Recent evidence indicates that tree swallows (Tachycineta bicolor) are expanding their range southwards. This is an unexpected pattern for these birds in light of climate change theory, which suggests most species will expand toward the poles. Isotope signatures vary across geographic regions, and have been used to differentiate between breeding populations. If this expansion is being driven from outside the breeding population, we expect to see differences in isotope signatures between returning birds and newly arrived birds. The null hypothesis is that returning birds and newly arrived birds will have similar isotope signatures, indicating that the newly arrived birds bred in the same area the previous year. To explore the drivers of this range expansion, we compared dD isotope values between returning and newly arrived individuals during the summers of 2017 and 2018. We took feather samples from a small population of breeding birds at Beaver Lake in Asheville, NC, with additional samples coming from returning birds and new arrivals from another breeding site in Boone, NC. Preliminary results from the Beaver Lake and Boone locations show a clear pattern differentiating isotope signatures of recaptured birds from others in the 2018 season, indicating that this expansion may be driven from outside the local breeding population.
