Within-Site Variation In Feather Stable Hydrogen Isotope (Delta H-2(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 (delta H-2(f)) can be linked to amount-weighted long-term, growing season precipitation delta H-2 (delta H-2(p)) surfaces to create delta H-2(f) isoscapes for assignment to molt origin. However, transfer functions linking delta H-2(p) with delta H-2(f) are influenced by physiological and environmental processes. A better understanding of the causes and consequences of variation in delta H-2(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 delta H-2(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 x species interaction effect on delta H-2(f). A model including species as single predictor received the most support (AIC weight = 0.74) in explaining variation in delta H-2(f). A species-specific variance parameter was part of all best-ranked models, suggesting variation in delta H-2(f) was not consistent among species. The second best-ranked model included a forage substrate x diet interaction term (AIC weight = 0.16). There was a significant year x species interaction effect on delta H-2(f) suggesting that interspecific differences in delta H-2(f) can differ among years. Our results suggest that within and among-year interspecific variation in delta H-2(f) is the most important source of variance typically not being explicitly quantified in geographic assignment tests using non-specific transfer functions to convert delta H-2(p) into delta H-2(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.