Establishing geographic links between different stages of the annual cycle of migratory species is fundamental to evaluating factors limiting their populations. Stable isotope measurements (δ13C, δ15N, and δ2H) of feathers combined with a knowledge of how these isotopes are structured spatially in foodwebs can be used to establish molt origins and migratory connectivity. Here, we investigated patterns of migratory connectivity between North American breeding grounds and South American wintering grounds of stable (n = 3) and declining (n = 9) populations of Barn Swallow (Hirundo rustica) in North America using a probabilistic assignment to multi‐isotope feather clusters derived for 488 winter‐grown feathers collected during the breeding season (2009 to 2012). Our study did not find evidence for differential degree of migratory connectivity between increasing/stable and declining populations of Barn Swallows but found a longitudinal structure in breeding and wintering ground origins of populations. Probable wintering areas for northern and western breeding birds corresponded with western regions of South America, while birds breeding in southern and eastern North America tended to occupy areas in north‐eastern South America. Possible factors contributing to the differential population trends between stable or increasing and declining breeding populations could be related to habitat quality of the different wintering areas, changes in climate, and the cost of long‐distance migration. The use of a multi‐isotope approach and the combination of prior information on geographic distribution of vegetation types, based on δ13C measurements, effectively constrained geographic origins of swallows and our approach can be applied to defining migratory connectivity for other species.

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BackgroundBarn Swallows (Hirundo rustica) breed almost exclusively in the Northern Hemisphere. However, since the early 1980's, a small disjunct breeding population has become established in eastern Argentina, presumably by birds previously derived from those breeding in North America. Currently, it is unknown where these individuals go following breeding and how they have adjusted to a reversal in phenology. Their austral wintering period corresponds to the breeding period of the northern ancestral population and so they can potentially return to these more traditional breeding sites or they may occupy other South American wintering regions left vacant by conspecifics returning to the Northern Hemisphere.Principal FindingsWe used a three-isotope (δ 13C, δ 15N, δ 2H) approach to investigate potential wintering areas in Central and South America of individuals breeding in Argentina. Feather isotope values differed from those expected and measured at local breeding sites in Argentina indicating molt after the austral breeding period and away from the breeding grounds. Potential molting origins were identified applying likelihood-based assignment methods to a δ 2H isoscape for South America and dichotomous prior information on the distribution of C3 and C4 vegetation types based on modeled vegetation-δ 13C values. Barn Swallows now breeding in Argentina have changed their migratory behavior but presumably use the same cues as those used by the ancestral population, molting their feathers during the austral winter, likely in north-eastern South America.

Molt is energetically demanding and various molt strategies (i.e., molt series, duration, intensity, timing, and location) have evolved to reduce the negative fitness consequences of this process. As such, molt varies considerably among species. Identifying where and when specific feathers are molted is also crucial to inform species‐specific studies using stable isotope markers to assign individuals to geographical regions where they molt. Using museum specimens, we examined the molt of three species of migratory swallows in the Americas: Bank Swallows (Riparia riparia), Barn Swallows (Hirundo rustica), and Cliff Swallows (Petrochelidon pyrrhonota). All three species have one primary and two secondary molt series. Bank and Cliff swallows had one rectrix molt series, and Barn Swallows molted the outer rectrix (R6) separately from the inner five rectrices (R1‐5). All three species have a relatively long flight feather molt duration (i.e., 140–183 days) and low molt intensity. Barn Swallows initiated flight feather molt in the fall, about 2 months later than Bank and Cliff swallows. Barn Swallows likely delay molt because of constraints associated with double brooding. For all three species, molt started with the primaries and inner secondaries and was closely followed by the rectrices and, finally, the outer secondaries. For those that began and then interrupted molt either in breeding areas or during fall migration, the first feathers molted were predominantly S8 and P1. All three species underwent body molt throughout the year, but most individuals molted their body plumage in wintering areas. We recommend that the most appropriate feathers for stable isotope research examining migratory connectivity and habitat use are either R2‐R4 or S2‐S4.