Integrating stable isotopes,parasite, and ring‐reencounter data to quantify migratory connectivity—A case study with Barn Swallows breeding in Switzerland,Germany, Sweden,and Finland

Ecosystems around the world are connected by seasonal migration. The migrant animals themselves are influenced by migratory connectivity through effects on the individual and the population level. Measuring migratory connectivity is notoriously difficult due to the simple requirement of data conveying information about the nonbreeding distribution of many individuals from several breeding populations. Explicit integration of data derived from different methods increases the precision and the reliability of parameter estimates. We combine ring‐reencounter, stable isotope, and blood parasite data of Barn Swallows Hirundo rustica in a single integrated model to estimate migratory connectivity for three large scale breeding populations across a latitudinal gradient from Central Europe to Scandinavia. To this end, we integrated a non‐Markovian multistate mark‐recovery model for the ring‐reencounter data with normal and binomial mixture models for the stable isotope and parasite data. The integration of different data sources within a mark‐recapture modeling framework enables the most precise quantification of migratory connectivity on the given broad spatial scale. The results show that northern‐breeding populations and Southern Africa as well as southern‐breeding populations and Western–Central Africa are more strongly connected through Barn Swallow migration than central European breeding populations with any of the African wintering areas. The nonbreeding distribution of Barn Swallows from central European breeding populations seems to be a mixture of those populations breeding further north and south, indicating a migratory divide.     

Incorporating breeding abundance into spatial assignments on continuous surfaces

Determining the geographic connections between breeding and nonbreeding populations, termed migratory connectivity, is critical to advancing our understanding of the ecology and conservation of migratory species. Assignment models based on stable isotopes historically have been an important tool for studying migratory connectivity of small‐bodied species, but the low resolution of these assignments has generated interest into combining isotopes with other sources in information. Abundance is one of the most appealing data sources to include in isotope‐based assignments, but there are currently no statistical methods or guidelines for optimizing the contribution of stable isotopes and abundance for inferring migratory connectivity. Using known‐origin stable‐hydrogen isotope samples of six Neotropical migratory bird species, we rigorously assessed the performance of assignment models that differentially weight the contribution of the isotope and abundance data. For two species with adequate sample sizes, we used Pareto optimality to determine the set of models that simultaneously minimized both assignment error rate and assignment area. We then assessed the ability of the top models from these two species to improve assignments of the remaining four species compared to assignments based on isotopes alone. We show that the increased precision of models that include abundance is often offset by a large increase in assignment error. However, models that optimally weigh the abundance data relative to the isotope data can result in higher precision and, in some cases, lower error than models based on isotopes alone. The top models, however, depended on the distribution of relative breeding abundance, with patchier distributions requiring stronger downweighting of abundance, and we present general guidelines for future studies. These results confirm that breeding abundance can be an important source of information for studies investigating broad‐scale movements of migratory birds and potentially other taxa.     

Novel statistical methods for integrating genetic and stable isotope data to infer individual‐level migratory connectivity

Methods for determining patterns of migratory connectivity in animal ecology have historically been limited due to logistical challenges. Recent progress in studying migratory bird connectivity has been made using genetic and stable‐isotope markers to assign migratory individuals to their breeding grounds. Here, we present a novel Bayesian approach to jointly leverage genetic and isotopic markers and we test its utility on two migratory passerine bird species. Our approach represents a principled model‐based combination of genetic and isotope data from samples collected on the breeding grounds and is able to achieve levels of assignment accuracy that exceed those of either method alone. When applied at large scale the method can reveal specific migratory connectivity patterns. In Wilson's warblers (Wilsonia pusilla), we detect a subgroup of birds wintering in Baja that uniquely migrate preferentially from the coastal Pacific Northwest. Our approach is implemented in a way that is easily extended to accommodate additional sources of information (e.g. bi‐allelic markers, species distribution models, etc.) or adapted to other species or assignment problems.     

Connecting breeding and wintering grounds of Neotropical migrant songbirds using stable hydrogen isotopes: a call for an isotopic atlas of migratory connectivity

There is an overdue and urgent need to establish patterns of migratory connectivity linking breeding grounds, stopover sites, and wintering grounds of migratory birds. Such information allows more effective application of conservation efforts by applying focused actions along movement trajectories at the population level. Stable isotope methods, especially those using stable hydrogen isotope abundance in feathers (δ²H_f) combined with Bayesian assignment techniques incorporating prior information such as relative abundance of breeding birds, now provide a fast and reliable means of establishing migratory connectivity, especially for Neotropical migrants that breed in North America and molt prior to fall migration. Here we demonstrate how opportunistic sampling of feathers of 30 species of wintering birds in Cuba, Venezuela, Guatemala, Puerto Rico, and Mexico, regions that have typically been poorly sampled for estimating migratory connectivity, can be assigned to breeding areas in North America through both advanced spatial assignment to probability surfaces and through simpler map lookup approaches. Incorporating relative abundance information from the North American Breeding Bird Survey in our Bayesian assignment models generally resulted in a reduction in potential assignment areas on breeding grounds. However, additional tools to constrain longitude such as DNA markers or other isotopes would be desirable for establishing breeding or molt origins of species with broad longitudinal distributions. The isotope approach could act as a rapid means of establishing basic patterns of migratory connectivity across numerous species and populations. We propose a large‐scale coordinated sampling effort on the wintering grounds to establish an isotopic atlas of migratory connectivity for North American Neotropical migrants and suggest that isotopic variance be considered as a valuable metric to quantify migratory connectivity. This initiative could then act as a strategic template to guide further efforts involving stable isotopes, light‐sensitive geolocators, and other technologies.     

Light‐level geolocators reveal migratory connectivity in European populations of pied flycatchers Ficedula hypoleuca

Understanding what drives or prevents long‐distance migrants to respond to environmental change requires basic knowledge about the wintering and breeding grounds, and the timing of movements between them. Both strong and weak migratory connectivity have been reported for Palearctic passerines wintering in Africa, but this remains unknown for most species. We investigated whether pied flycatchers Ficedula hypoleuca from different breeding populations also differ in wintering locations in west‐Africa. Light‐level geolocator data revealed that flycatchers from different breeding populations travelled to different wintering sites, despite similarity in routes during most of the autumn migration. We found support for strong migratory connectivity showing an unexpected pattern: individuals breeding in Fennoscandia (S‐Finland and S‐Norway) wintered further west compared to individuals breeding at more southern latitudes in the Netherlands and SW‐United Kingdom. The same pattern was found in ring recovery data from sub‐Saharan Africa of individuals with confirmed breeding origin. Furthermore, population‐specific migratory connectivity was associated with geographical variation in breeding and migration phenology: birds from populations which breed and migrate earlier wintered further east than birds from ‘late’ populations. There was no indication that wintering locations were affected by geolocation deployment, as we found high repeatability and consistency in δ¹³C and δ¹⁵N stable isotope ratios of winter grown feathers of individuals with and without a geolocator. We discuss the potential ecological factors causing such an unexpected pattern of migratory connectivity. We hypothesise that population differences in wintering longitudes of pied flycatchers result from geographical variation in breeding phenology and the timing of fuelling for spring migration at the wintering grounds. Future research should aim at describing how temporal dynamics in food availability across the wintering range affects migration, wintering distribution and populations’ capacity to respond to environmental changes.     

High individual repeatability and population differentiation in stable isotope ratios in winter-grown collared flycatcher Ficedula albicollis feathers

For migrants, we often lack complete information of their spatial distribution year round. Here, we used stable carbon, nitrogen and hydrogen isotope ratios extracted from feathers grown at the wintering sites of the long-distance migratory collared flycatcher Ficedula albicollis , to study how individuals from different breeding populations are distributed at the wintering sites. A sub-sample of birds was also sampled in two consecutive years to test for the repeatability of isotope ratios. Birds from the same breeding populations had more similar isotope ratios compared to birds from other nearby populations (10–100 km apart). Furthermore, isotope repeatability within individuals was high, implying that the observed pattern of isotope variation is consistent between years. We put forward two hypotheses for these patterns; 1) strong wintering site philopatry and migratory connectivity, suggesting that migratory connectivity may potentially be found on a much smaller spatial scale than previously considered, and 2) consistent interpopulation differentiation of feeding ecology at their wintering site.     

Roseate Terns Sterna dougallii on the southeast coast of South Africa: information on moult and migratory status

Determining the migratory status of seabird populations is crucial for addressing conservation concerns. The Roseate Tern Sterna dougallii is considered endangered in South Africa in view of its small breeding population and the threats to eggs and chicks. Earlier works based on capture-recapture data suggest that this population adopts a partial migratory strategy. Updated capture-recapture data were combined with new data on moult and stable isotopes of scapular feathers to determine whether subpopulations could be identified in the South African breeding population. Moult data on 404 individuals failed to identify subpopulations, but two groups were identifiable with the stable isotopes: one group of four individuals exhibiting low carbon and nitrogen stable isotope ratios and a second group (comprising the remaining 152 individuals) with significantly higher ratios. The isotope data suggest that birds from the two groups moulted in different areas. Comparisons with published studies showed that the 152 individuals had likely moulted their scapulars in South African waters, but the moulting area for the remaining four individuals could not be ascertained due to the absence of detailed isoscapes in the southern part of the Mozambique Channel. Further work investigating the genetic differentiation between South African and Madagascan Roseate Terns may bring new insight into the migratory behaviour of the South African population.     

A triple-isotope approach to predict the breeding origins of European bats

Despite a commitment by the European Union to protect its migratory bat populations, conservation efforts are hindered by a poor understanding of bat migratory strategies and connectivity between breeding and wintering grounds. Traditional methods like mark-recapture are ineffective to study broad-scale bat migratory patterns. Stable hydrogen isotopes (δD) have been proven useful in establishing spatial migratory connectivity of animal populations. Before applying this tool, the method was calibrated using bat samples of known origin. Here we established the potential of δD as a robust geographical tracer of breeding origins of European bats by measuring δD in hair of five sedentary bat species from 45 locations throughout Europe. The δD of bat hair strongly correlated with well-established spatial isotopic patterns in mean annual precipitation in Europe, and therefore was highly correlated with latitude. We calculated a linear mixed-effects model, with species as random effect, linking δD of bat hair to precipitation δD of the areas of hair growth. This model can be used to predict breeding origins of European migrating bats. We used δ(13)C and δ(15)N to discriminate among potential origins of bats, and found that these isotopes can be used as variables to further refine origin predictions. A triple-isotope approach could thereby pinpoint populations or subpopulations that have distinct origins. Our results further corroborated stable isotope analysis as a powerful method to delineate animal migrations in Europe.     

Advances in linking wintering migrant birds to their breeding-ground origins using combined analyses of genetic and stable isotope markers

An enduring problem in avian ecology and conservation is linking breeding and wintering grounds of migratory species. As migratory species and populations vary in the degree to which individuals from distinct breeding locales mix on stop-over sites and wintering grounds, establishing migratory connectivity informs our understanding of population demography and species management. We present a new Bayesian approach for inferring breeding grounds of wintering birds of unknown origins in North America. We incorporate prior information from analysis of genetic markers into geographic origin assignment based upon stable-hydrogen isotope analysis of feathers (δ(2)H(f)), using the Loggerhead Shrike (Lanius ludovicianus). Likely geographic origins derived from analyses of DNA microsatellites were used as priors for Bayesian analyses in which birds were assigned to a breeding-ground origin using their δ(2)H(f) values. As with most applications of Bayesian methods, our approach greatly improved the results (i.e. decreased the size of the potential area of origin). Area of origin decreased by 3 to 5-fold on average, but ranged up to a 10-fold improvement. We recommend this approach in future studies of migratory connectivity and suggest that our methodology could be applied more broadly to the study of dispersal, sources of productivity of migratory populations, and a range of evolutionary phenomena.     

Strong migratory connectivity and seasonally shifting isotopic niches in geographically separated populations of a long-distance migrating songbird

Whether migratory animals use similar resources during continental-scale movements that characterize their annual cycles is highly relevant to both individual performances and population dynamics. Direct knowledge of the locations and resources used by migrants during non-breeding is generally scarce. Our goal was to estimate migratory connectivity of a small Palaearctic long-distance migrant, the common nightingale Luscinia megarhynchos, and to compare resources used in non-breeding areas with resources used at the breeding grounds. We tracked individuals of three geographically separated populations and characterised their stable isotope niches during breeding and non-breeding over 2 years. Individuals spent the non-breeding period in population-specific clusters from west to central Africa, indicating strong migratory connectivity at the population level. Irrespective of origin, their isotopic niches were surprisingly similar within a particular period, although sites of residence were distant. However, niche characteristics differed markedly between breeding and non-breeding periods, indicating a consistent seasonal isotopic niche shift in the sampled populations. Although nightingales of distinct breeding populations migrated to different non-breeding areas, they chose similar foraging conditions within specific periods. However, nightingales clearly changed resource use between breeding and non-breeding periods, indicating adaptations to changes in food availability.     

The strength of migratory connectivity for birds en route to breeding through the Gulf of Mexico

The strength of migratory connectivity is a measure of the cohesion of populations among phases of the annual cycle, including breeding, migration, and wintering. Many Nearctic‐Neotropical species have strong migratory connectivity between breeding and wintering phases of the annual cycle. It is less clear if this strength persists during migration when multiple endogenous and exogenous factors may decrease the cohesion of populations among routes or through time along the same routes. We sampled three bird species, American redstart Setophaga ruticilla, ovenbird Seiurus aurocapilla, and wood thrush Hylocichla mustelina, during spring migration through the Gulf of Mexico region to test if breeding populations differentiate spatially among migration routes or temporally along the same migration routes and the extent to which within‐population timing is a function of sex, age, and carry‐over from winter habitat, as measured by stable carbon isotope values in claws (δ¹³C). To make quantitative comparisons of migratory connectivity possible, we developed and used new methodology to estimate the strength of migratory connectivity (MC) from probabilistic origin assignments identified using stable hydrogen isotopes in feathers (δ²H). We found support for spatial differentiation among routes by American redstarts and ovenbirds and temporal differentiation along routes by American redstarts. After controlling for breeding origin, the timing of American redstart migration differed among ages and sexes and ovenbird migration timing was influenced by carry‐over from winter habitat. The strength of migratory connectivity did not differ among the three species, with each showing weak breeding‐to‐spring migration MC relative to prior assessments of breeding‐wintering connectivity. Our work begins to fill an essential gap in methodology and understanding of the extent to which populations remain together during migration, information critical for a full annual cycle perspective on the population dynamics and conservation of migratory animals.     

Using stable hydrogen isotopes (δ2H) and ring recoveries to trace natal origins in a Eurasian passerine with a migratory divide

Despite recent advances in technology, it remains difficult to connect breeding and non‐breeding areas of populations of migratory organisms due to the challenges of year‐round tracking. Here, we used the Eurasian reed warbler Acrocephalus scirpaceus, a passerine with a pronounced migratory divide to demonstrate the promise of integrating several sources of information within the Bayesian modelling framework for the study of migratory connectivity. To this end, we combined data from stable hydrogen isotope ratios (δ²H) of feathers, ring recoveries, and the geographic delineation of sub‐populations on either side of the migratory divide. Feather δ²H measurements from local juvenile birds sampled across the breeding range tightly correlated with amount‐weighted mean annual precipitation δ²H values predicted for the natal sites. Predicted natal origins of birds intercepted en route in the Mediterranean region largely differed among the five stopover sites. Thanks to the different migratory pathways used by different breeding populations and the existence of a migratory divide, we were able to effectively narrow the assigned regions of origin. Our results show that spatial resolution of likelihood‐based assignments of geographic origins based on δ²H measurements may improve significantly when prior probabilities derived from population‐specific migratory directions are included. Integrating information from stable isotopes, ring recoveries, geolocators and other sources within the Bayesian modelling framework will provide an extremely useful toolbox for the study of animal movements in the future.     

Comparison of trace element and stable isotope approaches to the study of migratory connectivity: an example using two hirundine species breeding in Europe and wintering in Africa

Analyses of stable isotopes and trace elements in feathers may provide important information about location and habitat use during molt, thereby enabling the investigation of migratory connectivity and its ecological consequences in bird species that breed and winter in different areas. We have compared the conclusions arrived at based on the use of these two methods on the same samples of feathers from two migratory birds, the Sand Martin Riparia riparia and the Barn Swallow Hirundo rustica. We investigated the effects of location, age and sex on stable isotope (δ¹³C, δ¹⁵N, δD) and trace element profiles (As, Cd, Mg, Mn, Mo, Se, Sr, Co, Fe, Zn, Li, P, Ti, V, Ag, Cr, Ba, Hg, Pb, S, Ni and Cu). The feathers of adults at the breeding grounds were removed, forcing in birds to grow new feathers at the breeding grounds; this enabled a comparison of composition of feathers grown in Europe and Africa by the same individual. Stable isotope and trace element profiles varied geographically, even at micro-geographic scales, and also among age classes. The results of both methods suggest that food composition and/or source differs between adults and nestlings in the breeding area and that food and/or molting location changes with the age of individuals in Africa. In an attempt to determine the usefulness of data obtained from composition of feathers, we performed discriminant function analyses on information obtained on stable isotopes and trace elements in order to assess the correctness of the classification of group membership. When feathers molted in Africa were compared to those molted in Europe, trace element profiles of the 22 elements generally had a much greater resolution than the stable isotope profiles based on three stable isotopes. The proportion of correctly classified samples was also greater for analyses based on trace elements than for those based on stable isotopes.     

Transcontinental migratory connectivity predicts parasite prevalence in breeding populations of the European barn swallow

Parasites exert a major impact on the eco‐evolutionary dynamics of their hosts and the associated biotic environment. Migration constitutes an effective means for long‐distance invasions of vector‐borne parasites and promotes their rapid spread. Yet, ecological and spatial information on population‐specific host–parasite connectivity is essentially lacking. Here, we address this question in a system consisting of a transcontinental migrant species, the European barn swallow (Hirundo rustica) which serves as a vector for avian endoparasites in the genera Plasmodium, Haemoproteus and Leucocytozoon. Using feather stable isotope ratios as geographically informative markers, we first assessed migratory connectivity in the host: Northern European breeding populations predominantly overwintered in dry, savannah‐like habitats in Southern Africa, whereas Southern European populations were associated with wetland habitats in Western Central Africa. Wintering areas of swallows breeding in Central Europe indicated a migratory divide with both migratory programmes occurring within the same breeding population. Subsequent genetic screens of parasites in the breeding populations revealed a link between the host's migratory programme and its parasitic repertoire: controlling for effects of local breeding location, prevalence of Africa‐transmitted Plasmodium lineages was significantly higher in individuals overwintering in the moist habitats of Western Central Africa, even among sympatrically breeding individuals with different overwintering locations. For the rarer Haemoproteus parasites, prevalence was best explained by breeding location alone, whereas no clear pattern emerged for the least abundant parasite Leucocytozoon. These results have implications for our understanding of spatio‐temporal host–parasite dynamics in migratory species and the spread of avian borne diseases.     

Seasonal matching of habitat quality and fitness in a migratory bird

When species occupy habitats that vary in quality, choice of habitat can be critical in determining individual fitness. In most migratory species, juveniles migrate independently of their parents and must therefore choose both breeding and winter habitats. Using a unique dataset of marked black-tailed godwits (Limosa limosa islandica) tracked throughout their migratory range, combined with analyses of stable carbon isotope ratios, we show that those individuals that occupy higher quality breeding sites also use higher quality winter sites. This seasonal matching can severely inflate inequalities in individual fitness. This population has expanded over the last century into poorer quality breeding and winter habitats and, across the whole population; individual birds tend to occupy either novel or traditional sites in both seasons. Winter and breeding season habitat selection are thus strongly linked throughout this population; these links have profound implications for a wide range of population and evolutionary processes. As adult godwits are highly philopatric, the initial choice of winter habitat by juveniles will be critical in determining future survival, timing of migration and breeding success.     

Parasite assemblages distinguish populations of a migratory passerine on its breeding grounds

We attempted to establish migratory connectivity patterns for American redstarts Setophaga ruticilla between their breeding and wintering grounds by characterizing the composition of their haematozoan parasite assemblages in different parts of their range. We detected significant but limited geographic structuring of haematozoan parasite lineages across the breeding range of the redstart. We found that redstarts from the south-eastern (SE) region of the breeding range had a significantly different haematozoan parasite assemblage compared with populations sampled throughout the rest of the breeding range. Evidence using stable isotopes from feathers previously demonstrated that redstarts from the SE of the breeding range also have a unique and separate wintering range. Thus, although two methods of estimating migratory connectivity have now both shown the SE US breeding sub-population of redstarts to be distinct from other populations on both the breeding and wintering grounds, conclusive migratory connectivity for this species as a whole could not be established.     

A quantitative measure of migratory connectivity

The study of the extent of the connection between areas where populations of birds breed and areas where they winter has flourished in recent years mainly thanks to the development of new techniques, but also due to traditional ringing and recovery schemes, which allow tracking of individuals or populations linking wintering and breeding distributions. Currently, studies on migratory connectivity focus on retention of breeding population spatial structure on the non-breeding grounds and vice versa.Here we propose a method to quantify migratory connectivity based on Mantel correlation coefficients and to statistically test for deviations of the observed connectivity from a random mix of individuals. In addition, we propose a procedure, based on clustering algorithms, to identify whether observed connectivity depends on aggregation of individuals or on rigid transference of distribution patterns between areas.We applied this method to a large dataset of ringing recoveries of barn swallows (Hirundo rustica L) migrating from their Western Palearctic breeding areas to sub-Saharan winter quarters. We show that migration of barn swallow populations connects specific breeding and wintering areas, and that the “sub-populations” quantitatively identified by our method are consistent with qualitative patterns of migratory connectivity identified by studies of individual geographical populations based on other methods. Finally, we tested the performance of the method by running simulations under different scenarios. Such simulations showed that the method is robust and able to correctly detect migratory connectivity even with smaller datasets and when a strong geographical pattern is not present in the population. Our method provides a quantitative measure of migratory connectivity and allows for the identification of populations showing high connectivity between the breeding and wintering areas. This method is suitable for a generalized application to diverse animal taxa as well as to large scale analyses of connectivity for conservation purposes.     

Stable isotope and band-encounter analyses delineate migratory patterns and catchment areas of white-throated sparrows at a migration monitoring station

The Canadian Migration Monitoring Network consists of several fixed migration monitoring stations (MMS) that apply constant-effort protocols to track changes in the abundance of migratory birds. Such monitoring will be important for tracking long-term population trends of songbirds, especially for species breeding in remote areas such as the North American boreal forest. The geographical catchment sampled by individual MMS, however, remains largely unknown. Here, we used hydrogen isotope measurements (δD) of feathers of white-throated sparrows (Zonotrichia albicollis) moving through Delta Marsh MMS in Manitoba, Canada, to determine both wintering and breeding ground catchment areas monitored by this station. The δD of tail feathers, collected from spring and fall migrants delineated previous breeding or natal latitudes, ranging from the northern to the southern extremes of the western boreal forest. The δD values of head feathers grown on the wintering grounds and collected during spring migration revealed that individuals wintered in a broad region of the southeastern United States. The isotope data showed no relationship between estimated breeding/natal and wintering latitudes of white-throated sparrow populations. Stable isotope data provided little information on longitude. Band-encounter analyses, however, indicated a clear east–west segregation of these sparrows across Canada, supporting connectivity among breeding/natal and wintering longitudes over the entire scale of this species' range. Isotope analyses of multiple feather types representing different periods and geographic regions of the annual cycle can provide key information on migratory connectivity for species moving through dedicated MMS.     

Stable isotope analysis provides new information on winter habitat use of declining avian migrants that is relevant to their conservation

Winter habitat use and the magnitude of migratory connectivity are important parameters when assessing drivers of the marked declines in avian migrants. Such information is unavailable for most species. We use a stable isotope approach to assess these factors for three declining African-Eurasian migrants whose winter ecology is poorly known: wood warbler Phylloscopus sibilatrix, house martin Delichon urbicum and common swift Apus apus. Spatially segregated breeding wood warbler populations (sampled across a 800 km transect), house martins and common swifts (sampled across a 3,500 km transect) exhibited statistically identical intra-specific carbon and nitrogen isotope ratios in winter grown feathers. Such patterns are compatible with a high degree of migratory connectivity, but could arise if species use isotopically similar resources at different locations. Wood warbler carbon isotope ratios are more depleted than typical for African-Eurasian migrants and are compatible with use of moist lowland forest. The very limited variance in these ratios indicates specialisation on isotopically restricted resources, which may drive the similarity in wood warbler populations' stable isotope ratios and increase susceptibility to environmental change within its wintering grounds. House martins were previously considered to primarily use moist montane forest during the winter, but this seems unlikely given the enriched nature of their carbon isotope ratios. House martins use a narrower isotopic range of resources than the common swift, indicative of increased specialisation or a relatively limited wintering range; both factors could increase house martins' vulnerability to environmental change. The marked variance in isotope ratios within each common swift population contributes to the lack of population specific signatures and indicates that the species is less vulnerable to environmental change in sub-Saharan Africa than our other focal species. Our findings demonstrate how stable isotope research can contribute to understanding avian migrants' winter ecology and conservation status.     

Estimating migratory connectivity of birds when re‐encounter probabilities are heterogeneous

Understanding the biology and conducting effective conservation of migratory species requires an understanding of migratory connectivity – the geographic linkages of populations between stages of the annual cycle. Unfortunately, for most species, we are lacking such information. The North American Bird Banding Laboratory (BBL) houses an extensive database of marking, recaptures and recoveries, and such data could provide migratory connectivity information for many species. To date, however, few species have been analyzed for migratory connectivity largely because heterogeneous re‐encounter probabilities make interpretation problematic. We accounted for regional variation in re‐encounter probabilities by borrowing information across species and by using effort covariates on recapture and recovery probabilities in a multistate capture–recapture and recovery model. The effort covariates were derived from recaptures and recoveries of species within the same regions. We estimated the migratory connectivity for three tern species breeding in North America and over‐wintering in the tropics, common (Sterna hirundo), roseate (Sterna dougallii), and Caspian terns (Hydroprogne caspia). For western breeding terns, model‐derived estimates of migratory connectivity differed considerably from those derived directly from the proportions of re‐encounters. Conversely, for eastern breeding terns, estimates were merely refined by the inclusion of re‐encounter probabilities. In general, eastern breeding terns were strongly connected to eastern South America, and western breeding terns were strongly linked to the more western parts of the nonbreeding range under both models. Through simulation, we found this approach is likely useful for many species in the BBL database, although precision improved with higher re‐encounter probabilities and stronger migratory connectivity. We describe an approach to deal with the inherent biases in BBL banding and re‐encounter data to demonstrate that this large dataset is a valuable source of information about the migratory connectivity of the birds of North America.