Understanding bird migration and dispersal is important to inform full life-cycle conservation planning. Stable hydrogen isotope ratios from feathers (δ2 Hf ) can be linked to amountweighted long-term, growing season precipitation δ2 H (δ2 Hp ) surfaces to create δ2 Hf isoscapes for assignment to molt origin. However, transfer functions linking δ2 Hp with δ2 Hf are influenced by physiological and environmental processes. A better understanding of the causes and consequences of variation in δ2 Hf 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 δ2 Hf 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 δ2 Hf . A model including species as single predictor received the most support (AIC weight = 0.74) in explaining variation in δ2 Hf . A species-specific variance parameter was part of all bestranked models, suggesting variation in δ2 Hf 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 δ2 Hf suggesting that interspecific differences in δ2 Hf can differ among years. Our results suggest that withinand among-year interspecific variation in δ2 Hf is the most important source of variance typically not being explicitly quantified in geographic assignment tests using non-specific transfer functions to convert δ2 Hp into δ2 Hf . 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.
Nordell, C. J., Haché, S., Bayne, E. M., Sólymos, P., Foster, K. R., Godwin, C. M., … Hobson, K. A. (2016). Within-site variation in feather stable hydrogen isotope (ä2 Hf ) values of boreal songbirds: Implications for assignment to molt origin. PLoS ONE, 11(11). https://doi.org/10.1371/journal.pone.0163957