I investigated moult and migration in Bullock’s orioles (Icterus bullockii) using acombination of geolocators and stable hydrogen isotope analysis. Our results clearly demonstrate that Bullock’s orioles use a stopover site in the Mexican monsoon region to moult en route to their overwintering grounds. This migration strategy appears to be consistent across all age and sex classes of Bullock’s orioles from our population at the Northern extent of their range in Kamloops, BC. We also assessed how breeding abundance throughout the oriole’s breeding range varies with changes in primary productivity on their breeding, moulting, and overwintering grounds. Our data revealed that primary productivity on the breeding grounds, and potentially in some overwintering areas, affects subsequent season breeding abundance in several Bird Conservation Regions. This study has implications for other moult-migrant songbirds in Western North America and illustrates the importance of studying birds throughout their annual cycle.
