Within-Site Variation in Feather Stable Hydrogen Isotope (δ(2)H(f)) Values of Boreal Songbirds: Implications for Assignment to Molt Origin

Understanding bird migration and dispersal is important to inform full life-cycle conservation planning. Stable hydrogen isotope ratios from feathers (δ(2)H(f)) can be linked to amount-weighted long-term, growing season precipitation δ(2)H (δ(2)H(p)) surfaces to create δ(2)H(f) isoscapes for assignment to molt origin. However, transfer functions linking δ(2)H(p) with δ(2)H(f) are influenced by physiological and environmental processes. A better understanding of the causes and consequences of variation in δ(2)H(f) values among individuals and species will improve the predictive ability of geographic assignment tests. We tested for effects of species, land cover, forage substrate, nest substrate, diet composition, body mass, sex, and phylogenetic relatedness on δ(2)H(f) from individuals at least two years old of 21 songbird species captured during the same breeding season at a site in northeastern Alberta, Canada. For four species, we also tested for a year × species interaction effect on δ(2)H(f). A model including species as single predictor received the most support (AIC weight = 0.74) in explaining variation in δ(2)H(f). A species-specific variance parameter was part of all best-ranked models, suggesting variation in δ(2)H(f) was not consistent among species. The second best-ranked model included a forage substrate × diet interaction term (AIC weight = 0.16). There was a significant year × species interaction effect on δ(2)H(f) suggesting that interspecific differences in δ(2)H(f) can differ among years. Our results suggest that within- and among-year interspecific variation in δ(2)H(f) is the most important source of variance typically not being explicitly quantified in geographic assignment tests using non-specific transfer functions to convert δ(2)H(p) into δ(2)H(f). However, this source of variation is consistent with the range of variation from the transfer functions most commonly being propagated in assignment tests of geographic origins for passerines breeding in North America.