During the wintering months, corvids typically form foraging flocks. Seabirds use the islands as resting, breeding, and wintering sites. Other nightjar species occur in parts of the breeding and wintering ranges. It is strongly migratory, with almost all of the population wintering in Sub-Saharan Africa. Willow flycatchers travel approximately each way between wintering and breeding areas. In their wintering areas, they form flocks over 10,000. These birds migrate, wintering from central Mexico to Costa Rica. Vineyards and cotton replaced the cereal crops used by the wintering geese. Other significant areas include stop-over sites between the wintering and breeding territories. A smaller estimate is 236,000 birds wintering in North America. No population is known to use the same grounds for wintering and summering. The species is a long distance migrant predominantly wintering in northern Africa. Other species have an indication of their breeding and wintering status in Great Britain. It is a complete migrant, travelling between from its breeding grounds to its wintering sites. Pair-formation takes place before the breakup of the wintering flocks.