A multi-isotope (δ13C, δ15N, δ2H) approach to connecting European breeding and African wintering populations of barn swallow (Hirundo rustica)

A multi-isotope (δ13C, δ15N, δ2H) approach to connecting European breeding and African wintering populations of barn swallow (Hirundo rustica) Establishing links between breeding and wintering populations of longdistance migratory birds and other animals is fundamental to several aspects of migration research. However, severe limitations in our ability to track small-bodied migratory species still limits this field despite several recent technological breakthroughs. The measurement of naturally occurring stable isotopes of several elements in the tissues of migrants that travel across isotopic gradients or isoscapes has the potential to identify large scale migratory connectivity without some of the biases associated with the use of extrinsic markers. We investigated migratory connectivity between European breeding and African wintering populations of barn swallow (Hirundo rustica) by comparing feather isotope (δ13C, δ15N, δ2H) values with those expected from previously established feather isotopic clusters for Africa. We used a likelihood approach to assigning individuals to molt origins that also made use of prior information provided by ring recoveries as part of the EURING and SAFRING ringing efforts. We found evidence for strong isotopic spatial structure in the dataset, supporting the notion of a migratory divide in Europe with birds breeding in the Netherlands, Denmark, Germany and Eastern Europe wintering in southern Africa and virtually all samples from Switzerland westward being assigned to clusters in the northern portion of the species’ winter range. Individuals from the United Kingdom were assigned to areas including Namibia, Botswana, northern South Africa, and along the coast of Mozambique and Tanzania. Birds wintering in the northernmost region of the wintering grounds tended to breed in the southernmost region of the breeding grounds, providing some evidence of leap-frog migration. We detected a strong latitudinal threshold in feather δ13C in Europe for African-grown feathers, suggesting that birds breeding in southern Europe (< 50° Latitude) primarily used C3-dominated habitats in Africa, whereas birds in northern Europe (> 53° Latitude) primarily used C4-dominated habitats. Our results emphasize the power in using a multi-isotope approach to assign individuals and populations to known continental-scale isoscapes and the advantages of combining isotopic and conventional (ring recovery) information within a Bayesian assignment framework.