Moult migration in Bullock's orioles (Icterus bullockii) confirmed by geolocators and stable isotope analysis

In contrast to the majority of migratory songbirds in North America, which moult on or near their breeding grounds, the Bullock’s oriole (Icterus bullockii) is reported to stop during fall migration to moult en route to the wintering grounds. These birds seem to take advantage of food resources during the Mexican monsoon season in the Southwestern USA and Northwestern Mexico. We studied a population of Bullock’s orioles at the northern limit of their breeding range in Kamloops, British Columbia, Canada, using a combination of light-level geolocators and stable hydrogen isotope analysis. We found evidence that supports the existence of moult migration in this species, with geolocators indicating that all birds appeared to stay in the Mexican monsoon region for moult in an extended stopover period during fall migration. Feathers were isotopically enriched with deuterium compared to predicted breeding isotope values and were significantly more negative than winter-grown claws, confirming that moult occurred somewhere between the breeding and wintering grounds. Stable isotope data were consistent with complete prebasic stopover moult in adults and complete contour feather and variable tail feather moult in first-year orioles. Our results confirm that this northern population of Bullock’s orioles employs a moult migration strategy and highlight the usefulness of combining geolocator and stable isotope studies.