All populations are migratory and winter mainly at sea, usually in estuaries and bays, but often well offshore where fish are within diving reach near shallow banks or islands. The preferred passage and wintering habitat is water less than deep with a sand or gravel bottom, scattered rocks and patches of seaweed. During winter, birds typically feed alone and rarely aggregate into flocks, but on migration, concentrations of over 2000 individuals may occur at favoured staging sites. Migration is usually at night, but may occur during the day, especially when over water. This is particularly noticeable in autumn on the Great Lakes, when up to 18,000 birds may pass Whitefish Point on Lake Superior; these are thought to be Canadian breeders heading for the Atlantic Ocean to winter. This easterly route is longer than that to the Pacific, but avoids the Rockies.