A multi-isotope (δ2H, δ13C, δ15N) approach to establishing migratory connectivity of Barn Swallow (Hirundo rustica)

Abstract

Establishing geographic links between different stages of the annual cycle of migratory species is fundamental to evaluating factors limiting their populations. Stable isotope measurements (d13C, d15N, and d2H) 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. © 2014 García-Pérez and Hobson.