Partial Migration

Partial migration creates complex population structures in which resident and migratory individuals coexist year-round on wintering grounds. In European Blackbirds, resident Scandinavian birds migrate while British birds typically do not. Partial migration appears to be under polygenic genetic control with environmental modification. In lean winters, residents may gain a territorial advantage by holding year-round territories; in severe winters, migrants benefit from having fled to milder conditions. The balance shifts with climate change.