Seeing 50 or 60 yellow warblers on a spring day at Point Pelee was no big deal when Bob Huffman started bird-watching 10 years ago. Some days he’d see too many to count.

This spring, he’ll search all day and maybe find 10.

If it’s a good day, he might even see a couple of black-and-white warblers, little striped birds that used to be “everywhere.”

Don’t blame Huffman’s eyesight or birding skills. The devoted Livonia, Mich. naturalist, who invented a board game about bird-watching, is as sharp as ever. There just aren’t as many birds around as there used to be.

Throughout North America, bird-watchers have been saying they see fewer birds each year. But like Huffman, they’ve been going more on “vibes” than on hard data, said Sam Droege of the U.S. Fish and Wildlife Service.

Then, last November, came a study by Droege and three other researchers that confirmed bird-watchers’ intuition. Using data from the North American breeding bird survey, an annual roadside survey of U.S. and Canadian birds started in 1966, the researchers looked at increases and decreases in 62 migrant bird species in eastern North America. Between 1966 and 1978, about three-quarters of the species had increases. But from 1978 to 1987, three-quarters had significant drops.

In a continent-wide analysis, still under way, Droege’s group continues to find declines in certain groups of birds — those that prefer grassland or nest in scrubby areas, for instance.

What’s happening to the birds? Experts wag their fingers at everything from destruction of tropical rain forests and loss of family farms to suburban sprawl, cats, cars, picture windows and even backyard bird feeders.

Droege’s study, which linked declining migratory birds to destruction of the tropical forests, made headlines when it came out last fall. Most birds that breed in the forests of Canada and the United States winter in the tropics, where forests are being cleared at annual rates of at least 1 to 4 percent. Birds that can’t quickly adapt to the new landscape or find new places to live will die.

The study’s authors were careful to say that other factors, such as destruction of the birds’ breeding habitats in North America, probably contribute. But in the rush of publicity about the report, the tropical connection was overplayed, some experts now say.

True, some dramatic examples of bird extinctions can be linked directly to the loss of their winter homes. Bachman’s warbler, now believed extinct, spent its winters in Cuba. By the mid-1950s, all but 5 percent of the warbler’s forest wintering sites had been leveled to grow sugar cane, and the birds had nowhere else to go, said John Terborgh, director of Duke University’s Center for Tropical Conservation and author of “Where Have All the Birds Gone?” (Princeton University Press, $45; $14.95, paper).

But Terborgh doesn’t lay all the blame on tropical deforestation.

“It’s clearly the main cause of declines in some species … but there are many other species that have declined, some of them absolutely drastically, due to changes in the North American scene.”

One change, forest fragmentation, keeps coming up when people ponder what’s happening to the birds.

“What was once continuous forest is now broken up into patches,” said Richard Coles, a biology professor at Washington University in St. Louis. Coles has surveyed birds on a 2,000-acre wildlife refuge for the past 10 years. For seven of those years, numbers of birds and bird species have been dramatically lower than when he began.

Breaking up woodlands creates more forest edges, favorite hangouts for raccoons, house cats and other animals that prey on small birds. Fragmented forests also make patches of woods too small to support “area-sensitive” species, ones that need habitats of a certain minimum size to prosper. In a study of Connecticut woodlands converted to subdivisions, chickadees, blue jays and starlings increased as woods became suburbs. But area-sensitive wood thrushes, ovenbirds and worm-eating warblers declined.

The same thing is happening in Michigan. In parts of the state where woodlands have been preserved or cut very gradually, birds like cerulean warblers and wood thrushes are holding their own. Where subdivisions and agriculture have gobbled up forests, those species are suffering, said Ray Adams, coordinator of the Michigan Breeding Bird Atlas, a bird survey to be published next year.

Suburban sprawl also encourages pest birds like brown-headed cowbirds and blue jays, which can tolerate living around people.

Cowbirds lay their eggs in the nests of other birds, and the birds can’t tell the cowbird eggs from their own.

When a bird’s nest is taken over by cowbirds, “you find them raising more cowbirds and fewer of their own,” Coles said.

In Michigan, cowbirds practically wiped out the endangered Kirtland’s warbler before a control program rescued the species. Still, the balance is shaky.

“The endangered species program has to maintain an elaborate effort on Kirtland’s breeding grounds,” said Terborgh. “If they were to relax it for even five years, I daresay the species would go extinct.”

Blue jays, too, sabotage other bird species, not by pirating their nests, but by preying on their nestlings. As blue jay numbers have risen “spectacularly,” so has the rate of nest predation in suburban and semi-rural areas, Terborgh said. And backyard bird feeding in winter may encourage them.

Terborgh doesn’t discourage homeowners from putting out seeds and suet, but he does urge using feeders designed to let certain kinds of birds eat while keeping out jays and other pests.

Birds that nest in prairie grasslands are in trouble, too.

“We hardly have any prairie grasslands any more,” Terborgh said. “Corn and soybean are all across the Midwest.”

Glass-skinned skyscrapers, picture windows and even phone booths have devastating effects on birds that until recently were overlooked or grossly underestimated.