I usually walk my dog in Homer Park early in the morning just as the sun is breaking over the trees along the river. Traffic is light at that hour, and the park is big enough to mute the noise of what little there is.

This time of year, especially when the wind is from the north, the sounds that break the comparative silence are dry, almost insectlike chipping noises emanating from high overhead. These are the sounds of migrating songbirds passing through Chicago on their way to a winter in the tropics.

They have been flying all night–apparently using the calls to maintain contact with each other–and in the early morning they are looking for a place to feed and rest.

September migrants are mostly insect eaters like vireos, thrushes, and especially wood warblers, the tiny, brightly colored creatures that Roger Tory Peterson calls “the butterflies of the bird world.”

Close to 40 species of warblers pass through the Chicago area every year. In spring, they are a birder’s delight. The males in their breeding plumage are all intense colors and bold patterns, a snap to identify. By fall, the males have molted into their muted winter plumage, and the females and young are even more obscurely decorated. Peterson says it takes five years of fieldwork to master the fall warblers, and even then you are bound to miss a few. Leg color is a key field mark for a couple of species. But detecting the color of the legs on a four-inch bird–especially a four-inch bird flitting through the leaves 30 feet up in a tree–is a job for Superman.

Some of the tiny warblers passing overhead this month won’t stop flying until they reach Peru, but most species winter in southem Mexico, Central America, and the West Indies. A quick look at a map will show you that the birds must be rather crowded on their wintering grounds. North America is roughly funnel-shaped, with the wide end of the funnel to the north and the narrow end at the isthmus of Panama. In winter, birds whose nesting grounds stretch from the tree line in the north to Texas and Florida in the south, from Newfoundland in the east to Alaska in the west, are packed into a few islands and the relatively tiny area between Veracruz and the isthmus.

This crowding means that small changes in habitat on the wintering grounds can have large effects on populations. Clearing one acre of forest in Guatemala or Jamaica may condemn as many of our woodland birds as clearing ten acres here.

If you have been paying any attention at all to world affairs, you know that very large changes are under way in the tropics. Forests are being cut at a furious rate to make room for everything from coca fields to resort hotels. Thanks to a long-term survey of North American breeding birds, jointly sponsored by the U.S. Fish and Wildlife Service and its Canadian counterpart, we now have strong evidence that deforestation in the tropics is significantly reducing populations of birds that breed here and winter there, although the evidence also suggests that winter habitats are less crucial than breeding habitats.

The North American Breeding Bird Survey was started in 1966 by Dr. Chandler Robbins, who was then chief of the Migratory Nongame Bird Studies Section of the Bureau of Sport Fisheries and Wildlife, which is a part of the U.S. Fish and Wildlife Service. He has since retired from that position, but he still works as a wildlife biologist at the Fish and Wildlife Service’s Patuxent (Maryland) Wildlife Research Center, and he still administers the U.S. half of the breeding bird survey. Robbins is widely known among birders for, among other achievements, serving as principal author of the Golden Books guide Birds of North America, the first field guide to combine eastern and western birds in a single volume.

The breeding bird survey is carried out by volunteers, active birders who donate their time to the cause. Robbins picks a coordinator for each state (or, in Canada, each province), and the coordinators recruit as many skilled birders as possible. The coordinators do their recruiting through the informal network that birders maintain, a network that recognizes skill and experience and ignores formal credentials. Without the help of thousands of volunteers, the survey would collapse, since no government, state or federal, could afford to hire enough professionals to do the job.

Each volunteer covers one or more randomly located roadside routes. The procedure is to drive 50 miles, stopping every half-mile for three minutes and recording all birds seen or heard at each stop. Nearly 3,000 routes have been laid out, and 2,000 are covered each year, providing a look at every part of the two countries accessible by road.

Because this method misses a lot of birds–it undersamples places that aren’t near roads–it can’t be used to estimate total populations. But it does an effective job of recording changes in populations. Between 1966 and 1978, there weren’t many changes to record. Numbers of nearly all species fluctuated from year to year, but the differences were about what one would expect in wild populations. Species would be down one year and up the next, but the long-term averages were fairly stable.

One sad exception to this pattern was the cerulean warbler, a bird of the treetops whose upper parts are a deep, rich blue. The cerulean has been declining ever since the beginning of the survey, but until recently that decline seemed to be explained by events here on the bird’s breeding ground.

The cerulean is a bird of the forest interior. It is mainly midwestern. In fact, most of its population nests within the drainage of the Ohio River. As forests have been cut, it has been steadily losing breeding habitat. Robbins has supervised some detailed studies of the species and found that even where nesting pairs are trying, they are able to rear few young. In the small patches of forest that remain, the nests are too close to the forest edge. The birds are more vulnerable both to predation and to parasitism by cowbirds.

Cowbirds are brood parasites that lay their eggs in the nests of other birds, leaving the jobs of incubation and child care to the unwitting foster parents. Birds of woodland edges, which have long experience with cowbirds, have evolved some defenses against them. Robins and catbirds, for example, often remove cowbird eggs from their nests, and yellow warblers will build a new nest atop the old one, burying the cowbird egg in the floor of the new nest. Forest-interior species, which never before had to deal with cowbirds, have no such defenses. To make matters worse, the cerulean’s long migration flight–it winters on the eastern slope of the Andes–shortens its stay on the breeding grounds, giving it time to produce only one brood per year. And its nesting time coincides with the rather brief egg-laying period of the cowbird. Birds such as robins, which make much shorter migrations, raise two or even three broods a year, only one of which is vulnerable to cowbird parasitism.

For the rest of our warblers–and for vireos and thrushes as well–the period of stability ended about 1978. That year saw the start of a population decline that continues to this day, It is a general decline, affecting species like the Kentucky warbler and the worm-eating warbler, both of which nest mainly in the southern two-thirds of the United States, as well as the bay-breasted and black-throated green warblers, which nest in the northern United States and southern Canada. It affects birds of the treetops like the Cape May warbler, and birds of the forest floor like the ovenbird, a ground-dwelling warbler.

The fact that the decline has continued for ten years seems to rule out any kind of natural fluctuation.

“A ten-year decline could be half of a 20-year cycle,” Robbins told me, “but unless somebody could connect it with sunspots–which seems unlikely–it is hard to imagine a cause for such a long cycle.”

Most tellingly, the decline has affected only the species that breed in the United States and Canada and winter in the tropics. Birds resident here and short-distance migrants that spend the winter in the southern United States have not been affected.

Every winter, Robbins leads investigative teams to the tropics to study the winter habitats of our breeding birds. His technique is to compare two blocks of similar forest, one of them lying within a large, unbroken wooded area, the other isolated, a patch of surviving forest surrounded by development.

The results of these studies do give us reason to be slightly hopeful. “Our birds are more adaptable in their winter habitat than they are here,” Robbins said. “They can get along even in small, isolated wooded areas.” This suggests that they can sustain their reduced population levels as long as scattered islands of natural land remain untouched.

The bad news is that the resident birds, the species that nest in the tropics, cannot live in the small islands of habitat. For them, a woodlot in Mexico is as marginal and unsatisfactory as a similar place in Ohio would be for a cerulean warbler. They need space; without it they will not survive.

Meanwhile, John Fitzpatrick of the Field Museum has discovered that the decline of the cerulean warbler has more causes than just loss of its nesting habitat. Beset by problems here, it is also facing big trouble in the south. Fitzpatrick has found the bird’s winter home in the Andes. It is confined to a narrow range of altitude on the eastern slope; the land is forested but above the mosquito line. It is very attractive to humans, and settlers are cutting forests at a rapid rate to plant coffee and coca. The cerulean, catching it at both ends of its migration flight, faces a grim future.