Phylogeographical approaches to assessing demographic connectivity between breeding and overwintering regions in a Nearctic-Neotropical warbler (Wilsonia pusilla)

We characterized the pattern and magnitude of phylogeographical variation among breeding populations of a long-distance migratory bird, the Wilson's warbler (Wilsonia pusilla), and used this information to assess the utility of mtDNA markers for assaying demographic connectivity between breeding and overwintering regions. We found a complex pattern of population differentiation in mitochondrial DNA (mtDNA) variation among populations across the breeding range. Individuals from eastern North America were differentiated from western individuals and the eastern haplotypes formed a distinct, well-supported cluster. The more diverse western group contained haplotype clusters with significant geographical structuring, but there was also broad mixing of haplotype groups such that no haplotype groups were population specific and the predominance of rare haplotypes limited the utility of frequency-based assignment techniques. Nonetheless, the existence of geographically diagnosable eastern vs. western haplotypes enabled us to characterize the distribution of these two groups across 14 overwintering locations. Western haplotypes were present at much higher frequencies than eastern haplotypes at most overwintering sites. Application of this mtDNA-based method of linking breeding and overwintering populations on a finer geographical scale was precluded by the absence of population-specific markers and by insufficient haplotype sorting among western breeding populations. Our results suggest that because migratory species such as the Wilson's warbler likely experienced extensive gene flow among regional breeding populations, molecular markers will have the greatest utility for characterizing breeding-overwintering connectivity at a broad geographical scale.     

Winter commingling of populations of migratory species can cause breeding range underpopulation

We build a model with large-scale demographic consequences for migratory species. The model operates where four elements co-occur, and we rely on empirical research using migratory birds to demonstrate them. First, breeding ranges have internal structure flowing from natal philopatry. Second, fecundity varies geographically. Third, populations of different breeding provenances commingle during winter. And fourth, a population-limiting carrying capacity operates during winter. In the absence of breeding season population-limitation, only the breeding population with maximum fecundity persists. Consequently, some potential breeding areas that offer suitable and productive habitat are bereft of breeding birds because of the interplay between the geographical fecundity gradient and the shared winter quarters. Where breeding season population-limitation also plays a role for at least one population, one (or more) breeding population becomes permanently depressed, resulting in a density well below the carrying capacity of the productive breeding habitat that is occupied. In either case, not all populations fare equally well, despite net positive breeding season productivity. Changes in winter carrying capacity, for example habitat degradation in winter quarters, can lead to uneven effects on geographically defined breeding populations, even though there has been no change in the circumstances of the breeding range.     

Population genetic structure in migratory sandhill cranes and the role of Pleistocene glaciations

Previous studies of migratory sandhill cranes (Grus canadensis) have made significant progress explaining evolution of this group at the species scale, but have been unsuccessful in explaining the geographically partitioned variation in morphology seen on the population scale. The objectives of this study were to assess the population structure and gene flow patterns among migratory sandhill cranes using microsatellite DNA genotypes and mitochondrial DNA haplotypes of a large sample of individuals across three populations. In particular, we were interested in evaluating the roles of Pleistocene glaciation events and postglaciation gene flow in shaping the present-day population structure. Our results indicate substantial gene flow across regions of the Midcontinental population that are geographically adjacent, suggesting that gene flow for most of the region follows an isolation-by-distance model. Male-mediated gene flow and strong female philopatry may explain the differing patterns of nuclear and mitochondrial variation. Taken in context with precise geographical information on breeding locations, the morphologic and microsatellite DNA variation shows a gradation from the Arctic-nesting subspecies G. c. canadensis to the nonArctic subspecies G. c. tabida. Analogous to other Arctic-nesting birds, it is probable that the population structure seen in Midcontinental sandhill cranes reflects the result of postglacial secondary contact. Our data suggest that subspecies of migratory sandhills experience significant gene flow and therefore do not represent distinct and independent genetic entities.     

Delayed egg‐laying and shortened incubation duration of Arctic‐breeding shorebirds coincide with climate cooling

Biological impacts of climate change are exemplified by shifts in phenology. As the timing of breeding advances, the within‐season relationships between timing of breeding and reproductive traits may change and cause long‐term changes in the population mean value of reproductive traits. We investigated long‐term changes in the timing of breeding and within‐season patterns of clutch size, egg volume, incubation duration, and daily nest survival of three shorebird species between two decades. Based on previously known within‐season patterns and assuming a warming trend, we hypothesized that the timing of clutch initiation would advance between decades and would be coupled with increases in mean clutch size, egg volume, and daily nest survival rate. We monitored 1,378 nests of western sandpipers, semipalmated sandpipers, and red‐necked phalaropes at a subarctic site during 1993–1996 and 2010–2014. Sandpipers have biparental incubation, whereas phalaropes have uniparental incubation. We found an unexpected long‐term cooling trend during the early part of the breeding season. Three species delayed clutch initiation by 5 days in the 2010s relative to the 1990s. Clutch size and daily nest survival showed strong within‐season declines in sandpipers, but not in phalaropes. Egg volume showed strong within‐season declines in one species of sandpiper, but increased in phalaropes. Despite the within‐season patterns in traits and shifts in phenology, clutch size, egg volume, and daily nest survival were similar between decades. In contrast, incubation duration did not show within‐season variation, but decreased by 2 days in sandpipers and increased by 2 days in phalaropes. Shorebirds demonstrated variable breeding phenology and incubation duration in relation to climate cooling, but little change in nonphenological components of traits. Our results indicate that the breeding phenology of shorebirds is closely associated with the temperature conditions on breeding ground, the effects of which can vary among reproductive traits and among sympatric species.     

Long‐term continental changes in wing length,but not bill length,of a long‐distance migratory shorebird

We compiled a >50‐year record of morphometrics for semipalmated sandpipers (Calidris pusilla), a shorebird species with a Nearctic breeding distribution and intercontinental migration to South America. Our data included >57,000 individuals captured 1972–2015 at five breeding locations and three major stopover sites, plus 139 museum specimens collected in earlier decades. Wing length increased by ca. 1.5 mm (>1%) prior to 1980, followed by a decrease of 3.85 mm (nearly 4%) over the subsequent 35 years. This can account for previously reported changes in metrics at a migratory stopover site from 1985 to 2006. Wing length decreased at a rate of 1,098 darwins, or 0.176 haldanes, within the ranges of other field studies of phenotypic change. Bill length, in contrast, showed no consistent change over the full period of our study. Decreased body size as a universal response of animal populations to climate warming, and several other potential mechanisms, are unable to account for the increasing and decreasing wing length pattern observed. We propose that the post‐WWII near‐extirpation of falcon populations and their post‐1973 recovery driven by the widespread use and subsequent limitation on DDT in North America selected initially for greater flight efficiency and latterly for greater agility. This predation danger hypothesis accounts for many features of the morphometric data and deserves further investigation in this and other species.     

Effects of predation danger on migration strategies of sandpipers

We examine the potential selective importance of predation danger on the evolution of migration strategies of arctic‐breeding calidrid sandpipers. Adult calidrids truncate parental care for reasons not obviously related to levels of food abundance on the breeding areas or at migratory stopover sites, suggesting that a different trade‐off occurs between providing additional care and adult survivorship. The southward migrations of adult western sandpipers precede those of migratory peregrine falcons by almost a month. By moving early and quickly, adults remain ahead of migrant falcons all the way to their non‐breeding areas, where they rapidly moult flight feathers. They complete the moult just as falcons arrive in late September–October. By migrating early, they avoid exposure to falcons when they are unusually vulnerable, due to the requirements for fuelling migratory flight and of wing feather moult. Juvenile western sandpipers migrate south just as falcon numbers start to increase, but do not moult flight feathers in their first winter. Pacific dunlin use an alternative strategy of remaining and moulting in Alaska after falcons depart, and migrating to their overwintering sites after migrants have passed. East of the Rocky Mountains, the southbound migration of falcons begins 4–6 weeks later. Southbound semipalmated sandpipers make extended migratory stopovers, but their lengths of stay shorten prior to falcon migration to the sites in September. Predation danger also may affect the evolution of migration routes. Southbound western sandpipers fly directly from Alaska to southern British Columbia, in contrast to the multi‐stage journey northward along the Alaska panhandle. We estimate that a direct flight would be more economical on northward migration, but may be avoided because it would expose sandpipers to higher mass‐dependent predation danger from migratory falcons, which travel north with sandpipers. By contrast, few raptors are present in Alaska during preparation for the southward flight. A temporal and spatial window of safety may also permit semipalmated sandpipers to become extremely vulnerable while preparing for trans‐Atlantic southward flights. Danger management may account for the these previously enigmatic features of calidrid migration strategies, and aspects of those of other birds.     

Western sandpipers have altered migration tactics as peregrine falcon populations have recovered

The presence of top predators can affect prey behaviour, morphology and life history, and thereby can produce indirect population consequences greater and further reaching than direct depredation would have alone. Raptor species in the Americas are recovering since restrictions on the use of dichlorodiphenyltrichloroethane (DDT) and the implementation of conservation measures, in effect constituting a hemisphere-wide predator-reintroduction experiment, and profound effects on populations of their prey are to be expected. Here, we document changes in the behaviour of western sandpipers (Calidris mauri) at migratory stopover sites over two decades. Since 1985, migratory body mass and stopover durations of western sandpipers have fallen steadily at some stopovers in the Strait of Georgia, British Columbia. Comparisons between years, sites and seasons strongly implicate increasing danger from the recovery of peregrine falcons (Falco peregrinus) as a causal factor. A decade-long ongoing steep decline in sandpiper numbers censused on our study site is explained entirely by the shortening stopover duration, rather than fewer individuals using the site. Such behavioural changes are probably general among migratory shorebird species, and may be contributing to the widespread census declines reported in North America.     

Migration of two calidrid sandpiper species on the predator landscape: how stopover time and hence migration speed vary with geographical proximity to danger

The effects of relative fuel load on migration speed and on vulnerability have been investigated, but the effects of seasonal variation in predation danger on the amount of fuel and duration of stopover have not been considered. We analyzed seasonal patterns of stopover residence times for western and semipalmated sandpipers Calidris mauri and C. pusilla on southward migration in relation to the passage of migratory peregrine falcons Falco peregrinus. We predicted that individuals on stopover far in advance of the seasonal arrival of falcons would adjust stopover length and hence relative fuel load to migrate slowly and cautiously. We predicted that individuals on stopover later in the season would increase migratory speed as the arrival of migratory falcons came closer, while individuals on stopover under or behind the passage of falcons would migrate slowly. Adult and juvenile semipalmated and adult western sandpipers migrated prior to seasonal increases in peregrine abundance, and as predicted, the seasonal patterns of their stopover durations are consistent with an increase in the speed of migration as the date of peregrine arrival approached. Juvenile western sandpipers, in contrast, migrating concurrently with falcons, slowed their speed of migration as predator abundance increased. Stopover patterns differ between species due to different relative fuel loads. The results fit predictions made based on a ‘mortality‐minimizing’ migration strategy.     

Plasma metabolites and migration physiology of semipalmated sandpipers: refueling performance at five latitudes

Long-distance bird migration is fueled by energy gathered at stopover sites along the migration route. The refueling rate at stopover sites is a determinant of time spent at stopovers and impacts the overall speed of migration. Refueling rate during spring migration may influence the fitness of individuals via changes in the probability of successful migration and reproduction during the subsequent breeding season. We evaluated four plasma lipid metabolites (triglycerides, phospholipids, β-OH-butyrate, and glycerol) as measures of refueling rate in free-living semipalmated sandpipers (Calidris pusilla) captured at non-breeding areas. We described the spatial and temporal variation in metabolite concentrations among one winter site in the Dominican Republic and four stopover sites in the South Atlantic and Mid-Atlantic Coastal Plain regions of North America. Triglycerides and β-OH-butyrate clearly identified spatial variation in refueling rate and stopover habitat quality. Metabolite profiles indicated that birds had higher refueling rates at one site in the Mid-Atlantic Coastal Plain than at three sites on the South Atlantic Coastal Plain and one site in the Dominican Republic. Temporal variation in lipid metabolites during the migration season suggested that male semipalmated sandpipers gained more weight at stopovers on the South Atlantic Coastal Plain than did females, evidence of differential migration strategies for the sexes. Plasma lipid metabolites provide information on migration physiology that may help determine stopover habitat quality and reveal how migratory populations use stopover sites to refuel and successfully complete long-distance migrations.     

Ecological factors regulating brood attendance patterns of the Western Sandpiper Calidris mauri

Parental brood attendance patterns vary greatly among shorebird species. For monogamous calidridine species, biparental care with female-first brood departure is most common. It is believed that adult sandpipers balance potential individual survival costs associated with extended parental care against the benefit gained by their brood of prolonged parental care. These costs and benefits are difficult to quantify and factors affecting the termination of parental brood attendance are unclear. We compared clutch size, nesting phenology, and parental attendance patterns of Western Sandpipers Calidris mauri at Nome and Kanaryarmiut, Alaska, sites separated by three degrees of latitude. The sites differed in breeding density and duration of breeding season, but the distribution of clutch sizes did not differ between sites or between nesting attempts. Parental attendance patterns were similar between sites, suggesting that parental attendance is a highly conserved life-history trait in Western Sandpipers. Male Western Sandpipers attended broods longer than females, and the duration of parental attendance decreased at a similar rate for both sexes as the season progressed. Male and female Western Sandpipers undertake differential migrations to their non-breeding grounds, with males typically settling at more northerly locations and females at more southerly sites, a migration pattern shared by certain other monogamous calidridine species. These same species exhibit similar parental brood attendance patterns, suggesting the strong role of overall migration distance in shaping the expression of parental attendance behaviours. A contrast of more geographically disjunct sites coupled with a better understanding of the migratory connectivity between Western Sandpiper breeding and non-breeding populations would elucidate the role of cross-seasonal effects on parental brood attendance decisions.     

Effects of migration distance on life history strategies of Western and Semipalmated sandpipers in Perú

Migration distances of shorebird species correlate with life history strategies. To assess age‐specific migratory preparation and adult wing‐molt strategies, we studied Western Sandpipers (Calidris mauri) and Semipalmated Sandpipers (C. pusilla) with different migration routes at the Paracas National Reserve in Perú, one of the most austral non‐breeding areas for these sandpipers, from 2012 to 2015. Western Sandpipers breed near the Bering Sea, ~11,000 km from Paracas. Semipalmated Sandpiper populations at Paracas are a mixture of short‐billed birds from western Arctic breeding sites, plus long‐billed birds from eastern sites, ~8000 km distant. Adults of both species arrive in October with primary feathers already partially renewed so wing molt starts at sites further north. Semipalmated Sandpipers with longer bills completed wing molt later than shorter billed birds. Adults of both species prepared for migration in February and March. No juvenile Western Sandpipers prepared for migration, confirming the “slow” over‐summering life history strategy of more southerly non‐breeding populations. Juvenile Semipalmated Sandpipers showed bimodality in strategies. Most showed no migratory preparation, but, during three non‐breeding periods, from 27% to 31% fattened, molted, and partially replaced outer primaries during the pre‐migratory period. Juveniles with longer culmens were heavier and tended to have more alternate plumage. Juveniles that were partially molting primaries had longer culmens and more alternate plumage. Juvenile Semipalmated Sandpipers from eastern‐breeding populations thus have a higher propensity for a fast life history strategy, and western birds a slow one, at this non‐breeding site in Peru. Western‐breeding Semipalmated Sandpiper populations thus resemble Western Sandpipers, suggesting a common, possibly distance‐related, effect on life history strategy.     

Winter rainfall predicts phenology in widely separated populations of a migrant songbird

Climate change is affecting behaviour and phenology in many animals. In migratory birds, weather patterns both at breeding and at non-breeding sites can influence the timing of spring migration and breeding. However, variation in responses to weather across a species range has rarely been studied, particularly among populations that may winter in different locations. We used prior knowledge of migratory connectivity to test the influence of weather from predicted non-breeding sites on bird phenology in two breeding populations of a long-distance migratory bird species separated by 3,000 km. We found that winter rainfall showed similar associations with arrival and egg-laying dates in separate breeding populations on an east–west axis: greater rainfall in Jamaica and eastern Mexico was generally associated with advanced American redstart (Setophaga ruticilla) phenology in Ontario and Alberta, respectively. In Ontario, these patterns of response could largely be explained by changes in the behaviour of individual birds, i.e., phenotypic plasticity. By explicitly incorporating migratory connectivity into responses to climate, our data suggest that widely separated breeding populations can show independent and geographically specific associations with changing weather conditions. The tendency of individuals to delay migration and breeding following dry winters could result in population declines due to predicted drying trends in tropical areas and the tight linkage between early arrival/breeding and reproductive success in long-distance migrants.     

Niche dynamics of shorebirds in Delaware Bay: Foraging behavior,habitat choice and migration timing

Niche differentiation through resource partitioning is seen as one of the most important mechanisms of diversity maintenance contributing to stable coexistence of different species within communities. In this study, I examined whether four species of migrating shorebirds, dunlins (Calidris alpina), semipalmated sandpipers (Calidris pusilla), least sandpipers (Calidris minutilla) and short-billed dowitchers (Limnodromus griseus), segregate by time of passage, habitat use and foraging behavior at their major stopover in Delaware Bay during spring migration. I tested the prediction that most of the separation between morphologically similar species will be achieved by differential migration timing. Despite the high level of overlap along observed niche dimensions, this study demonstrates a certain level of ecological separation between migrating shorebirds. The results of analyses suggest that differential timing of spring migration might be the most important dimension along which shorebird species segregate while at stopover in Delaware Bay. Besides differences in time of passage, species exhibited differences in habitat use, particularly least sandpipers that foraged in vegetated areas of tidal marshes more frequently than other species, as well as short-billed dowitchers that foraged in deeper water more often than small sandpipers did. Partitioning along foraging techniques was less prominent than segregation along temporal or microhabitat dimensions. Such ranking of niche dimensions emphasizes significance of temporal segregation of migratory species – separation of species by time of passage may reduce the opportunity for interspecific aggressive encounters, which in turn can have positive effects on birds' time and energy budget during stopover period.     

GLOBAL MITOCHONDRIAL DNA PHYLOGEOGRAPHY OF HOLARCTIC BREEDING DUNLINS (CALIDRIS ALPINA)

Comparison of mitochondrial DNA (mtDNA) control-region sequences of 155 dunlins from 15 breeding populations confirmed the existence of five major phylogeographic groups in the circumpolar breeding range of this migratory shorebird species. Time estimates of the origin of groups, based on sequence divergences and a molecular clock for birds, suggest a scenario of repeated fragmentation of populations in isolated tundra refugia during the late Pleistocene. The distribution of about three-quarters of all detected molecular variance between phylogeographic groups attests to the strongly subdivided genetic population structure in dunlins that is being maintained by natal philopatry. Each mtDNA phylogeographic group can be related to a morphometrically defined subspecies, but several other recognized subspecies are not supported by monophyletic mtDNA lineages within their purported ranges. More detailed analysis of several European populations reveals low amounts of gene flow and the partitioning of a substantial fraction of molecular variance between them. This ongoing evolution of population-genetic structuring within the European phylogeographic group most likely started with the last retreat of the ice sheets some 10,000 years ago. Dunlins thus provide one of the clearest examples of the linkage between historical and contemporary components of mtDNA phylogeographic structuring in birds.     

Malarial parasites as geographical markers in migratory birds?

We tested the hypothesis that malarial parasites (Plasmodium and Haemoproteus) of black-throated blue warblers (Dendroica caerulescens) provide sufficient geographical signal to track population movements between the warbler's breeding and wintering habitats in North America. Our results from 1083 warblers sampled across the species' breeding range indicate that parasite lineages are geographically widespread and do not provide site-specific information. The wide distribution of malarial parasites probably reflects postnatal dispersal of their hosts as well as mixing of breeding populations on the wintering range. When compared to geographically structured parasites of sedentary Caribbean songbirds, patterns of malarial infections in black-throated blue warblers suggest that host-malaria dynamics of migratory and sedentary bird populations may be subject to contrasting selection pressures.     

Migratory connectivity in a declining bird species: using feather isotopes to inform demographic modelling

Aim Conservation programmes for endangered migratory species or populations require locating and evaluating breeding, stopover and wintering areas. We used multiple stable isotopes in two endangered European populations of wrynecks, Jynx torquilla L., to locate wintering regions and assess the degree of migratory connectivity between breeding and wintering populations. Location Switzerland and Germany. Methods We analysed stable nitrogen (δ¹⁵N), carbon (δ¹³C) and hydrogen (δD) isotopes from wing feathers from two populations of wrynecks to infer their wintering origins and to assess the strength of migratory connectivity. We tested whether variation in feather isotopic values within the Swiss population was affected by bird age and collection year and then considered differences in isotopic values between the two breeding populations. We used isotopic values of summer‐ and winter‐grown feathers to estimate seasonal distributions. Finally, we calculated a species‐specific δD discrimination factor between feathers and mean annual δD values to assign winter‐grown feathers to origin. Results Bird age and collection year caused substantial isotopic variation in winter‐grown feathers, which may be because of annually variable weather conditions, movements of birds among wintering sites and/or reflect asynchronous moulting or selection pressure. The large isotopic variance in winter‐grown feathers nevertheless suggested low migratory connectivity for each breeding population, with partially overlapping wintering regions for the two populations. Main conclusions Isotopic variance in winter‐grown feathers of two breeding populations of wrynecks and their geographical assignment point to defined, albeit overlapping, wintering areas, suggesting both leapfrog migration and low migratory connectivity. On this basis, integrative demographic models can be built looking at seasonal survival patterns with links to local environmental conditions on both breeding and wintering grounds, which may elucidate causes of declines in migratory bird species.     

Lines in the mud; revisiting the boundaries of important shorebird areas

Many shorebird populations are declining throughout the world, concurrent with declines and degradation of wetland habitats. Such declines necessitate a more consistent approach towards conserving habitats used by shorebird populations. Individuals of many shorebird species congregate in specific areas during their non-breeding season. Worldwide, non-breeding areas are designated as ‘important’ for shorebird conservation based primarily on the abundance of birds found in an area. However, the boundaries of any area are often defined with incomplete information regarding how shorebirds use that habitat. This paper discusses examples in Australia where improved knowledge of shorebird habitat use led to the identification of very different boundaries of important shorebird areas than those identified originally. We highlight how simple questioning of those who count shorebirds in an area, led to an improved understanding of which areas were apparently used by the same local population of non-breeding shorebirds. Subsequent analysis of available count, recapture and/or home range data of particular shorebird species is needed to verify expert opinion regarding most of these boundaries. We review how enhanced boundaries improve the ability of shorebird monitoring to detect population changes; allow management of shorebird habitats at relevant spatial scales; and lead to appropriate designations of important areas. While the kinds of approaches to boundary setting described here are not new, they are not consistently applied worldwide. We suggest additional guidelines to those produced under the Ramsar Convention in regard to designating important areas. We also call for more studies on the movements of migratory shorebirds during the non-breeding season to direct more consistent boundary setting around important non-breeding habitats used by local populations of migratory shorebirds.     

Population structure of Purple Sandpipers (Calidris maritima) as revealed by mitochondrial DNA and microsatellites

The Purple Sandpiper (Calidris maritima) is a medium‐sized shorebird that breeds in the Arctic and winters along northern Atlantic coastlines. Migration routes and affiliations between breeding grounds and wintering grounds are incompletely understood. Some populations appear to be declining, and future management policies for this species will benefit from understanding their migration patterns. This study used two mitochondrial DNA markers and 10 microsatellite loci to analyze current population structure and historical demographic trends. Samples were obtained from breeding locations in Nunavut (Canada), Iceland, and Svalbard (Norway) and from wintering locations along the coast of Maine (USA), Nova Scotia, New Brunswick, and Newfoundland (Canada), and Scotland (UK). Mitochondrial haplotypes displayed low genetic diversity, and a shallow phylogeny indicating recent divergence. With the exception of the two Canadian breeding populations from Nunavut, there was significant genetic differentiation among samples from all breeding locations; however, none of the breeding populations was a monophyletic group. We also found differentiation between both Iceland and Svalbard breeding populations and North American wintering populations. This pattern of divergence is consistent with a previously proposed migratory pathway between Canadian breeding locations and wintering grounds in the United Kingdom, but argues against migration between breeding grounds in Iceland and Svalbard and wintering grounds in North America. Breeding birds from Svalbard also showed a genetic signature intermediate between Canadian breeders and Icelandic breeders. Our results extend current knowledge of Purple Sandpiper population genetic structure and present new information regarding migration routes to wintering grounds in North America.     

Can intrinsic factors explain population declines in North American breeding shorebirds? A comparative analysis

Many shorebirds that breed in North America are declining. These trends reflect global patterns in shorebird populations. Here we ask what factors make some shorebird species more prone to decline than others. Specifically, we test the influence of migratory behaviour (route and distance), biogeography (population size and range), life history (body size, clutch size) and sexual selection (social mating system and testis size) on population trends in North American breeding shorebirds. Using phylogenetic comparative methods, we show that species that migrate across continental North America are more prone to decline than species that do not. Our finding that continental migrants are associated with population decline indicates that intrinsic factors may play an important role in predisposing a species to decline. Previous studies within the class Aves have failed to identify migration route as a correlate of decline or extinction risk. Two other intrinsic factors (oceanic migrants and threats on the non-breeding grounds) were also important in our overall models, although neither was significant alone. The moderate explanatory power of our variables indicates that other factors are also important for explaining shorebird declines. We suggest that contemporary threats, most notably habitat loss and degradation at migratory stopover sites, are likely to be important.     

SEX-BIASED GENE FLOW IN SPECTACLED EIDERS (ANATIDAE): INFERENCES FROM MOLECULAR MARKERS WITH CONTRASTING MODES OF INHERITANCE

Abstract Genetic markers that differ in mode of inheritance and rate of evolution (a sex‐linked Z‐specific micro‐satellite locus, five biparentally inherited microsatellite loci, and maternally inherited mitochondrial [mtDNA] sequences) were used to evaluate the degree of spatial genetic structuring at macro‐ and microgeographic scales, among breeding regions and local nesting populations within each region, respectively, for a migratory sea duck species, the spectacled eider (Somateria fisheri). Disjunct and declining breeding populations coupled with sex‐specific differences in seasonal migratory patterns and life history provide a series of hypotheses regarding rates and directionality of gene flow among breeding populations from the Indigirka River Delta, Russia, and the North Slope and Yukon‐Kuskokwim Delta, Alaska. The degree of differentiation in mtDNA haplotype frequency among breeding regions and populations within regions was high (φ_CT= 0.189, P < 0.01; φ_SC= 0.059, P < 0.01, respectively). Eleven of 17 mtDNA haplotypes were restricted to a single breeding region. Genetic differences among regions were considerably lower for nuclear DNA loci (sex‐linked: φ_ST= 0.001, P > 0.05; biparentally inherited microsatellites: mean θ= 0.001, P > 0.05) than was observed for mtDNA. Using models explicitly designed for uniparental and biparentally inherited genes, estimates of spatial divergence based on nuclear and mtDNA data together with elements of the species' breeding ecology were used to estimate effective population size and degree of male and female gene flow. Differences in the magnitude and spatial patterns of gene correlations for maternally inherited and nuclear genes revealed that females exhibit greater natal philopatry than do males. Estimates of generational female and male rates of gene flow among breeding regions differed markedly (3.67 × 10^‐4 and 1.28 × 10^‐2, respectively). Effective population size for mtDNA was estimated to be at least three times lower than that for biparental genes (30,671 and 101,528, respectively). Large disparities in population sizes among breeding areas greatly reduces the proportion of total genetic variance captured by dispersal, which may accelerate rates of inbreeding (i.e., promote higher coancestries) within populations due to nonrandom pairing of males with females from the same breeding population.