Population connectivity and the causes and consequences of differential migration in a long-distance migratory shorebird, the western sandpiper

Predicting how changes in the environment will impact migratory populations requires an understanding of how different ecological factors shape migratory behaviour across the annual cycle and determining how events in one season might influence individuals in later seasons. My dissertation investigates the causes of variation in migratory behaviour of a long-distance migratory shorebird, the western sandpiper (Calidris mauri), and examines the potential consequences of this variation for breeding performance. Western sandpipers are partially latitudinally segregated during the non-breeding season by sex, morphology, and reproductive life history strategy: females, large birds, and individuals choosing to delay breeding by a year tend to migrate further south. Genetic population differentiation has been proposed as a mechanism maintaining latitudinal variation in morphology and life history strategy. By estimating population connectivity, I found that wintering populations mix substantially during the breeding season and are thus unlikely to be genetically distinct, suggesting trait-dependent differential migration is a more likely alternative to explain latitudinal patterns during the non-breeding season. Western sandpipers are a strongly bill-dimorphic species (female bills are longer), and niche differentiation according to bill length on the non-breeding grounds has been proposed as a mechanism promoting differential migration by bill morphology and therefore sex. I found little evidence to suggest that diet, as inferred from stable isotope analysis, varies with bill length, either within sites or across latitudes. I related variation in migration behaviour to potential reproductive consequences by investigating whether variation in migration distance from non-breeding areas or winter habitat carry over to influence the timing of breeding, a key determinant of reproductive success. I found little indication that either aspect of the non-breeding season covaried with the timing of clutch initiation. My findings have significant implications for conservation management, since population connectivity estimates across the species’ range can now be incorporated in demographic models. The results challenge several earlier hypotheses explaining differential migratory behaviour of western sandpipers. Future work should focus on testing alternative explanations and on investigating whether other events outside of the breeding season affect reproductive success.