This species is partly migratory, northernmost populations deserting their breeding areas in winter. Birds winter in Europe and Asia south of the breeding range. Birds in northern Fennoscandia and Russia vacate their territories between late August and early November, with most leaving in late September to mid-October as the first cold weather arrives. Adverse conditions may lead to disorientation, large numbers gathering on ships on overcast or wet nights. Large influxes include 15,000 birds on the Isle of May in October 1982, and nearly 21,000 birds through a single site in Latvia during September and October 1983. Spring migration is complete by late March on the Mediterranean islands, but continues to late April or early May in northern Europe. The spring passage is much lighter than in autumn, suggesting high mortality on migration.