Geographic variation in strontium and hydrogen isotopes in Avian tissues: Implications for tracking migration

Isotopes can be powerful tools for answering fundamental questions about the ecology and evolution of migratory organisms because the origin of individuals can theoretically be estimated from a single capture. However, there is still remarkably little information available about how and why isotopes vary in animal tissues, especially over large spatial scales. Here, I describe variation in both stable-hydrogen (deltaDF) and strontium (87Sr/ 86SrF) isotopic compositions in the feathers of a migratory songbird, the Tree Swallow (Tachycineta bicolor), across 18 sampling sites in North America and then examine potential mechanisms driving this variation. I found that deltaDF was strongly correlated with latitude of the sampling site whereas 87Sr/86Sr F was strongly correlated with longitude. deltaDF was related to deltaD of meteoric waters where molting occurred and 87Sr/ 86SrF was influenced primarily by the geology of the molting area. Using simulation models, I then assessed the utility of combining both markers to estimate the origin of individuals. Using 13 geographic regions, I found that the number of individuals correctly assigned to their site of origin increased from less than 40% using either deltaD or 87Sr/ 86Sr alone to 74% using both isotopes. My results suggest that these isotopes have the potential to provide predictable and complementary markers for estimating long-distance animal movements. Combining isotopes influenced by different processes may allow researchers, to link the population dynamics of migratory animals across large geographic ranges.