Singing the Blues for Songbirds
Bird lovers lament as experts ponder the decline of dozens of forest species
Ten mornings every spring, Bob Askins gets up at five, jumps in his rusty Datsun and drives to the Connecticut College Arboretum, a 460-acre patch of forest in the midst of suburban New London. Once there, he heads on foot for the 70-acre plot of mature oak, red maple and hemlock that he calls his "study area."
As he walks, he listens with the experienced ear of an ornithologist for bird song--the see-me of a red-eyed vireo, the weesy-weesy-weesy of a black-and-white warbler, an ovenbird's teacher-teacher. When he hears a bird singing--"a male hanging out a No Trespassing sign," as he puts it-he carefully marks the spot on a detailed map of the forest plot.
By the end of June, Askins has what amounts to a census of the area's songbird population. He and his colleagues, Wendy Dreyer and Margarett Philbrick, are primarily interested in the forest nesters among a group of species called neotropical migrants. These creatures, most of them songbirds, make up roughly half of the 650 bird species that range in this country. Of all neotropical migrants, about half breed in North American woods or shrubs during the spring and summer, then fly for the winter to Mexico, the Caribbean and points further south.
The census, taken at least every three years since the 1950s and annually since Askins' arrival at Connecticut College in 1982, tells an eye-opening story. During the 1970s, when forests south of the arboretum were cleared for development, migrant populations in the reserve fell dramatically. As farmland and other areas to the north of it returned to woodland, the birds' populations turned around, rising almost to their levels before the development.
For Askins, the message is clear: As go the woods, so go the birds. It is a formula that could have dire implications for thousands of neotropical migrants. Like Askins, many experts are concerned that plunging populations of migratory songbirds in much of the Northeast and parts of the Midwest point to increasingly silent springs--certainly in those areas and possibly across much of the country.
Figures from the U.S. Fish and Wildlife Service's Breeding Bird Survey (BBS) seem to bear out those fears: Almost three-quarters of all forest migrants that breed in the Northeast have suffered severe population declines in the past ten years.
Even more alarming are indications that for many species this trend represents a reversal. The Breeding Bird Survey numbers show that during the previous decade at least 23 of the northeastern species now in decline had actually been on the upswing. For some birds, that about-face was especially dramatic. The bay-breasted warbler, for example, registered a 0.9 per cent increase between 1968 and 1978, then declined by 12.1 percent during the ensuing decade. Similarly, the blackpoll warbler showed a 4.4 percent increase during the earlier decade, only to drop by 5.4 percent.
Until recently, most ornithologists attributed this downward trend to massive destruction of forests in the birds' tropical wintering grounds. "That's where forests are being lost at the fastest rate," says Askins, adding that tropical deforestation still looms as a tremendous potential threat to migratory songbirds.
But a growing number of studies, including Askins' own, make it clear that the problem is not nearly so simple. Many experts now think songbird declines are due to a mosaic of factors, one of the most important of which may be a slicing up of forests right in the United States.
Forest fragmentation, as scientists call the intentional felling of woodland, is actually two processes. In populated areas such as the Atlantic seaboard, it means reduction in the size of forest tracts, usually due to suburbanization and development. In less inhabited areas--northern New England, for example--forest fragmentation refers to isolation of one patch of forest from another by logging, or by the building of roads or power lines.
Some birds, such as chickadees and titmice, thrive in and around the fragmented forests typical of a 'suburban landscape. But many, including warblers, vireos and other neotropical migrants, require large, unbroken woodlands for protection from predators lurking on forest fringes.
The forests of New Hampshire are good-weather home and breeding grounds for two species of warbler, the American redstart and the black-throated blue, both of which have been studied intensively by ornithologist Richard Holmes of Dartmouth College and colleague Thomas W. Sherry of Tulane University. Like Askins, these scientists walk the forest, calculating the songbird populations by mapping the territories of singing males.
The results of their work, soon to be published in the book Ecology and Conservation of Neotropical Migrant Land-birds, are surprising. Over the 23-year study period, a large percentage of the birds that migrated to Jamaica in the fall returned the following spring; evidently, their numbers had not been significantly thinned by tropical deforestation. Yet during the same period, the New Hampshire populations of the two warbler species showed a steady decline. This suggests that the immediate source of the problem does not lie in the destruction of the birds tropical retreats. "Loss of winter habitat in Jamaica is a potential problem for the birds," says Holmes, "but at present it doesn't seem to be the major threat."
The real problem, he says, lies with the spread of people in the birds' North American home. Clearing trees to build houses, roads and shopping malls depletes the birds' habitat, leaving smaller forest patches and thus limiting the number of birds that can successfully reproduce.
Forest fragmentation also opens up remaining adjacent woodlands to the animals that follow people--not just cats and dogs, but opossums, skunks, squirrels, chipmunks, blue jays, raccoons and even black bears. These predators then feast on the eggs and young of migrant songbirds, many of which build open nests vulnerably close to the ground.
One of the migrants' greatest enemies, at least in the Midwest, wears not fur but feathers: the brown-headed cowbird, a New World blackbird that made its way from the Great Plains to the East about a century ago. The bird congregates near farms and along forest fringes--just the sort of habitat created by fragmentation--where it perpetrates an insidious form of exploitation called egg parasitism.
When a cowbird female comes across a nest belonging to another bird, she deposits an egg or two of her own, sometimes even tossing out a few of the nest's rightful occupants. Cowbird chicks hatch early and grow quickly, running their adopted parents ragged to feed them while "legitimate" chicks often languish and die. Breaking up forests into smaller patches means more cowbirds and thus, some experts believe, a growing threat to neotropical migrant populations.
In fragmented forests in central Illinois, Scott Robinson and Cheri Trine of the Illinois Natural History Survey found an astonishing rate of cowbird parasitism. As many as four-fifths of the songbird nests they examined had been "colonized" by cowbirds. Wood thrushes were frequent victims; many nests, in fact, contained more cowbird than thrush eggs. In some midwestern areas, says Robinson, "thrushes are raising nothing but cowbirds."
The cowbird threat is less severe in areas to the east, perhaps because there are simply fewer cowbirds. Even so, spot-checks by researchers throughout the East and Northeast suggest a clear connection between forest fragmentation and declining neotropical migrant populations.
In the Washington, D.C., area, for example, naturalists have been counting birds at three small wooded parks--Rock Creek Park, Cabin John Island and Glover-Archbold Park--for 35 to 40 years. Here the story is one of obvious decline.
During the 1940s and 1950s, neotropical migrants such as the red-eyed vireo and ovenbird made up 65 to 80 percent of the total bird populations in the parks. By the mid-1970s those figures had fallen to as low as 20 percent.
Over roughly the same period (between 1950 and 1985), forests in the four adjacent Maryland counties were reduced by 30 to 40 percent as trees were cleared to make way for a burgeoning human population. Evidently, then, the fate of songbird populations in the area is linked directly to that of its forests.
In southern Connecticut, Robert Askins' findings tell essentially the same story, but with a curious twist. Between 1953 and 1976, he says, the Connecticut College Arboretum forest became increasingly isolated as woods to the south gave way to houses, a shopping center and apartment complex. Sure enough, populations of several migrant species, including the red-eyed vireo, wood thrush and scarlet tanager, took a nose dive.
Between 1975 and 1985, however, as abandoned farms to the north began to revert to their natural forest state, surrounding woodland actually increased. During that period, Askins reports, the total number of songbird species in the arboretum also grew, as did populations of the ovenbird, eastern wood-pewee, red-eyed vireo and hooded warbler.
Studies in larger, unfragmented northeastern forests tend to uphold the connection between available woodlands and songbird populations. In New Hampshire's relatively pristine Hubbard Brook Forest, for instance, populations of 8 of the 14 songbird species that nest there remained stable between 1969 and 1986, and the numbers of one species--the ovenbird--actually rose.
In large forest tracts in the Great Smoky Mountains of Tennessee and North Carolina, David Wilcove of the Environmental Defense Fund compared bird counts made in the late 1940s with others taken in the early 1980s and found "no evidence of a widespread decline in neotropical migrants." And in the heavy forests of the White Memorial Foundation, a private nature preserve in northwestern Connecticut, surveys showed that between 1965 and 1988 numbers of neotropical migrants, especially the veery, red-eyed vireo and ovenbird, climbed significantly.
When experts put these local studies together, a pattern begins to take shape: Most neotropical migrant songbird species thrive in large forests and suffer in small ones. The cutting up of larger forests into smaller ones produces "heavy local effects on songbird populations," at least in some parts of the Northeast, says Sam Droege, an ornithologist with the U.S. Fish and Wildlife Service and former coordinator of the Breeding Bird Survey.
Still, the case is far from closed. On a regional and national scale, the link between forest fragmentation and migrant songbird populations becomes fuzzier. Censuses by the BBS showed that between 1966 and 1988, overall songbird numbers in the Midwest remained relatively stable (9 of 63 species increased; 13 declined). Meanwhile, populations in the West were generally on the upswing: 12 of 48 species increased and only 2 declined.
"We're finding substantial geographic variations where the declines are occurring," says Fran James, an ornithologist at Florida State University. "It's going to take a lot more work to figure out which areas had more forest fragmentation, and whether fragmentation is really the cause where declines did occur."
Even in the Northeast, where the trend is obviously down, no region-wide estimates of forest fragmentation rates exist, says Sam Droege. He admits, "We can't put our population counts and our habitat data together and make any real statement about the effects of fragmentation."
Droege and his colleagues have spent hours poring over Forest Service aerial photos of northeastern forest tracts. As their preliminary findings suggest, he says, "environments that are currently fragmented are losing neotropical migrants at a faster rate than areas that are currently unfragmented." Still, he concedes, "we need to do a lot more work before we can stand firmly behind this."
Clearly, not all experts are satisfied with the notion that forest fragmentation is the overriding factor in declining neotropical migrant populations. To Fran James, the matter is "still an open story. There's not going to be any one cause for decline," she says, "and it's likely to be more complicated than it's usually portrayed." She thinks some of the drop may be due to forces that scientists--who, after all, are relatively new to the bird-counting game--don't yet fully understand. "Twenty-five years is not a long time to sort out these complex factors," she says.
James points out that bird populations can be affected by fluctuations in weather and by changes in the forests where the birds breed. Food supply in particular is subject to dramatic ups and downs; some studies suggest that the rapid reversal in populations of Tennessee, blackpoll and bay-breasted warblers was due to fluctuations in the numbers of budworms, the birds' favorite food.
Sam Droege agrees that much of the songbird decline in the Northeast could be due to the push-pull of factors that have little to do with human influence. "During the next ten years, the birds might all come back," he says. But, more likely, he continues, "we're going to see an exacerbation of these forest fragmentation problems. It's better to raise a yellow flag of warning than to sit around doing nothing."
In some quarters, that yellow flag has already turned into a green light for action. Two years ago, the Fish and Wildlife Service launched the Neotropical Migratory Bird Conservation Program, which includes not only such domestic agencies as the U.S. Forest Service, National Park Service and Bureau of Land Management, but also budding international partnerships with Canada, Costa Rica, St. Lucia, Venezuela and other countries that share a concern for the songbirds' fate.
The project, currently financed by Congress to the tune of more than $2.6 million, sponsors research and education efforts. Officials hope to institute national and international programs to manage and preserve the birds' wintering grounds in the tropics as well as their large-forest breeding grounds in North America.
To experts like Bob Askins, these efforts are coming none too soon. The danger isn't so much extinction, he says--at least not yet. The birds will have a place to live as long as large forest tracts in the Appalachians and northern New England remain intact. Rather, the danger is that the songbirds may disappear from huge portions of the northeastern coastal plain.
His hope for their future comes with a warning: "If we want to see these birds even within a day's drive, we're simply going to have to protect our forests."
Bill Lawren wrote this article to the accompaniment of migrant songbirds on the marsh near his New England home.