Variation in the Stable-Hydrogen Isotope Composition of Northern Goshawk Feathers: Relevance to the Study of Migratory Origins

The analysis of stable-hydrogen isotope ratios in feathers (δDf) allows researchers to investigate avian movements and distributions to an extent never before possible. Nonetheless, natural variation in δDf is poorly understood and, in particular, its implications for predictive models based on stable-hydrogen isotopes remain unclear. We employed hierarchical linear modeling to explore multiple levels of variation in the stable-hydrogen isotope composition of Northern Goshawk (Accipiter gentilis) feathers. We examined (1) inter-individual variation among goshawks from the same nest, and (2) intra-individual variation between multiple feathers from the same individual. Additionally, we assessed the importance of several factors (e.g., geographic location, climate, age, and sex characteristics) in explaining variation in δDf. Variation among individuals was nearly eight times the magnitude of variation within an individual, although age differences explained most of this inter-individual variation. In contrast, most variation in δD values between multiple feathers from an individual remained unexplained. Additionally, we suggest temporal patterns of δD in precipitation (δDp) as a potential explanation for the geographic variability in age-related differences that has precluded the description of movement patterns of adult raptors using δDf. Furthermore, intra-individual variability necessitates consistency in feather selection and careful interpretation of δDf-based models incorporating multiple feather types. Finally, although useful for describing the movements of groups of individuals, we suggest that variability inherent to environmental and intra-individual patterns of δDp and δDf, respectively, precludes the use of stable-hydrogen isotopes to describe movements of individual birds