by Joe Eaton
Thirty-six years have passed since the publication of Silent Spring, Rachel Carson's landmark study of pesticide impacts. Thanks in part to action inspired by her work, spring is not yet silent. But it may be quieter than it once was; some voices may be absent from the chorus. Even if you are only a casual observer of birds, you may have missed the wood thrushes that used to sing at dusk in the woodlot down the road, or the warblers that once crowded the trees in the neighborhood park. But how can you tell if these losses are strictly local or part of a larger pattern of decline? And if the decline is real, what's causing it?
Part of the answer may lie in the 32 years' worth of data collected by the Breeding Bird Survey (BBS). Launched in 1966 by US Fish and Wildlife Service ornithologist Chandler Robbins, the BBS has expanded from an initial 600 survey routes to more than 3700 throughout the US and Canada; northern Mexico may be included soon. Survey methodology is straightforward. Participants, either professional biologists or amateur birders, travel a 25-mile route early on a May or June morning, stopping every half mile to scan and listen for birds for a three-minute period. Unlike atlasers (see sidebar), BBS census-takers do not look for evidence of nesting; they simply record a species as present. Completed tally sheets go to the Patuxent Environmental Science Center in Maryland for compilation and analysis. The BBS web site <http://www.mbr.nbs.gov/bbs/> provides access to trend data by species or group of species, geographic area, or habitat.
Some cautions are necessary in looking at BBS trends. Routes always follow roads, so birds that avoid edge environments are likely to be under-represented in the count. Nocturnal species (owls and nightjars), waterfowl, seabirds, and other groups are also undercounted; we must rely on Christmas Bird Counts and other tools for an idea of their numbers. Moreover, BBS routes are not uniformly distributed throughout the range of any given bird species, so regional differences may be distorted.
Despite the caveats, however, researchers see the survey as the best available way of keeping track of North American bird populations, particularly those of migratory songbirds, a group of special interest and concern. As ornithologist Scott Robinson puts it: "Like political polls and other sampled censuses, the BBS has known limitations, but there is no other comparable tool for monitoring trends in the numbers of any other group of living things - other than people themselves." Other projects, such as the Cornell University Laboratory of Ornithology's on-line warbler census and state and county atlasing efforts, are gradually filling in details, but the BBS remains the prime source for data that covers years and seasons with continent-wide scope.
Although the overall picture is mixed, with some bird species doing well in some regions and poorly in others, several clear trends emerge from the survey data:
- A handful of species have experienced a serious decline in numbers, including some not yet officially listed as endangered. Some, like the wood thrush, eastern wood-pewee, Kentucky warbler, and cerulean warbler, are forest-nesting neotropical migrants - birds that spend their breeding seasons in temperate woodlands and their winters in a variety of habitats in Central and South America and the West Indies. Chandler Robbins' studies indicate that forest-wintering species are faring worst. Wood thrush numbers, for instance, have dropped an average of 1 to 3 percent each year over the first 30 years of BBS data. Bobolinks and other grassland birds show comparably alarming patterns, as do a few others with broader habitat requirements, such as the loggerhead shrike. All told, 23 migrant species are declining over most of their US breeding range.
- In some parts of the eastern US, notably the Adirondacks and the Smokies, most migratory songbirds are declining. Western populations of neotropical migrants seem more stable than eastern populations, although their total numbers are smaller.
- Some birds that nest by preference in early successional habitat - regrowing clearcuts, abandoned farm fields - may also be in trouble. These include the eastern towhee and the golden-winged and prairie warblers.
- Looking at the North American continent as a whole, it's hard to generalize about neotropical migrants. Some members of this "guild" are holding steady or even increasing their population, while others diminish in numbers. Upward trends have been recorded for the western kingbird, blue grosbeak, and cliff swallow, among others.
When it comes to possible causes of these observed trends, we have enough suspects to fill an Orient Express parlor car. Here are the most frequently cited factors.
Loss of nesting habitat: Both forest- and grassland-nesting birds have suffered directly from shrinkage of suitable habitat. Changing agricultural practices, notably the conversion of pastureland to row crops and a change to earlier hay harvests, have hurt grassland birds that had adapted to farm environments. Some human activities have had secondary effects: birds partial to old-field habitat seem to have flourished when New England and Appalachian farms were taken out of production, then slumped as weedy early-successional growth was replaced by forests. Fire suppression and flood control have also worked against these early-successional specialists.
Fragmentation of nesting habitat: As forests are broken up by roads, power lines and other developed areas, not enough habitat may be left to meet minimum requirements of territory size. The "edge effect" comes into play here as well: forest birds nesting near a road or field are more vulnerable to predators and nest-robbers - jays, raccoons, snakes - and to parasitization by cowbirds (see sidebar). Fragmentation of riparian habitat in the Southwest by overgrazing has pushed the least Bell's vireo, a frequent cowbird victim, to the edge of extinction. Studies of grassland birds suggest similar risk factors.
Loss of wintering habitat: The destruction of tropical rainforest may be the single most publicized threat to migratory songbirds, and indeed some declining species depend on primary rainforest for winter habitat. But the picture is more complex than that. Other tropical habitats used by North American migrants, such as the dry and semi-deciduous forests of Mexico and Central America and the mangrove forests of South America, are even more imperiled than rainforests. And while some birds depend on a specific type of winter habitat, others are more adaptable. Some do well in shade-grown coffee and cacao plantations; others, such as the indigo bunting, in farm fields. But quality does count. According to a recent study by Peter Marra of the Smithsonian Migratory Bird Center, American redstarts wintering in prime Jamaican mangrove habitat go on to have better reproductive success than those using less desirable scrublands.
While neotropical migrants have received more attention, winter habitat loss also affects some short-distance migrants (Henslow's sparrow, sedge wren) that travel only as far as the grasslands of the southern US. With few exceptions, a species' winter range tends to be more geographically restricted than its summer range, so the loss of even small tracts of suitable wintering habitat has a disproportionate impact.
Loss of stopover habitat: Birds also need rest and refueling stops along their migration routes. The residential and commercial development of coastal areas from Texas to Florida deprives trans-Gulf migrants of a badly needed beachhead. Birds relying on beachfront habitat along the Atlantic Coast have also been affected, and the loss of riparian growth in the interior West has affected some overland migrants as well.
Cowbird parasitization: The reproductive success of some neotropical migrants has been seriously compromised by the brown-headed cowbird (see sidebar). If wood thrushes raise more cowbird chicks than thrush chicks, the thrush population will not be able to replace itself.
Pesticides and other toxics: Chemicals like DDT and DDE, banned in the US but exported to poorer nations, affect migrants on their wintering grounds. Researcher Stanley Temple discovered that dickcissels, members of a grain-eating, grassland-nesting species, were being directly poisoned by Venezuelan farmers who consider the birds' huge winter flocks a threat to rice and sorghum crops. Not just songbirds suffer; Argentine scientists found that pesticides used to control grasshoppers take a major toll on the insectivorous Swainson's hawks that winter in the Pampas.
Climate change: Not yet a major causal factor, global warming has the potential to devastate songbird populations. Those with narrow habitat requirements, like the jackpine-dwelling Kirtland's warbler and other warblers tied to Eastern montane spruce and fir forests, would bear the brunt of the disruption as their habitat trees die out.
Other mortality sources: Researchers estimate that hundreds of millions of songbirds fall prey to domestic cats each year, while as many fatally collide with broadcasting towers and high-rise buildings.
What can be done to halt these declines? Using BBS and other population data, wildlife agencies and environmental organizations are assessing the size of the problem and devising strategies for saving what they can. Since 1991 an initiative called Partners in Flight has brought together government entities and environmental groups like the National Audubon Society and the American Bird Conservatory to identify species needing close attention and to designate critical locales. These include Important Bird Areas, which could be the localized breeding habitat of a rare species or a significant stopover point for migrants, and Bird Conservation Areas, larger regions with healthy bird populations or the potential to sustain such populations. Government agencies taking part in Partners in Flight are also expected to keep migratory birds in mind when shaping their land-use policies; the US Forest Service, US Fish and Wildlife Service, and Bureau of Land Management have developed new guidlines toward this end. Partners in Flight has also fostered educational programs like One Bird - Two Habitats, in which students in North America and the tropics share information about migratory birds.
In his highly readable Where Have All the Birds Gone? (1989), John Terbrogh suggests further goals for citizen action to protect migrants. Among them: supporting environmental groups that work at the international level; advocating foreign aid tied to sustainable farming and forestry practices; opposing domestic grazing subsidies and the use of the National Forests as tree farms; promoting conservation easements and greenbelt initiatives; taking part in local bird censuses and surveys. "If we wish to prevent calamity, we must be well informed on both the status of breeding populations here in North America and the rapidly changing patterns of land use in the tropics," says Terbrogh. This is clearly not an arena for the scientists and politicians alone: the contributions of dedicated amateurs to the BBS, local atlasing projects, and other efforts can play a vital part in saving our hemisphere's migratory birds.
Featured species: cowbirds
The brown-headed cowbird is one species that Breeding Bird Survey census-takers hope not to find on their routes. This relative of the blackbirds is a brood parasite like the Old World cuckoos, dumping its eggs in the nests of other species and sometimes tossing out the host's own eggs. Female cowbirds can lay a prodigious 20-40 eggs each nesting season. Their large, aggressive young outcompete their nestmates for food, and the host's offspring usually fail to survive.
Before the colonial settlement of North America, cowbirds followed the bison to feed on insects stirred up by their hooves. They once parasitized as few as 50 host species that nested in wooded areas on the margins of the Great Plains. Many of these had evolved defenses such as ejecting the cowbird eggs from the nest or abandoning the nest to start a second brood.
But the spread of farming created vast new expanses of suitable habitat, and the nest parasites were quick to exploit it. They are now found from coast to coast and victimize at least 200 other species, and the newer victims lack innate defensive behavior. Birds nesting at the forest edge are at special risk.
In the 1960s, before control measures were begun, cowbirds laid their eggs in 70 percent of the nests of the Kirtland's warbler, a vulnerable species with a small population, narrow habitat requirements, and limited breeding and wintering ranges. Only cowbird removal has prevented the warbler's extinction. The least Bell's vireo and black-capped vireo have been hit nearly as hard. Some songbirds that show rapid declines in BBS data, such as the prairie warbler and wood thrush, also suffer high rates of cowbird parasitization. As Scott Robinson summarized an Illinois study, some wood thrushes "are doing nothing but raising cowbirds." Cowbird parasitization and loss of nesting and wintering grounds constitute a potent triple whammy for these host species.
The cowbird phenomenon is not limited to neotropical migrants. A related species, the shiny cowbird, has expanded from South America into the Caribbean and has almost wiped out the Puerto Rican yellow-shouldered blackbird.
Mapping the thing with feathers
At the other end of the scale from the Breeding Bird Survey, Breeding Bird Atlas projects provide baseline data at the state or county level. Atlasers divide their target area into survey blocks of equal size and attempt to verify every nesting species in each block. The resulting richly detailed picture of breeding bird distribution complements the broad-gauge BBS trend data. In his introduction to the published results of a county-level California atlas, W. David Shuford underlined the importance of such projects: "The fine-scale distribution data and supplemental natural history information of the Marin County Breeding Bird Atlas can be used by local conservationists as an aid to preserving and protecting our remaining valuable wildlife habitats… [A] basic understanding of the distribution, abundance, and habitat requirements of all our native fauna is essential for protecting our heritage of biological diversity."
Pioneered in Great Britain in the 1950s, atlasing was launched in North America in 1971 when Maryland's Montgomery and Howard Counties were surveyed. Subsequent projects have covered Vermont, New York, Pennsylvania, Michigan, Ontario, and the Maritimes, as well as at least a dozen counties in California.
Atlasers use more stringent criteria for counting birds than do BBS surveyers. Birds at a nest or parents feeding fledglings count more heavily than the sight or sound of a singing male. There is a whole hierarchy of behavior in between: courtship, territorial defense, carrying nesting material. Some common species such as the turkey vulture can be surprisingly hard to catch in the act.
In September of 1998 I spoke with David Fix, Jude Power, Tom Leskiw and John Hunter, who have almost completed an atlas project for Humboldt County in northwestern California. Hunter described this area as presenting a unique set of challenges: "You've got the spotted owl, the marbled murrelet, timber companies, marijuana growers - it's a volatile mix."
The survey's 200-plus participants ranged from Humboldt State University faculty and US Forest Service biologists to high school students and retirees. They tried for county-wide coverage rather than restricting themselves to known birding hotspots; as Fix put it, "Atlasing washes away the bias that birders give a landscape." The atlasers were able to census all but a few sites where landowners refused access. Most of Humboldt's timber companies, sensitive to public-relations implications in the wake of old-growth forest conflicts, proved cooperative.
Over the five years of the project, atlasers tracked the spread of the eastern barred owl into spotted owl territory, discovered several nesting species not previously known to breed in the county, and found some rarities nesting in unexpected habitat: snowy plovers on gravel beds in the Eel River, willow flycatchers in early-successional scrub - new shrubby growth that often follows land disturbances. When it is published, the Humboldt County atlas will add an important piece to the mosaic of knowledge about North American bird populations and create a valuable local benchmark for future changes.
Joe Eaton is a California natural history writer.
Silent Spring II