. Since 2003, weights of chicks near fledging have been lower than during the 1990s, and average breeding success has been reduced.
Great Skuas Stercorarius skua and Arctic Skuas S. parasiticus were counted on their breeding grounds in Orkney during June 2010. Great Skua numbers had fallen by 23% overall since the previous census in 2000. However, the decline was not uniform across Orkney, the major colony on Hoy declining by 32% but with many smaller colonies actually increasing. Possible reasons for this difference are discussed. Arctic Skua numbers fell by 47% in the same time period and by 64% from their peak in 1992. We found evidence of intra-specific negative density dependence in trends of both skua species, suggesting that declines may have been driven by low food availability. In addition, we found a negative relationship between Arctic Skua trends between 2000 and 2010 and the numbers of Great Skuas present in each colony in 2000. These findings suggest that whilst scarcity of food may have contributed to Arctic Skua declines, predation by Great Skuas is likely to have played an additional role. Predation on Arctic Skuas, both adults and young, is also likely to be ultimately attributable to a scarcity of alternative fish prey for Great Skuas.
Arctic Terns (Sterna paradisaea) and Common Terns (S. hirundo) are similar in many aspects of their breeding ecology, but Common Terns generally lay three eggs per clutch whereas Arctic Terns lay two. In our study, Common Terns had a higher rate of food delivery and energy supply to the nest and higher nest attendance, indicating that they made trips of shorter average duration. This suggests that the number of chicks raised by these two species was primarily limited by the rate at which parents could supply food. However, estimated daily metabolizable energy intake of chicks was about 30% higher in Common Terns than in Arctic Terns. Common Tern chicks apparently spent a higher proportion of daily energy intake on maintenance of body temperature. It remains unknown whether this difference was because Common Tern parents could not brood three chicks as effectively as Arctic Terns brooded two or because the energy requirements for heat production in the third-hatched Common Tern chick were particularly high. If brooding did play a less important role in the energy budgets of Common Terns, the number of chicks that Arctic Terns could raise may have been limited not only by the rate at which parents could supply food to the nest but also by the requirements of chicks for brooding. We suggest that more detailed studies on the role of brooding constraints in limiting brood size in these species are required to clarify this matter.
In waterfowl, offspring survival and the effects of extrinsic (i.e. weather, hatching date) and intrinsic (i.e. physical and nutritional traits of individual females and ducklings, brood sizes) factors on it are poorly understood. In 2000 and 2001, we estimated duckling and brood survival of White-winged Scoters (Melanitta fusca deglandi) at Redberry Lake, Saskatchewan, Canada, to 30 days of age to examine relationships between duckling survival and (1) hatch date, (2) initial brood size at hatch, (3) duckling size and body condition at hatch, (4) offspring sex, (5) maternal female size and body condition at hatch, and (6) weather. We estimated survival with Cormack-Jolly-Seber models, in program MARK, from observations of individually marked adult females (n = 94) and ducklings (n = 664). Most mortality (i.e. 0.80 and 0.95 for each year, respectively) occurred within six days of hatch in both years. Duckling survival probability decreased with advancing hatch date; increased with larger initial brood sizes; was higher for larger, better-conditioned ducklings; and increased with favorable weather. Brood survival decreased with advancing hatch date, increased with larger initial brood sizes, and increased with favorable weather. For 2000 and 2001, our models predict survival probabilities of ducklings (0.0061 and 0.0027, respectively) and broods (0.015 and 0.00048, respectively) that are lower than any previously reported. We suspect that intense gull (Larus spp.) predation shortly after hatch had the largest influence on duckling survival, though results also underscore the significance of intrinsic factors.
Effets des Facteurs Intrinsèques et Extrinsèques sur la Survie les Jeunes de Melanitta fusca deglandi
Population surveys indicate a trend of declining abundance of scoters (Melanitta spp.) in North America. Little is known about changes in life-history traits that may be responsible for the recent population decline of White-winged Scoters (Melanitta fusca deglandi). Therefore, we studied nesting ecology of White-winged Scoters at Redberry Lake, Saskatchewan, during the summers of 2000–2001. We found 198 nests and examined nest-site selection by comparing habitat features of successful nests, depredated nests, and random sites. Discriminant function analysis differentiated habitat features—measured at hatch—of successful nests, depredated nests, and random sites; lateral (r = 0.66) and overhead (r = 0.35) concealment were microhabitat variables most correlated with canonical discriminant functions. We also modeled daily survival rate of nests as a function of year, linear and quadratic trends with nest age, nest initiation date, and seven microhabitat variables. Nest survival from a time-constant model (i.e. Mayfield nest-success estimate) was 0.35 (95% CL: 0.27, 0.43). Estimates of nest success were lower than those measured at Redberry Lake in the 1970s and 1980s. Nest survival increased throughout the laying period and stabilized during incubation, and showed positive relationships with nest concealment and distance to water and a negative relationship with distance to edge. Considering those factors, a model-averaged estimate of nest survival was 0.24 (95% CL: 0.09, 0.42). We concluded that White-winged Scoters selected nesting habitat adaptively, because (1) successful sites were more concealed than depredated sites; (2) nest sites (both successful and depredated) had higher concealment than random sites; and (3) nest sites were on islands, where success is greater than on the mainland.
Most of the Great Skuas (Stercorarius skua) breeding at Hermaness, Shetland, exhibit dietary specialization: a small proportion feed almost exclusively upon seabird prey, a small proportion feed as generalists, and most feed on fishery discards. We investigated the foraging dynamics, reproductive performance, and survival of Great Skuas that specialized in depredating other seabirds compared with those feeding predominantly on fish. Around half of the specialist bird predators defended combined breeding and feeding territories that included a section of seabird colony; the remainder of the predatory skuas foraged away from breeding territories. Specialist bird predators retained their feeding habit and, if present, feeding territory, across years. Time budgets revealed that specialist bird predators spent less time foraging than skuas feeding predominantly on fish. Results of radio-telemetry indicated that bird-specialist skuas have smaller home ranges than other birds. In a comparison of reproductive performance, specialist bird predators consistently hatched earlier among years. They also showed larger clutch volumes and improved chick condition, but these were subject to annual variations. Hatching success and fledging success for specialist bird predators and specialist fish predators were similar. Specialist bird predators showed similar annual survival compared with fish-feeders over the same period. Specializing as a bird predator may be limited to the best birds in the population, but their poorer than predicted breeding success reveals the need for further study into the relationship between diet and reproductive success in this species.
Consecuencias Reproductivas para Individuos de Stercorarius skua que se Especializan como Depredadores de Aves Marinas
Resumen. La mayoría de los individuos de Stercorarius skua que se reproducen en Hermaness, Shetland, presentan una especialización de la dieta: una proporción pequeña se alimenta casi exclusivamente de aves marinas, otra proporción pequeña generalista y la mayoría se alimenta de desechos de pesqueros. Se investigó la dinámica de forrajeo, el desempeño reproductivo y la supervivencia de individuos de S. skua que se especializan en la depredación de otras aves marinas comparándolo con otros que se alimentan predominantemente de pescado. Cerca de la mitad de los depredadores especialistas de aves defienden de manera combinada territorios para reproducción y para alimentación que incluyen una sección de la colonia: el resto de los individuos depredadores forrajean lejos de los territorios de reproducción. Los aves depredadores especialistas de aves mantienen sus há bitos alimenticios y, en caso de presentarse, el territorio para alimentación, a través de los años. Los presupuestos de tiempo revelaron que los depredadores especialistas de aves pasan menos tiempo forrajeando que los que se alimentan predominantemente de pescado. Los resultados de radio-telemetría indicaron que las aves especialistas tienen rangos de hogar menores que otras aves. En una comparación de desempeño reproductivo, los depredadores especialistas de aves presentaron fechas de eclosión más tempranas, las cuales se repitieron a traves de los años. Así mismo, mostraron volúmenes de puesta más grandes y mejor condición de las crías, pero éstos estuvieron sujetos a variaciones anuales. El éxito de eclosión y el éxito de emplumamiento de los depredadores especialistas de aves y de los depredadores especialistas de pescado fueron similares. Los depredadores especialistas de aves mostraron una supervivencia anual similar a la de los que se alimentan de pescado durante el mismo periodo. La especialización como depredador de aves puede limitarse a las mejores aves de la población, pero el bajo éxito reproductivo encontrado, contrario a lo predicho, revela la necesidad de estudios adicionales sobre la relación entre la dieta y el éxito reproductivo en esta especie.
Red-throated Loon (Gavia stellata) numbers in Alaska have fluctuated dramatically over the past 3 decades; however, the demographic processes contributing to these population dynamics are poorly understood. To examine spatial and temporal variation in productivity, we estimated breeding parameters at 5 sites in Alaska: at Cape Espenberg and the Copper River Delta we estimated nest survival, and at 3 sites within the Yukon-Kuskokwim Delta we estimated nest survival and productivity. Nest survival varied broadly among sites and years; annual estimates (lower, upper 95% confidence interval) ranged from 0.09 (0.03, 0.29) at Cape Espenberg in 2001 to 0.93 (0.76, 0.99) at the Copper River Delta in 2002. Annual variation among sites was not concordant, suggesting that site-scale factors had a strong influence on nest survival. Models of nest survival indicated that visits to monitor nests had a negative effect on nest daily survival probability, which if not accounted for biased nest survival strongly downward. The sensitivity of breeding Red-throated Loons to nest monitoring suggests other sources of disturbance that cause incubating birds to flush from their nests may also reduce nest survival. Nest daily survival probability at the Yukon-Kuskokwim Delta was negatively associated with an annual index of fox occurrence. Survival through the incubation and chick-rearing periods on the Yukon-Kuskokwim Delta ranged from 0.09 (0.001, 0.493) to 0.50 (0.04, 0.77). Daily survival probability during the chick-rearing period was lower for chicks that had a sibling in 2 of 3 years, consistent with the hypothesis that food availability was limited. Estimates of annual productivity on the Yukon-Kuskokwim Delta ranged from 0.17 to 1.0 chicks per pair. Productivity was not sufficient to maintain population stability in 2 of 3 years, indicating that nest depredation by foxes and poor foraging conditions during chick rearing can have important effects on productivity.