IN a few weeks, millions of intercontinental migrants will begin appearing in the northern latitudes. Colorfully plumed and with stout hearts, they will have come hundreds - in many cases thousands - of miles to breed and nest. And for those touched by their beauty and unique ability to leave Earth's bounds at will, these spring and summer visitors will provide considerable enjoyment and inspiration.
But populations of once-abundant birds have been declining steadily, and some now face the possibility of extinction. Amateur bird watchers have suspected as much for 10 or 20 years, but experts now confirm unofficial reports from the field.
``Increasingly now, we have the real data to back up those anecdotal observations,'' says Stan Senner, director of the National Audubon Society's migratory-bird conservation program.
According to the United States Fish and Wildlife Service, 70 percent of all neotropical bird species that summer in the eastern United States have declined over the past decade. (Between 300 and 400 species of neotropical migrants spend the winter in Central and South America, Mexico, and the Caribbean then head north to the US and Canada.)
The number of cerulean warblers has been dropping about 3 percent a year; olive-sided flycatchers, nearly 4 percent a year; yellow-billed cuckoos, 1.5 percent a year. Rose-breasted grosbeaks have declined by more than 40 percent overall; blackpoll warblers by 60 percent.
``It's hard to believe all these favorite birds could be declining,'' says Peter Stangel, director of the neotropical bird program at the National Fish and Wildlife Foundation in Washington, D.C.
The news from the Midwest is troubling as well. Last June, the Fish and Wildlife Service reported that seven of the 12 bird species considered endemic to the Great Plains grasslands have declined over the past 25 years, with ``statistically significant'' dropoffs for the mountain plover, Franklin's gull, Cassin's sparrow, and lark bunting.
And in the Western US, the northern spotted owl has become a household familiarity, joined recently in the news by the marbled murrelet and the gnatcatcher as species threatened by industrial forestry or housing development. The number of native Hawaiian bird species has dropped by half (from 140 to 70) since Polynesians first arrived in the islands. Nobody has seen a Molokai thrush since 1988.
Edward O. Wilson, a Harvard University professor of science and an expert on the decline of species, estimates that 1/5 of bird species worldwide have gone extinct. ``Thus instead of 9,040 species alive today, there probably would have been about 11,000 species if left alone,'' he writes in his 1992 book ``The Diversity of Life'' (Harvard University Press).
Professor Wilson notes a study by the International Council for Bird Preservation (now known as Bird Life International) showing that 11 percent or 1,029 of the surviving species are endangered.
There are many reasons for the decline in bird populations. The World Wildlife Fund reported last month that some 500 species of birds are being traded in Southeast Asia, many in violation of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
``The wild bird trade has been estimated to involve somewhere between two and five million specimens per year and the largely undocumented trade in Chinese song birds, mainly to East and Southeast Asian destinations, may add another one to three million birds to the total,'' states the report, titled ``Sold for a Song.''
``No one realized until now the enormity of this trade,'' says Ginette Hemley, who heads the World Wildlife Fund's trade monitoring arm. More than 600,000 live parrots are traded every year, according to the World Resources Institute, most of those to the US.
Loss of environment cited
Domestic cats (and pets that have gone wild) are estimated to kill hundreds of millions of birds each year. Over-hunting continues in some countries. Environmental disasters like the Exxon Valdez oil spill in Alaska and the Gulf war kill hundreds of thousands (perhaps millions) of birds as well.
But the main source of bird declines is the loss of habitat.
Cutting and burning rain forests in Central and South America for timber and cattle ranches, plowing up prairies to produce grain in the North American Midwest, urban sprawl and industrial development, draining wetlands, the fragmenting of forests all over the world, damming and diverting rivers, acid rain, and toxic runoff from mining and farm chemicals - all have contributed to the degradation and destruction of bird habitats.
Some 42 million acres of tropical forests are lost each year (an area about the size of Washington State). More than half of all temperate rain forests (the kind found along the Pacific coast of North America) are gone. The United States is losing some quarter-million acres of wetlands a year. (Ninety-five percent of California's Central Valley wetlands along the Pacific flyway have disappeared over the past century.)
Reporting last summer on the decline in native grassland birds, the US Fish and Wildlife Service noted the impact of plowing: ``In North Dakota, for example, less than 25 percent of the native prairie remains. In Iowa, where 30 million acres of tall-grass prairie are estimated to have existed at the beginning of the 19th century, only a small fraction remains in isolated tracts scattered throughout the state. Fire control and water management practices also altered the grasslands by making conditions favorable for the growth of shrubs and small trees.''
The loss of habitat in turn makes migratory birds more vulnerable to predators and competitors like jays, crows, and cowbirds. Neotropical migrants are particularly susceptible to forest fragmentation, since many nest near the ground.
``Migratory birds are under stress as never before, both on their North American breeding grounds and on their tropical and south temperate wintering ground,'' writes Duke University biologist John Terborgh in his collection of essays titled ``Where Have All the Birds Gone?'' (Princeton University Press).
``There are lots of danger signs,'' agrees Mr. Senner of the National Audubon Society's migratory-bird conservation program in Boulder, Colo. ``The good news is that most species are still abundant,'' he adds. ``The bad news is we're seeing declining species and populations across the continent in virtually every kind of bird.''
In recent years, efforts to protect birds and restore their habitat have been stepped up.
In 1992, the US government enacted the Wild Bird Conservation Act, which restricts the import of captured birds. The federal government also has begun to limit inappropriate activities on the nation's wildlife refuges, including military exercises, resource extraction, and some forms of recreation.
Much of the field work to restore bird habitat is being done by private conservation groups. Audubon Society volunteers are cutting down exotic species like salt cedar in the riparian areas of central California to encourage the growth of native willows and shrubs. The Nature Conservancy recently acquired 80 acres in the Cayman Islands from an anonymous donor to protect the endangered West Indian parrot.
All concerned agree that international efforts are needed, particularly since so many declining bird species are international travelers.
One such effort is ``Partners in Flight - Aves de las Americas,'' the neotropical migratory-bird conservation program set up in 1990 by the National Fish and Wildlife Foundation.
The foundation was established by Congress in 1984 as a public-private nonprofit organization. It has grown steadily since then to include 15 federal agencies, 60 state and provincial wildlife agencies, several dozen conservation groups and academic organizations, and representatives of the forest products industry.
Using government funds matched with private donations, the foundation has issued 864 conservation grants totaling more than $100 million. These have included 145 grants totaling $11.6 million to help protect neotropical birds.
Many of these projects are to survey and analyze bird populations and their habitat. Some involve habitat management and acquisition.
Others provide training for resource managers. Such projects have been funded in the United States, Canada, Mexico, Panama, Costa Rica, Belize, Venezuela, Bahamas, Puerto Rico, and Dominican Republic.
``It's far too big a problem for any one organization or for that matter any one country to undertake,'' says Mr. Stangel, who directs Partners in Flight.
Interior Secretary Bruce Babbitt cites the decline in bird populations as a prime reason to create a ``National Biological Survey.'' This would provide better data on trends and help head off what he calls ``train wrecks,'' or species that have dwindled to the point of being endangered.
``The National Biological Survey would provide exactly the kind of information we need for proactive and preventative measures,'' says the Audubon Society's Senner. His organization's annual ``Christmas Bird Count,'' which involves some 43,000 participants in the Western Hemisphere and Pacific islands, provides key data to government agencies.
``But in the end,'' he adds, ``the NBS itself won't save anything. We still need a commitment to act.''