An evaluation of migration fidelity of Ruby-throated Hummingbirds inferred from stable isotope methods

Knowledge of spatial connectivity between breeding and non-breeding locations of migratory birds and their breeding site fidelity are important for avian conservation. Ruby-throated Hummingbirds (RTHU, Archilochus colubris) breed in eastern Canada west to the Rocky Mountains and in the USA east of the Mississippi River and spend the non-breeding period in Mexico, Central America, and southern Florida, USA. We measured the hydrogen and oxygen stable isotopic compositions of adult RTHU tailfeathers (fourth rectrix) from three breeding locations in North America to estimate migratory connectivity between breeding and non-breeding grounds where feathers are grown. Feather δ ²H values showed no statistical difference among the three sampling locations as well as disparate geographic assignments from one location on the non-breeding grounds in Costa Rica. Therefore, only weak evidence of migratory connectivity between breeding and non-breeding grounds could be ascertained for our sample of this species. The lack of migratory connectivity detected for Ruby-throated Hummingbirds using stable isotopes is consistent with origins from broad regions on the non-breeding grounds. However, it may also imply that precipitation δ ²H values on the non-breeding grounds do not vary enough to detect a difference among our study populations. Sampling of additional populations in the eastern and southern portion of the species' breeding range and the non-breeding grounds may reveal differences in migratory connectivity among populations and requires further investigation. The δ ¹⁸O values of feathers correlated poorly to their δ ²H values, an effect that may reflect the balance between metabolically driven processes and environmental water on the δ ¹⁸O values of hummingbird tissues. This study provides the foundations for further investigations into migratory connectivity of RTHU using δ ²H_f values and suggests potential avenues of study for use of δ ¹⁸O values of tissues in metabolic research.