The Nearctic Neotropical migration system

Approximately half of all neotropical migrants winter in Mexico and the Antilles. . . . relative to the size of the breeding ground, this is a very restricted area. Populations are consequently highly compressed. (John Terborgh 1989.)

The New World migration system differs from the Eurasian-African one in several respects. First, the presence of a land bridge between North and South America and of sizeable islands in the Caribbean mean that the journey is potentially far less arduous than for the Old World migrants which must cross large areas of hostile desert or sea. The continuum of habitable ground from North through Central to South America means that there is no large geographical break between potential breeding and wintering areas, which for many species are contiguous or overlapping. Moreover, in the New World, the mountain chains run mainly north-south, rather than east-west, and thus present less of an obstacle to the passage of birds. Second, most of the North American migrants winter in Central America and the Caribbean islands, and a relatively small proportion extend in South America south of the equator. Even more than in Africa, therefore, the majority of species crowd into an area smaller than their overall breeding range. Third, most migrants from North America are forest species which winter in forest, rather than in dry savannah or scrub, adding to some of the richest bird communities on earth. For these reasons, the idea that population sizes might be limited primarily on the wintering grounds seems at first sight just as tenable for the New World as for the Old. Yet, as explained later, many of the migratory species that have declined in recent decades are thought to have done so because of changes in the breeding rather than in the wintering areas.

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