STRONTIUM AND HYDROGEN ISOTOPES CAN EFFECTIVELY ESTIMATE NATAL ORIGIN FOR MIGRATORY RAPTORS

Habitat loss and fragmentation have reduced forest cover throughout North America. Identifying key breeding grounds and mobility of forest-dependent species, such as birds of prey, is increasingly urgent. Typical methods for obtaining this kind of geographical information, such as banding and recapturing or satellite- and radio-tracking, may be prohibitively expensive or logistically challenging. Isotopic analysis is highly complementary to other methods for tracking mobility and geographic provenance. Feathers can be collected relatively non-invasively during normal banding efforts and can provide information about an individual’s history during a single capture event. Hydrogen isotope (δD) values, which are primarily influenced by climate, are routinely used to provide broad geographic estimates of natal region. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr), which primarily reflect bedrock geology, can considerably enhance the precision of geospatial estimates when coupled with δD. Here, we use a dual isotope predictive model to estimate most likely natal origin for juvenile migratory raptors that were trapped in southwestern Idaho during their fall migration. The model can distinguish local Idaho birds from those that migrated from further north. Moreover, the geographic estimates for the natal regions for most individuals are remarkably precise. This powerful geochemical approach is readily adaptable to other localities or species. Additionally, isotopic data can be combined with other types of data, including trace metal analysis or genetics, to investigate where birds are exposed to toxins and examine population connectivity. Lastly, because samples retain their biogenic isotope values more or less indefinitely, it should be possible to look at shifts in breeding ranges over time using museum specimens.