Assigning birds to geographic origin using feather hydrogen isotope ratios (δ2H): importance of year, age, and habitat

Abstract

In North America, gradients in the ratio of stable hydrogen isotopes in amount-weighted, growing-season mean precipitation (2H:1H; depicted as δ2Hp) form a largely latitude-sensitive isoscape that can be used to estimate the geographical origin of animals. Feathers are metabolically inert following growth and δ2Hfvalues retain information on geographical origins. However, there are important assumptions underlying this approach that can only be tested using birds of known origin. Here, we investigated sources of variation in δ2Hfmeasurements from Ovenbirds ( Seiurus aurocapilla (L., 1766)) associated with year, age class, feather type, season, and habitat type in New Brunswick, Canada. The observed δ2Hfgenerally followed that predicted from the Global Network of Isotopes in Precipitation database. However, we found a strong year × age interaction on δ2Hf. Season, habitat type, and feather type explained only a small portion of the overall variation in δ2Hf. These results show the advantages of using annual δ2Hpisoscapes and age-specific corrections when converting δ2Hpto δ2Hf. We submit that the interaction effect can be modeled by accounting for interannual variation in the amount of precipitation during the breeding season. These procedures will allow for more precise estimates of the geographical origin of individual birds, especially for studies conducted near oceanic coasts, which are subject to significant annual variations in growing season δ2Hp.