Abstract
Nearly one in five bird species is migratory, but not all individuals within a migratory species necessarily migrate: in partially migratory species, some do and some do not. Such within-species variability provides a natural experiment for investigating the mechanisms driving bird migration. Previous studies at the species level suggest that migrating provides a way to escape harsh winters, and to secure an increased access to resources, particularly important during the breeding season. Urbanization, by altering local temperatures (‘heat island’ effect) and resource availability (e.g. through garbage or garden feeders) can buffer the effects of winter harshness and modify breeding-season resource availability, potentially affecting individual migratory strategies. Here, we use ringing data from twelve North American partially migratory bird species to investigate the effects of natural environmental conditions (winter temperature, breeding season resource surplus) and urbanization on the propensity of individuals to migrate. We find strong support for the hypothesis that individuals migrate to avoid harsh winters, with, for eleven species, significantly higher probabilities of residency in areas with milder winters. We also found (significant for five species) that resource surplus in the breeding season reduces the propensity to migrate. Finally, urbanization increased the likelihood that individuals remain year-round in their ranges, avoiding to migrate away from their breeding range (four species) or their wintering areas (eight species), after controlling for climate and resources. Our results thus indicate that bird migratory strategies will respond to global change – in climate and land use – and indeed are already doing so.