The study of individual movement patterns using stable hydrogen isotopes (δ 2H) as a natural marker has grown; however, recent studies have suggested that measurement of δ 2H in feathers (δ 2Hf) may prove unreliable as a means for identifying region of origin of migrating or wintering birds, particularly raptors. In this study, we examine whether differences in body condition could explain some of the variability in δ 2H in feathers. We analyzed growing feathers of 21 Swainson's Hawks breeding in northern CA for δ 2H, nitrogen (δ 15N), and carbon (δ 13C) stable isotopes in relation to body condition. We found that δ 2H was variable (range = 40‰), and that variability was significantly associated with body condition. Raptors derive most or all of their moisture from prey. Therefore, we suggest that individuals in poor condition have an enriched pool of body water relative to individuals in good condition, due to fractionation of body water stores during respiratory water loss and metabolic processes. Body condition was also negatively correlated with δ 15Nf. However, δ 2Hf, δ 15Nf, and δ 13Cf were not correlated, suggesting that the relationship between δ 2Hf and body condition is a result of physiological processes rather than differences in dietary δ 2H. We used an isotopic basemap of δ 2Hf values to assess individual origin as if they were encountered naively on the migration or wintering grounds, and all individuals fell within the 95% confidence interval of our study area. Conversely, the 95% confidence interval of δ 2Hf values obtained encompassed almost the entire breeding range of this species, indicating little ability to differentiate origins of this species.
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Vol. 51 • No. 2