Now these trees are being wiped out. Growers, encouraged by strong demand for coffee and by financial help from the U.S. government, are cutting down the overhanging trees and converting their land to densely planted ``sun plantations.'' Since 1978, forests and fruit groves have been removed from nearly half the region's coffee-growing land.
As the shade trees have fallen, so too have the U.S. populations of these migrant species. Some of that decline is linked to the loss of forest and grassland in the United States. But without the fruit, insects and shelter provided by the winter resting grounds, some migratory birds die, and others lack the energy to produce young in the spring.
About one-third of the wood thrush population has vanished since 1966, according to the National Biological Service. The Baltimore oriole's decline has been even faster; the population has been cut by one-fifth in the last decade.
Russell Greenberg, director of the Smithsonian migratory center, thinks there is a direct link between the loss of the coffee plantations' shade trees and the birds' decline.
Scientific studies have found as many as 150 bird species in traditional coffee plantations, second only to the abundance found in virgin tropical forests. That number was reduced by half in the sunny plantations of Guatemala, Greenberg found, and some high-tech fields were ``almost devoid of birds.''
``More and more, what kind of coffee you drink makes a statement about who you are,'' Greenberg said. ``So we want people, when they ask for their whole-bean French roast top shelf coffee, to say, `But I also want it to be shade-grown.' ''
In fact, Americans' growing taste for fine coffee may benefit the conservationists' cause. Gourmet coffee now accounts for about one-sixth of the 2.4 billion pounds sold each year in this country.
Ted Lingle, executive director of the Specialty Coffee Association of America, said that most of the Central American gourmet coffees probably come from shade-grown plantations, even though they're not labeled that way. Most grocery-store mass-market brands are from sun plantations, because the price is lower and the supply is bigger.
The songbirds are not likely to go extinct in our lifetime. Although exact numbers aren't known, these birds' North American flocks probably number in the millions, said a Cornell University ornithologist, Ken Rosenberg.
But, given the steady decline across all parts of the country of birds like the wood thrush, scientists say it is important to conserve their habitats in the United States and abroad.
Right now there is no such thing as coffee with a shade-grown label on the U.S. market, though some coffee boutiques carry coffees labeled ``bird-friendly.'' The new Sustainable Coffee Coalition, an unlikely assembly of wildlife scientists, coffee companies and activists, is working to develop a shade-labeling program.
Larger coffee companies say they need to learn more about the issue before making any changes.
Jeanne McKay, a spokeswoman for Starbucks, the largest specialty coffee chain, said the company knows of no reliable source for shade-grown coffee. Although the Seattle-based company has signed a pledge of environmental responsibility, it declines to sell organic coffee, which experts say is almost always shade-grown.
``We don't want to confuse our customers'' with too many choices, McKay said. ``We're very concerned about the land in coffee-producing countries, but our first and foremost concern has to be coffee quality.''