CORRELATION BETWEEN FEATHER ISOTOPES AND BODY CONDITION FOR SWAINSON'S HAWKS, AND IMPLICATIONS FOR MIGRATION STUDIES

The study of individual movement patterns using stable hydrogen isotopes (delta H-2) as a natural marker has grown; however, recent studies have suggested that measurement of delta H-2 in feathers (delta H-2(f)) 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 delta H-2 in feathers. We analyzed growing feathers of 21 Swainson's Hawks breeding in northern CA for delta H-2, nitrogen (delta N-15), and carbon (delta C-13) stable isotopes in relation to body condition. We found that delta H-2 was variable (range = 40 parts per thousand), 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 delta N-15(f). However, delta H-2(f), delta N-15(f), and delta C-13(f) were not correlated, suggesting that the relationship between delta H-2(f) and body condition is a result of physiological processes rather than differences in dietary delta H-2. We used an isotopic basemap of delta H-2(f) 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 delta H-2(f) values obtained encompassed almost the entire breeding range of this species, indicating little ability to differentiate origins of this species.