Numbers of Migratory Birds in Sharp Decline : Environment: Suburban territories are encroaching on feeding grounds. Shorebirds and songbirds have been hit particularly hard.

Shorebirds that migrate 7,000 miles between their winter homes in Argentina and their Arctic nesting grounds make a pit stop every spring on New Jersey beaches to feast on horseshoe crab eggs.

New Jersey also provides “safe houses” between wintering and nesting grounds for many other types of migrating birds, including hawks that follow the coastline, the Delaware River and Appalachian ridges and tiny warblers looking for food in woodlands and coastal thickets.

Naturalists have noticed alarming declines in the feathered wayfarers. The number of sanderling, a species of migrating shorebird, has dropped 50% in a decade. Sharpshin hawk populations have fallen dramatically since the New Jersey Audubon Society started counting in 1976, and songbirds are disappearing.

New Jersey, the most densely populated state, is crossed by several major bird routes, and birds are colliding with a human exodus to rural and shore areas. Experts say the disappearing migrants are not only a loss for the Garden State, but should serve as an early warning for the entire nation.

Audubon ornithologists blame the drop on increased development of rural and coastal areas, which often eliminates bird habitat and erects such hazards as airports, highways and plate-glass windows (which birds often fly into).

“The thing that humanity really has to do is ask itself a question: Are birds important enough to want to protect?” said Pete Dunne, a veteran birdwatcher. “If the answer is yes, then we’d better get with it, because it’s going to be a long road back.”

Migrants are hard to protect because they are vulnerable at so many points. Such shorebirds as the red knot and the sanderling could be wiped out by damage to their nesting sites high above the Arctic Circle or to their wintering range.

But they are at special risk during their long journey, when they cluster at refuges like the Delaware Bay, where 80% of North America’s sanderling and 40% of the red knot gather in mid-May. The birds could not survive the grueling flight without a chance to rest.

“Each link in the migratory path offers them something,” said Kathleen Clark, a zoologist with New Jersey’s Division of Endangered and Non-Game Species. “Delaware Bay is critical because the food supply is there in such abundance.”

At Cape May, an average of 36,000 sharpshin hawks have been counted each year since 1976, said Paul Kerlinger, director of the Audubon Society’s Cape May observatory. But the 1988 count was 19,000 and last year’s was just above 10,000.

Kerlinger said the number of sharpshins counted at some inland observation points has also gone down.

No one knows why sharpshin hawks are disappearing or even if the decline is permanent or temporary. One hypothesis is that the songbirds they depend on for food are dying off, Kerlinger said. Another factor could be plate-glass windows, which appear to be a hawk-killer second only to hunting.

Sidney Gauthreaux, a Clemson University zoologist, uses old National Weather Service radar records to track songbird migrations across the Gulf of Mexico from Central America to Louisiana. He estimates that the number of birds making the trip has declined 50% in the last 35 years.

Songbirds, which live mostly in deep woods, have been hit by deforestation in the tropics and in the United States. On the U.S. Gulf Coast, strips of woodland along beach areas provide resting places for migrants before and after the flight to Central America, Gauthreaux said, but those “little refuges” are now being cut.

Increasing suburbanization of New Jersey’s rural areas, with office parks and landscaped housing developments replacing forests, provides ideal habitat for some wild species. Deer, raccoons, Canada geese and cow birds have become pests. Both raccoons, which eat eggs, and cow birds, which use other species’ nests for their own eggs, play a part in killing off songbirds.

Chandler Robbins, a wildlife biologist at the Patuxent National Wildlife Refuge in Maryland, observed that many species are doing just fine, spreading into new territories. Cardinals have begun nesting in Massachusetts, while a new species of cow bird is penetrating Southern states after moving from South America to Puerto Rico, where it is crowding out native species.

But thrushes, vireos and many types of warblers are down.

To protect migrants, the Audubon Society suggests:

* Leave rural and shore property as natural as possible, protecting native plants that provide food for birds.

* Limit or ban spraying for gypsy moths and mosquitoes. In addition to killing pests, insecticides wipe out bugs that some birds eat.

* Cluster housing to preserve woodlands and limit development near the shore.

* Keep cats, which can be efficient bird killers, indoors. Prevent dogs from running free.

* Design man-made hazards to accommodate birds. Plate-glass windows can be angled downwards so that they do not reflect trees and sky. Red warning lights on broadcast towers and skyscrapers attract birds and might be replaced with white strobe lights, although that may aggravate humans.