As temperatures warm and landscapes turn green, 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 are returning from winter retreats in Central and South America to breed. Because they migrate to the neotropical regions during the nonbreeding season they are known as neotropical migratory birds. Some of these birds are common to us — the ruby-throated hummingbird, gray catbird, purple martin, barn swallow and chimney swift. Others, such as the red-eyed vireo, scarlet tanager, wood thrush and Cape May warbler, may be familiar only to bird watchers.

The importance of migratory birds cannot be overlooked. Birds are our best natural insect control, eating tons of insects annually. As green leaves emerge each spring, so do millions of caterpillars and insects. Coinciding with this event, an array of birds: Orioles, vireos, flycatchers, warblers and swallows return to feast on them.

With the arrival of songbirds and other migratory shorebirds, waterfowl and raptors comes the emergence of a particular breed of person — the bird watcher or birder. Bird watching is becoming a leading recreational industry. Americans devote a great deal of time and money to enjoy the sights and sounds of their favorite birds.

In the report, “1991 National Economic Impact of Nonconsumptive Wildlife-Related Recreation,” which estimates the economic value of wildlife watching in the United States, it states that more than 79 million people spent $18.1 billion to watch, photograph and feed wildlife that year.

Equipment and other expenditures accounted for $10.6 billion and trip-related expenses accounted for $7.5 billion. These expenditures generated $40 billion in economic activity and supported 766,000 jobs!

Despite their importance to our environment and economy, many species are declining. Large tracts of fields, forests and wetlands are disappearing to development. Habitat loss throughout North, Central and South America has impacted neotropical bird populations the most.

Today, many birds must nest in smaller, fragmented habitats. This leaves eggs and chicks vulnerable to predation by other birds, such as blue jays and crows, and mammals, such as raccoons and cats. Fewer chicks survive to replace adults killed by natural causes, which include the perilous winter and spring migrations between North and Central and South America. Wintering habitats in Central and South America are also being altered and are disappearing, in some cases, faster than breeding habitats.

To identify the status of different bird species, researchers developed the Breeding Bird Survey, conducted since 1966. The Breeding Bird Survey documents the number of each species noted along specific survey routes. By analyzing the data, researchers can tell if a species is declining. Though some annual changes may seem small, the effects over time can be devastating. For example, a species declining at 2 percent per year translates into a more than 50 percent decline over 25 years.

Looking at the data collected from 1966-1994, researchers have documented the changes in bird populations within the Chesapeake Bay/Susquehanna River region. The yellow-billed cuckoo, gray catbird, least flycatcher, summer tanager, indigo bunting and barn swallow are some of the species declining at less than 1 percent per year. The red-headed woodpecker, Baltimore oriole, wood thrush, Eastern wood peewee, chipping sparrow and scarlet tanager are examples of birds declining between 1 percent and 3 percent per year. Field sparrows, Eastern meadowlarks and Eastern screech owls are all declining at more than 4 percent per year.

More startling is the decline in warblers. Many of these small, colorful birds, favorites of bird watchers, breed in forests and swamps. The golden-winged warbler is declining at an average rate of 7 percent per year; the black-and-white warbler 5.5 percent; the Cerulean warbler by more than 4 percent; and the black-throated blue warbler and the Prothonotary warbler by more than 3 percent.

What can you do to protect migratory birds? If you are a cat owner, don’t let your cat roam free. Americans keep an estimated 60 million cats as pets. Studies show that millions of birds are killed by cats each year. Fledglings are especially vulnerable.

If you're a coffee lover, consider buying shade-grown coffee. Coffee grown on clearcut plantations not only destroys rainforest, but also critical wintering habitat for migratory birds. For more information about shade-grown coffee, contact ECO-O.K. Certification Program, c/o Rainforest Alliance at 212- 677-1900.

Communities can provide much needed habitat by protecting existing grassland, wetlands and woodlands and replanting native grasses, shrubs and trees in developed areas. Continuous strips of vegetation, known as greenways, provide safe corridors for birds to travel.

Homeowners can reduce their lawns and add native grasses, flowers, shrubs. Seeds, berries, fruits and nectar are important food for birds. Forests, fence rows, hedges and meadows provide nesting and roosting habitat as well as protection from predators.

To help people become more aware of migratory birds, the second Saturday in May is designated as International Migratory Bird Day (IMBD). This year IMBD falls on May 10.

For information on IMBD events, see the related box on this page or contact U.S. Fish and Wildlife Service, Office of Migratory Bird Management, (703) 358-2318, the National Wildlife Foundation, (202) 857-0166 or your state's Partners in Flight Program; Delaware: Lisa Gelvin-Innvaer, Division of Fish and Wildlife, 302-653-2882; Maryland: Glenn D. Therres, Nongame and Urban Wildlife Program, 410-827-8612; Pennsylvania, Dan Brauning, State Ornithologist, 717-547-6938; Virginia: Rick Reynolds, Department of Game and Inland Fisheries, 703-248-9386; West Virginia: Kathleen Leo, Nongame Wildlife Program, 304-637-0245; and New York: Robert Miller, Wildlife Resources Center, 518-439-0198.