But gradually, unnoticed by many, the colors have become more muted, the sounds less audible.
"You don't see the massive spring migrations you did in the past," said Robbins, 71, a wildlife biologist at the Patuxent Wildlife Research Center in Laurel, Md. "There are far fewer good days for birdwatchers."
Studies published by Robbins and other scientists with the U.S. Fish and Wildlife Service, the National Zoo and Duke University during the last six months show that populations of migrating songbirds in Pennsylvania, New Jersey and the eastern United States have dropped dramatically in recent years.
David Wilcove, a senior ecologist with the Washington-based Wilderness Society, said that overall declines of 30 percent to 70 percent of many species of songbirds have been recorded during the last 40 years in New Jersey, New York, Maryland, other eastern states and Illinois.
"And these declines appear to have accelerated since the late 1970s," he said. "The springs don't look the same and they don't sound the same. Part of the soul of the spring has been lost."
Robbins and three other scientists found that between 1978 and 1987, the populations of 20 migrating songbirds that breed in the eastern United States and Canada dropped significantly while only four species increased.
During this period, northern orioles declined by 26 percent, wood thrushes by 31 percent, yellow-billed cuckoos by 37 percent, Tennessee warblers by 67 percent and bay-breasted warblers by 79 percent.
Other significant declines occurred among scarlet tanagers, Acadian flycatchers, veeries, white-eyed vireos, indigo buntings and 10 other species. The population of yellow warblers, black-and-white warblers, yellow-breasted chats and blue grosbeaks increased.
"We don't really understand why the declines have been so great since 1978," Robbins said. "But they are alarming. These birds are an indicator of what is going on in the environment. The loss of these birds means that we are losing a lot of animals as well. Ultimately this will have a major impact on people."
Wilcove said that studies have shown a loss of more than one-third of the songbirds in Greenbrook Sanctuary in Alpine, N.J., and as much as 70 percent of the songbirds in Rock Creek Park in Washington during the last 40 years.
Scientists say that the greatest losses have occurred among forest-dwelling songbirds that winter in the Caribbean and Latin America. These migratory birds account for about two-thirds of the birds within forests. They help keep insect populations under control and pollinate flowers, shrubs and trees.
"These are the birds that birdwatchers adore, the creatures who fill the woods each May," Wilcove said.
While scientists don't fully understand the decline, most blame it on the fragmentation of forests in the United States, the loss of tropical forests, increases in songbird predators, pesticides, air pollution, changed farming practices and even the increase in windows.
"On the whole it has become a more hostile world for songbirds and it is getting more hostile each year," Wilcove said.
Robbins' study, published in October in the Proceedings of the National Academy of Sciences, found that the loss of tropical forests in the Caribbean and Central America was a major reason for the declines. These forests, which
serve as the winter homes for hundreds of bird species, are being lost at a rate of between 1 percent and 3.5 percent a year as land is cleared to raise cattle and grow crops, he said.
Dramatic changes in the ecology of forests in the eastern United States also have occurred as large forest tracts have been carved up to build roads, housing developments and shopping malls.
Many nest-robbing species, such as blue jays, grackles, raccoons and oppossums thrive along the edges of fragmented forests, where they have ready access to the songbirds that live in the interior. Many migratory songbirds construct open, cuplike nests, which leave their eggs vulnerable to predators.
The population of two other songbird enemies, cats and dogs, has soared as housing developments have sprung up.
Jim Bednarz, a researcher at the Hawk Mountain Sanctuary Association in Kempton, Pa., has demonstrated how the fragmentation of forests affects the population of ovenbirds.
He found that one breeding male lives on every four acres of land in the protected 20,000-acre Hawk Mountain Sanctuary, about 100 miles northwest of Philadelphia. But when he examined five smaller forest tracts nearby, he found that those with between 40 and 200 acres had only one or two male ovenbirds and that those with fewer than 40 acres had no ovenbirds at all.
"These findings are alarming because on a 40-acre tract we would expect to find 10 male ovenbirds," he said.
Phillips Street, a past president of the Delaware Valley Ornithological Club, said that agricultural changes also have contributed to the songbird decline. The average size of the nation's farms has grown larger as small family farms have been sold to large corporations. This has reduced the number of hedgerows, where many songbirds breed, he said.
"And pesticides have probably also contributed to the decline," Street added. "DDT, which causes reproductive failure in many birds, is banned in the United States. But it is still being used in Latin American countries where songbirds winter."
Still another factor may be the increase in windowpanes, as the number of homes, shopping centers and office buildings has increased. Daniel Klem Jr., a biologist at Muhlenberg College in Allentown, estimates that between 98 million and 976 million birds are killed each year in the United States when they smash into windows.
"The majority of the birds being killed are songbirds," he said. "Birds don't perceive glass as a barrier. They hit windows in homes, high-rises, cars and even telephone booths."
What, if anything, can be done to save the brightly patterned songbirds, which give so much pleasure to tens of millions American birdwatchers and help keep insects in check?
Klem said architects should work with biologists to design windows that are less lethal to birds. He said awnings, patterned windows and nets could reduce songbird deaths.
Wilcove said that reforestation efforts should be expanded and that logging in national forests should be halted. He said that international agencies such as the World Bank should stop loaning money for the construction of roads through tropical forests and should instead offer financial incentives to preserve forests.
"The opportunity exists to halt the decline in the loss of songbirds, but it won't last forever," he said. "It is a profoundly sad experience to enter the woods on a sunny spring day and not hear the songs of warblers, vireos or thrushes. It is not merely a silent spring. It's as though spring never came."