As the weather warms up and spring vegetation emerges, the air is filled with the sounds of birds 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 here. Some are common to suburbia - 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, are less familiar.

The importance of songbirds cannot be overlooked. Birds, as a whole, are our best form of natural insect control, eating tons of insects annually. As vegetation emerges each spring, so do millions of caterpillars and insects. At the same time, an array of birds, like orioles, vireos, flycatchers, warblers and swallows, return to North America and feast on the insects.

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 a leading recreational industry. Americans devote a great deal of time and money to enjoy the sights and sounds of their favorite birds.

In 1991, more than 24 million Americans took trips for the purpose of watching wild birds, and more than 63 million Americans fed birds in their backyards. That same year, $18.1 billion was spent on expenses related to birds, including memberships, optical and photographic equipment, and birding trips. In the United States, $2.5 billion is spent annually on bird seed, feeders, bird baths and nesting boxes.

Although millions of migratory birds return to their breeding areas each year, many species are declining. Habitat loss throughout North, Central and South America has had the greatest impact on their populations. In North America, large tracts of fields, forests and wetlands are disappearing as the result of development.

Many songbirds nest and breed in the forest. Today, instead of the protection of the dense forest interior, many birds are forced to 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 replace adults killed by natural causes. Those that do must survive the often-perilous winter and migrations between North and South America. The birds' winter habitats in Central and South America are also being altered and are disappearing, in some cases, faster than breeding habitats.

One way that researchers track birds is through 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 whether a species is declining or not. Though some annual changes may seem small, the effect over time can be devastating. For example, a bird species declining at a rate of 2 percent a year translates into a decline of more than 50 percent over 25 years.

Looking at the data collected in the past 29 years, researchers have documented the changes in bird populations in the Chesapeake Bay watershed. The yellow-billed cuckoo, gray catbird, least flycatcher and Baltimore oriole are declining at less than 1 percent per year. The wood thrush, whip-poor-will, Eastern wood peewee, chipping sparrow, barn swallow and scarlet tanager are declining between 1 percent and 3 percent a 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 a year; the black-and-white warbler between 4 percent and 5 percent; the Cerulean warbler by more than 4 percent; the black-throated blue warbler by more than 3 percent; and the Prothonotary warbler between 2 percent and 3 percent.

Even if you are not an avid birder, you can help protect 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 cats kill millions of birds each year. Fledglings are especially vulnerable.

Communities can provide much needed habitat by protecting existing woodlands and re-establishing trees in developed areas. Continuous strips of vegetation, known as greenways, provide safe corridors for birds to travel.

Small pockets of backyard habitat can contribute to the survival of many bird species. Landowners can reduce the amount of lawn and add native grasses, flowers, shrubs and trees to their own yards. Their seeds, berries, fruits and nectar are important sources of 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 has been designated International Migratory Bird Day. This year IMBD falls on May 11. To welcome back these long distance fliers and learn more about migratory birds, consider attending one of the following events:

  • May 11-12 - The Delmarva Birding Festival provides an opportunity to see warblers and shorebirds on Maryland's Eastern Shore and includes trips to Assateague and Deal islands and Blackwater and Eastern Neck National Wildlife Refuges. For information, call the Maryland Department of Natural Resources at (410) 974-3195.
  • May 11 - International Migratory Bird Day will be celebrated at the National Aquarium in Baltimore, Pier 3. For information, call Carrey Rowsom at (410) 576-3827.
  • May 11- International Migratory Bird Day will be celebrated at the Salisbury Zoo, Salisbury Maryland. For information, call Carrie Samison at (410) 742-2640.

For information on other IMBD events, contact the U.S. Fish and Wildlife Service, Office of Migratory Bird Management, (703) 358-1714; the National Wildlife Foundation, (202) 857-0166; or your state's Partners in Flight Program (usually found within a state's game or wildlife department).

In addition, the American Birding Association has put together a directory of volunteer opportunities for birders. For information, call the ABA at (800) 634-7736.