Fifty years later, University of California biologist Robert Seib did a landmark study of snake diversity in Guatemalan coffee farms, renewing the connection. A spate of recent studies has also shown a clear link between coffee production and bird biodiversity. There are about 250 species of birds that breed mainly in the temperate region of North America and winter mainly in the tropics, including 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, for example, the Mexican conservation group Pronatura Veracruz counted 4.5 million raptors passing over a single town.
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. But all too often the home tree has been replaced by a mall or housing development in the north or a cattle pasture in the south.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 disappearance of the forest fits into a deadly mosaic that also includes nest predation by cowbirds, uncontrolled hunting and pesticides. Some are clearly in decline.