Flexibility in migratory strategy contrasts with reliance on restricted staging and overwintering grounds for Sabine’s gulls from the Canadian High Arctic

Abstract Sabine’s gulls (Xema sabini) undertake the longest migration of any gull, a trans-equatorial journey between Arctic breeding and southern hemisphere wintering areas. For such long-distance migrants, quantifying within- and between-individual variation in migratory strategy is key towards understanding resilience to environmental variability encountered over migration. We tracked 22 birds on 32 migrations from the Canadian Arctic to evaluate strategies and quantify flexibility among individuals and years. All birds undertook extended stopovers in a geographically-restricted staging area halfway through migration in the California Current System in both directions. Individuals were otherwise flexible in most aspects of migration but were repeatable in arrival date and duration of the southbound staging phase. Routes taken during southbound migration and overlap in overwintering areas were significantly larger within the same year than among years. Overall, birds showed high individual flexibility in migratory strategies but made similar decisions to one another in the same years. Every year, all birds showed repeatable, consistent reliance on the staging grounds as a key stopover site in both directions. This suggests Sabine’s gulls adjust to environmental change in many aspects of their migration but may be vulnerable to climate change and other anthropogenic influences during critical stages of the journey.