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Hydrogen stable-isotope analysis of feathers is an increasingly popular method for estimating the origins of migrating and wintering birds. Use of this method requires that investigators know which feathers are grown on breeding grounds and how the hydrogen stable-isotope ratios of feathers ([delta][D.sub.f]) relate to those of local precipitation ([delta][D.sub.p]). In this study, we measured [delta][D.sub.f] of adult (primaries 1, 3, and 10) and nestling Cooper's Hawks (Accipiter cooperii) in Wisconsin, North Dakota, and British Columbia, Canada. As previously shown, [delta][D.sub.f] of nestling feathers were related to [delta][D.sub.p]. In contrast, the [delta][D.sub.f] of adult feathers grown on the breeding grounds were substantially greater than those of their nestlings, and varied significantly across primary feathers and study areas. Our findings suggest that it is not possible to use hydrogen stable-isotope analysis of feathers to learn the origins of migrating adult Cooper's Hawks (or possibly adults of other large-bodied species with extended molting periods) until more is learned about the physiological or ecological mechanisms underlying these isotopic discrepancies. Key words: Accipiter cooperii, migration, molt, raptor.