Feather stable isotope (δ2H) measurements suggest no historical variation in latitudinal origin of migrants in two declining songbirds

Abstract

The geographic origin of migratory birds can be traced using measurements of stable-hydrogen isotopes (δ2H) in their feathers. In continental Europe, local amount-weighted mean annual δ2H in precipitation varies with latitude, so that assignments of geographical origin of bird feathers are linked to latitude. Consequently, a potential northward latitudinal shift in the distribution of a species’ abundance is expected to translate into a shift in δ2H values for feathers grown on the breeding grounds. We analyzed δ2H in feathers from migrant individuals of Brambling (Fringilla montifringilla) and Ortolan Bunting (Emberiza hortulana) collected in France from the mid-nineteenth century to recent years (2015), to assess potential temporal trends in feather δ2H that may indicate changes in source populations or a shifting breeding range. These two species have shown long-term population declines in Europe with subsequent range retractions, but we did not find obvious differences in feather δ2H values in samples collected over the last 160 years. One potential explanation for this result is that population declines occurred similarly at all latitudes across the breeding ranges without involving a measurable latitudinal shift in abundance that could translate into changes in feather δ2H values. However, this result may also be due to potential temporally shifting terrestrial spatial δ2H gradients (i.e., isoscape) similar to a shift in bird abundances, or to limited northward range shifts that do not translate into detectable trends in feather δ2H.