There are about 250 species of birds that breed mainly in the temperate region of North America and winter mainly in the tropics. These travelers include waterfowl, shorebirds, raptors and songbirds. As they funnel down through Mexico and the narrow waist of the Central American isthmus, the birds are compressed into spaces much smaller than their breeding area. In 1996, the Mexican conservation group Pronatura Veracruz, for example, counted 4.5 million raptors passing over a single town. The migration is a marvelous thing.
Scientists guess that between two and five billion birds make the annual journey.
Some take the trip slowly, stopping frequently to rest and refuel, but many go the distance without stopping. The blackpoll warbler, for example, makes an incredible leap from Alaska to the Amazon that includes a thousand miles over open water. But all too often the home tree has been replaced by a mall or housing development in the north or a cattle pasture in the south. The status of these birds is much studied and debated. Some are clearly in decline.