We addressed a United States Fish and Wildlife Service (USFWS) commitment to undertake hydrogen isotopic analyses of feathers (“Stable hydrogen isotope ratios from feathers”) collected from falconer-harvested passage peregrine falcons (Falco peregrinus) by: (1) developing a map using feathers collected from falcons of known origin, (2) linking falconer-harvested passage peregrines to natal origin, and (3) estimating the degree to which the predicted make-up of the harvest in the USFWS Final Environmental Assessment (EA) was correct (i.e., greater than 64 percent of peregrines originated from high latitudes). The USFWS coordinated the collection of 125 feathers in the United States from known-origin nestlings and unknown-origin passage peregrine falcons from 2009 to 2011. To supplement the USFWS sample, we used existing “Stable hydrogen isotope ratios from feathers” data from a collection of feathers coordinated by the Canadian Wildlife Service (CWS) from 1993 to 2002. The CWS sample included 172 known-origin feathers from throughout North America and 105 feathers from unknown-origin passage peregrine captured on migration at South Padre Island, Texas by Earthspan. Differences in analytical methodology between laboratories used by the USFWS and CWS to analyze samples yielded results that could not be compared. We, therefore, examined the USFWS and CWS samples separately using single factor ANOVA and Tukey post-hoc tests to undertake multiple comparisons of mean “Stable hydrogen isotope ratios from feathers” values for feathers collected from known-origin nestlings from three management populations (eastern, western, and northern) and unknown origin passage peregrine falcons. In general, isotope ratios of feathers from passage peregrines in both the CWS and USFWS samples differed significantly from those of feathers from populations of eastern and western peregrines. Isotope ratios of feathers from passage peregrines in both the CWS and USFWS samples were not different from those of northern peregrines. We then used a published stable hydrogen isotope-in-precipitation isoscape to regress “Stable hydrogen isotope ratios from feathers” values from the CWS sample against mean annual mean annual stable hydrogen ratio values in precipitation to develop a “Stable hydrogen isotope ratios from feathers” isoscape. We used the resulting isoscape to show that the majority (greater than 75 percent) of unknown-origin passage peregrines captured on South Padre Island likely originated from northern regions. This analysis was hampered by (1) the paucity of known-origin feathers from the northern management population in the USFWS sample (i.e., only two known–origin samples were collected from latitudes north of 49 degrees N), (2) the fact that the USFWS and CWS samples were not comparable, and (3) that our estimate is reported without an associated measure of uncertainty. Although some degree of uncertainty remains as a result of these weaknesses, we believe that the results reported here are sufficient to answer, in a preliminary way, the primary question posed in the EA, and they support the prediction that the majority of passage peregrine falcons taken by falconers in the period 2009-2011 originated from areas north of 54 degrees N.
