Stable-isotope analyses of feathers of migrant birds can help identify the locations where these birds bred or molted (i.e., breeding or molt origins). We examined stable hydrogen isotope (δD) values in feathers of American Kestrels (Falco sparverius) wintering at sites east (South Carolina) and west (Tennessee) of the Appalachian Mountains to assess their breeding origins. We used the dataset provided by Lott, and Smith (2006, Auk 123:822–835) to create a feather isotope basemap that considered error propagation associated with uncertainty in feather δD values and used the predicted mean growing-season precipitation δD of Bowen et al. (2005, Oecologia 143:337–348). We also measured feather δ18O on a subset of wintering kestrels and found that the general linear relationship found between feather δD and δ18O measurements deteriorates for feather δD more positive than −20‰. We suggest that δ18O measurements can be used to screen raptor feather isotope datasets to identify possible outliers that may unduly influence the assignment of birds to origin using feather δD basemaps. Our analyses indicated that the wintering population consisted of 13.1% ± 5.5% (SD) residents for the Tennessee site and 9.3% ± 3.5% (SD) residents for South Carolina. Migrants originated from the north-central portion of the U.S. breeding range (east of the Rocky Mountains) and southeastern Canada. This was consistent with leapfrog migration in this species.
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