High in a California live oak, a Wilson's warbler - a small, yellow-green songbird with a glistening black cap - engages in a ritual of spring. The warbler belts out his sharp, staccato song. "This is my territory," he is saying, beckoning a female to join his leafy kingdom.
With equal ritual, Julian Wood, a young biologist, carefully notes the call and its location. Around him, among the scrub brushes and stands of oak and pine on this bluff above the Pacific Ocean, stakes carefully demarcate sections in which scientists keep count of every breeding pair of birds.
For 30 years, scientists here at the Palomarin field station of the Point Reyes Bird Observatory have been counting and studying the breeding birds.
But since the 1980s, scientists here, as elsewhere, have grown alarmed at what their studies are showing - widespread and significant declines in the numbers of neotropical migratory birds. These birds winter in the tropics of Central and South America and breed in the northern part of the hemisphere.
Hardest hit are songbirds - sparrows, warblers, orioles, and other denizens of river basins, grasslands, and forests. Often invisible to the eye, their lilting burbles fill the early morning air.
"A half a dozen years ago, when news of the declines hit the scientific world and the public, people were aghast," says Peter Stangel of the National Fish and Wildlife Foundation. "A lot of birdwatchers knew something was wrong, but they did not know the scale of the problem."
The numbers can be dramatic. Of 30 songbirds that breed at the Palomarin field station, 17 species are in significant decline. The Wilson's warbler population has dropped by almost 10 percent each year over the past 15 years, while farther inland, in California's Central Valley, the species has virtually stopped nesting.
"These birds are getting hammered," says Daniel Evans, executive director of the Point Reyes Bird Observatory. "Migration as we know it is going to disappear in our lifetime."
But scientists debate both the scope and the causes of the problem. While the specifics may vary depending on the species, most point to habitat loss at both ends of the migratory path due to development as a major reason for the declines. Northern forests have been fragmented by the sprawl of suburban development, while tropical rain forests have been wiped away to make room for cattle ranches in Mexico and Central America.
In the breeding grounds, changes in habitat have most severely affected birds that nest in grasslands and scrub, says biologist Sam Droege. In the West and Midwest, grassland species such as sparrows, meadowlarks, and bobolinks have been severely affected by farming techniques including the use of herbicides and pesticides.
In the East, birds that nest low to the ground in young trees and scattered bushes, such as yellow-breasted chats and white-eyed vireos, are vulnerable to a proliferation of predators, including house cats, raccoons, and possums.
Forest fragmentation, particularly in the Eastern United States, has contributed to the growth of another threat - the brown-headed cowbird. The cowbird acts as a parasite, laying its own eggs in the nests of other birds, and because most birds nest near the forest edge, forest fragmentation has made their nests more accessible. The result: The cowbird now affects more than 200 species and is considered responsible for the near extinction of the Kirtland's warbler.
Even when habitat is preserved in the US, the birds can face equally severe threats in their wintering grounds from habitat loss, mainly in tropical forests. In the mountains of the northern Andes, the wintering grounds of the the blue and white cerulean warbler have been largely deforested, contributing to the rapid decline of that bird, biologists report.
"Very few bird species can tolerate this gantlet of problems," says Mr. Droege, the biologist. "When people move in, birds, with few exceptions, have to move out."
And so they have.
Plenty of scientific evidence exists to support the gravity of the situation. Much of it comes from records and information gathered nationwide each spring in a massive, volunteer-staffed effort first organized by the US Fish and Wildlife Service in 1965, called the Breeding Bird Survey (BBS).
Using the BBS and other monitoring data, conservationists have placed 35 of 157 North American neotropical migrants on a "watch list" of birds at risk. Another revealing study, looking at high-resolution radar images of the nocturnal clouds of migratory songbirds along the Gulf Coast, showed a 50 percent decline in the size of the flocks during the past two decades.
Some scientists have written of a new "silent spring," a reference to Rachel Carson's famous clarion call for action in the early 1960s against the deadly effect of pesticides. But for the most part, these birds are still present in large numbers, well beyond the levels that would put them on federal lists of threatened or endangered species.
"I have a hard time using the word crisis," says Russell Greenberg, director of the Smithsonian Migratory Bird Center. Instead there is a "persistent, gradual erosion," he explains.
Scientists don't agree on the scale of the problem, partly because data collection has been sketchy. For example, scientists do not know where many of the North American birds go in the winter.
But almost all scientists concur that migratory bird populations are in substantial, and in some cases, drastic decline.
Mr. Evans compares the problem to that of global warming. "We don't have the definitive answer, but we know we have a problem."