High individual consistency in migratory strategies of American Herring Gulls (Larus smithsonianus) across multiple years

Migration is a fundamental component of the annual cycle of many bird species, largely driven by the need to access resources with seasonal availability. Birds employ diverse migratory strategies, undertaking short to long-distance migrations, presumably balancing energetic costs and risks with benefits of more efficiently meeting resource needs. American Herring Gulls (Larus smithsonianus) show high variation in migratory strategies among regions and individuals, including partial migration within some populations. However, the extent to which migratory decisions of individuals are flexible or stable over time remains poorly understood. In this study, we tracked American Herring Gulls from five breeding colonies in Nova Scotia, Canada, over 2–5 years and used spatial and temporal metrics to examine repeatability in their migratory decisions. We expected to find significant variation among individuals in migratory decisions at the regional and colony level, with some individuals showing generalist migratory strategies (i.e., low inter-annual repeatability in spatiotemporal metrics) and others showing specialized migratory strategies (i.e., high repeatability from fixed migratory decisions among years). As expected, we found high variability among individuals, with some birds remaining relatively local in Nova Scotia and others overwintering in regions of the United States ranging from Maine to South Carolina. Contrary to our expectations, we found that individual migratory decisions were highly consistent across years, in duration and distance travelled during migration and overwintering, as well as in migratory routes and areas used during overwintering. Our findings suggest that American Herring Gulls may rely on learned migratory strategies rather than adjusting dynamically to changing environmental conditions. This carries implications not only for understanding migratory decisions, site fidelity, and adaptability to environmental changes or anthropogenic pressures, but also in terms of relative exposure to risks in a species that typically exhibits a high degree of behavioural plasticity.