AS the weather warms and spring vegetation emerges, the air fills with sounds almost forgotten. Birds are returning to the forests, fields and wetlands of North America. Absent for months, more than 360 species of migratory songbirds, known as neotropical migrants, have returned from winter retreats in Central and South America to breed. Some of these birds are common to suburbia - the ruby-throated hummingbird, gray catbird, purple martin and chimney swift. Others, such as the red-eyed vireo, scarlet tanager, wood thrush and Cape May warbler, may only be familiar to bird enthusiasts.

Familiar or not, many of this nation's neotropical migrants are declining. Threats to populations are many and complex, although habitat loss throughout North, Central and South America has had the greatest impact on their declines. In North America, large tracts of fields, forests and wetlands are disappearing as the result of development. Smaller parcels cannot support as many breeding birds.

Many songbirds nest and breed in the forest. Today, instead of the protection of the dense forest interior, many birds must nest in smaller forest fragments and thinner edge habitat. This leaves eggs and chicks more vulnerable to predation by other birds, such as blue jays and crows, and mammals, such as raccoons and cats. Fewer chicks survive to adulthood to replace the adults killed by natural causes. Those that do must still survive the often perilous winter and spring migrations between North and South America. The birds' wintering habitats in Central and South America are also being altered and are disappearing, in some cases, faster than breeding habitats.

Another serious threat faced by many songbirds is nest parasitism by brown-headed cowbirds. Brown-headed cowbirds once followed buffalo, feeding on undigested seeds in manure and insects stirred up by the herds. These nomadic birds left their eggs in the nests of other birds. As the country was settled, native grasslands disappeared, forests were cut and wetlands drained. This change in habitat composition allowed the cowbirds to find and parasitize more nests, expanding their range. Cowbirds invade fragmented forests, leaving their eggs in the nests of native birds. The surrogate mothers unknowingly incubate the eggs and care for the young cowbirds. Slower hatching and less aggressive, the songbird's real chicks are often neglected and die of starvation.

One of the ways that researchers are able to track neotropical birds is by the Breeding Bird Survey which has been conducted since 1966. Initially instituted by the U.S. Fish and Wildlife Service and now under the authority of the National Biological Service, the Breeding Bird Survey documents the number of each species noted along specified survey routes. By analyzing the data, researchers can establish the how much a particular species is increasing or declining per year. Though some of the annual changes may seem small, the trend over several years can be devastating. For instance a bird species declining 2 percent per year translates into a 50 percent decline in that population over 25 years.

Looking at data collected in the last 28 years, researchers have documented the changes in bird populations within the Chesapeake Bay watershed. The yellow-billed cuckoo, gray catbird, scarlet tanager, summer tanager, least flycatcher and barn swallow are declining at less than 1 percent per year. The wood thrush, whip-poor-will, Eastern wood peewee, chipping sparrow and Baltimore oriole are declining between 1 percent and 3 percent per year. Even more startling are the declines in warblers. Many of these small, colorful birds, favorites of bird watchers, breed in forests and swamps. The golden-winged warbler is declining by an average of 7 percent per year; the black-and-white warbler by more than 5 percent; the Cerulean warbler by 4 percent; and the Prothonotary and black-throated blue warblers are both declining by more than 3 percent.

The importance of songbirds cannot be overlooked. Neotropical birds are some of our best insect controllers, eating tons of insects annually. As green vegetation emerges each spring, so do millions of caterpillars and insects. Coinciding with this event, an array of birds, like orioles, vireos, flycatchers, warblers, and swallows return to North America and feast upon the abundant insects.

With the arrival of these travelers, many of them songbirds, also comes the emergence of a special breed of person - the bird watcher or birder. Bird watching is becoming a leading recreational industry. Americans devote a great amount of time and money just so that they can enjoy the sights and sounds of their favorite species. Sixty-five million Americans enjoy bird watching and bird feeding. In 1991, Americans spent $18.1 billion on memberships, optics and photographic equipment, and birding trips.

The hardest value to quantify, yet probably the most important, is how much these birds contribute to the overall quality of life. Many of this nation's most beautiful birds, both in color and song, are neotropical migrants. How dreary our landscapes would be without the beauty of the tanagers, buntings, orioles, warblers, vireos, flycatchers and thrushes. How desolate the fields and forests would be without their songs. In her book Silent Spring, Rachel Carson wrote "There was a strange stillness. The birds, for example - where had they gone? Many people spoke of them, puzzled and disturbed. The feeding stations were deserted ... It was a spring without voices." Although this book illustrated the impact of pesticides on wildlife, the loss of breeding and wintering habitat on birds may produce the same dreadful outcome - a silent spring.

Everyone can participate in activities that protect songbirds and other migratory birds. If you are a cat owner, do not let your cat roam free. Americans keep an estimated 60 million cats as pets. Studies show that hundreds of millions of birds are killed by cats each year. Fledglings are especially vulnerable.

Communities can provide much needed habitat by protecting existing woodlands or reestablishing trees in developed areas. Continuous strips of vegetation, known as greenways, provide safe corridors forbirds to travel along.

Small pockets of backyard habitat can contribute to the survival of many migratory bird species. Landowners can offset the loss of bird habitat by reducing the amount of lawn and adding native grasses, flowers, shrubs and trees to their own yard. Plants that produce seeds, berries, fruits and nectar provide important food for birds. Forests, fence rows, hedges and meadows provide nesting and roosting habitat, as well as escape routes from predators.

For those interested in doing more to conserve migratory birds, the second Saturday in May has been designated as International Migratory Bird Day. This year it is scheduled for May 13 and will continue throughout the following week. For information about International Migratory Bird Day events, contact U.S. Fish and Wildlife Service, Office of Migratory Bird Management, (703) 358-1714, National Fish and Wildlife Foundation, (202) 857-0166 or your state's Partners In Flight program (usually within the state's game and or wildlife departments).