ORIGINS OF AMERICAN KESTRELS WINTERING AT TWO SOUTHERN US SITES: AN INVESTIGATION USING STABLE-ISOTOPE (δD, δ18O) METHODS

Stable-isotope analyses of feathers of migrant birds can be help identify the locations where these birds bred or molted (i.e., breeding or molt origins). We examined stable hydrogen isotope (delta D) values in feathers of American Kestrels (Falco sparverius) wintering at sites east (South Carolina) and west (Tennesee) 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 base map that considered error propagation associated with uncertainty in feather delta D values and used the predicted mean growing-season precipitation delta D of Bowen et al. (2005, Oecologia 143:337-348). We also measured feather delta O-18 on a subset of wintering kestrels and found that the general linear relationship found between feather delta D and delta O-18 measurements deteriorates for feather delta D more positive than -20 parts per thousand. We suggest that delta O-18 measurements can be used to screen raptor feather isotope datasets to identify possible outliers 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.