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 ( 2 H: 1 H; depicted as δ 2 H p ) form a largely latitude-sensitive isoscape that can be used to estimate the geographical origin of animals. Feathers are metabolically inert following growth and δ 2 H f values 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 δ 2 H f measurements from Ovenbirds ( Seiurus aurocapilla (L., 1766)) associated with year, age class, feather type, season, and habitat type in New Brunswick, Canada. The observed δ 2 H f generally followed that predicted from the Global Network of Isotopes in Precipitation database. However, we found a strong year × age interaction on δ 2 H f . Season, habitat type, and feather type explained only a small portion of the overall variation in δ 2 H f . These results show the advantages of using annual δ 2 H p isoscapes and age-specific corrections when converting δ 2 H p to δ 2 H f . 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 δ 2 H p .