CORRELATES OF DEUTERIUM (δD) ENRICHMENT IN THE FEATHERS OF ADULT AMERICAN KESTRELS OF KNOWN ORIGIN

Many populations of the American Kestrel (Falco sparverius) are in decline; in northern populations it is unknown whether low recruitment and rates of return are due to mortality, poor detection, or dispersal. Ratios of stable hydrogen isotopes in feathers (delta D-f) have been used widely to estimate origins of birds, in some cases providing estimates of adults' dispersal between breeding seasons and post-natal dispersal of young. We attempted to use delta D in feathers, grown during breeding, to identify returning and immigrant birds and to quantify dispersal. We used birds of known origin to establish the expected local delta D-f but found a high degree of deuterium enrichment in these individuals relative to the local delta D of rainfall (delta D-p) and a significant difference between delta D-f of nestlings and adults, which complicated the distinguishing of local birds from immigrants. We subsequently compared the delta D of primary feathers and plasma of adults and tested for relationships among the delta D of adult feathers and adult mass, body size, and reproductive effort at the time of growth to explore the cause of deuterium enrichment in adult feathers. Adults' feathers were significantly more deuterium-enriched than plasma, and their distribution did not overlap with that of nestlings' feathers. Larger males and females that fledged female nestlings of greater mass had feathers that were more deuterium-enriched, while males whose mates laid clutches of greater volume had less enriched feathers. We discuss our results with respect to the prevailing hypotheses for deuterium enrichment of raptor feathers, particularly the evaporative-cooling hypothesis.