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Stable-hydrogen isotope ratios (deuterium:protium; δD) in feathers enable researchers to evaluate patterns of avian movement and to estimate the source areas of migratory birds. However, variation in feather δD remains inadequately described, thus confounding inferences of avian movement and origin. We assessed variation within a feather and among feathers within and between tracts in three species of immature raptors. Within contour feathers, measurements of δD increased from a distal section to an adjacent, proximal section; the magnitude of δD increase varied with raptor species. Furthermore, contour and flight feathers differed systematically in their δD content. Two explanations for intrafeather and intraindividual variation warrant further investigation: (1) hydrogen isotope fractionation associated with feather growth rate, and (2) the incorporation of temporal variation in environmental δD into growing feathers. We consider these explanations for raptors and passerines, which seemingly differ in the incorporation of deuterium into feathers. Additionally, corresponding sections of multiple contour feathers exhibited better measurement repeatability than multiple sections within a contour feather; the variability of multiple δD measurements within a feather tract (geometric SD: ±3.5%) suggests that biological effects on the repeatability of δD measurements from concurrently grown feather material are difficult to distinguish from analytical effects. In most cases, intrafeather and intraindividual variation can be minimized by informed sample selection decisions, but both sources of variation must be considered when stable-hydrogen isotopes are used to infer the geographic origins of migrants, ascertain migratory connectivity, and facilitate avian conservation decisions.

INTRAFEATHER AND INTRAINDIVIDUAL VARIATION IN THE STABLE-HYDROGEN ISOTOPE (δD) CONTENT OF RAPTOR FEATHERS