Feather isotopes (62Hf) and morphometrics reveal population-specific migration patterns of the Blackpoll Warbler (Setophaga striata)

Blackpoll Warblers (Setophaga striata) have declined precipitously according to the North American Breeding Bird Survey, but that survey's coverage of the boreal breeding range is limited. Migration monitoring offers an attractive tool for additional assessment because migrants from inaccessible portions of the breeding range are included in counts. However, for site-specific trends to be combined into regional or range-wide population trends, the breeding ground origin of the migrants counted at each site must be known. Blackpolls have a loop migration pattern in which spring and fall migrants follow different paths, but very little is known about population-specific routes within North America. We used stable hydrogen isotope assays of tail feathers (52Hf, 4th rectrix) and wing-length measurements from migration monitoring sites across Canada and the northeastern United States to broadly delineate breeding/natal origins of blackpolls captured at those sites. Blackpolls captured on spring migration in southern Ontario and western Quebec had characteristics expected of birds from breeding range west of the Great Lakes. These birds travel northward from the eastern Gulf of Mexico to Canada east of the Great Lakes before turning westward to reach their final destination between northwestern Ontario and eastern portions of the Northwest Territories. Many birds sampled at Great Lakes sites prior to 2010, but not thereafter, had 52Hf and wing-length characteristics expected of breeding range in eastern Canada, suggesting differential rates of population change among regions. Estimates of migratory connectivity indicated considerable mixing of populations from different portions of the breeding range during migration. Our results both corroborate and refine the known clockwise loop migration pattern, provide new insight into spring migration routes across North America, and provide a foundation for incorporating breeding ground origins into estimations of range-wide population trends based on standardized migration counts.

Automated summary

By using stable hydrogen isotope assays and wing-length measurements, we were able to broadly delineate the breeding/natal origins of Blackpoll Warblers captured at migration monitoring sites across Canada and the northeastern United States. Our results confirmed the known clockwise loop migration pattern, provided new insights into spring migration routes in North America, and laid the foundation for incorporating breeding ground origins into estimations of range-wide population trends based on standardized migration counts.