The Decline of Eastern Songbirds
photograph of Brown-headed Cowbirds (top) by Rohan Kamath
photograph of an American Crow (bottom) by Tom Grey
the end of World War II there has been a decline in forest
songbird populations over much of the eastern United States.
For example, in Rock Creek Park in the middle of Washington,
D.C., populations of Red-eyed Vireos have dropped by 79
percent and Ovenbirds by 94 percent. Acadian Flycatchers,
Yellow-throated Vireos, Black-and-white Warblers, and Hooded
Warblers have disappeared entirely. The decline has not been
uniform for all species; the Acadian Flycatcher and others
that migrate long distances to tropical America have
suffered more than residents or those like robins and
towhees that can overwinter in the southern United States.
Nor has the decline been equal in all types of forest; the
loss of species from woodlots and small forest tracts
exceeds the loss from large stretches of forest such as
those of the Great Smoky Mountains National Park.
One suspected cause is, quite naturally, the rapid destruction of tropical forests where many migrants overwinter. Perhaps deforestation in Mexico, Costa Rica, and Jamaica, for instance, is responsible for the decline of some species, such as the Worm-eating Warbler. But in the last century about half of the forest breeding habitat of that species in eastern North America was destroyed, while there was relatively much less loss of tropical forests in that period. The result may well have been a surplus of wintering habitat. More recent deforestation has wiped out on the order of half of the tropical forests, and perhaps has just about restored the balance between available breeding and wintering habitat.
Other possible explanations
of declines in eastern migratory songbirds have to do with
changes within North America. They include increased cowbird
parasitism, loss and fragmentation of habitat, and increased
nest predation in habitat patches. When Christopher Columbus
landed, cowbirds are thought to have been largely confined
to open country west of the Mississippi, because the
continuous forests of the eastern United States did not
provide suitable habitat for their ground feeding or social
displays. As the forests were cleared, cowbirds extended
their range, occupying most of the East but remaining rare
until this century. Then increased winter food supply,
especially the rising abundance of waste grain in southern
rice fields, created a cowbird population explosion. The
forest-dwelling tropical migrants -- especially vireos,
warblers, tanagers, thrushes, and flycatchers -- have proven
very vulnerable to cowbird parasitism. And that
vulnerability is highest for those birds nesting near the
edge of wooded habitat and thus closest to the open country
preferred by the cowbirds.
This provides one explanation for the much sharper decline of songbirds in forest fragments than in large areas of continuous forest: nest sites in a forest fragment are on average closer to open land than those in continuous forest because there is more "edge" per unit area. In addition, there is evidence that fragmentation per se, with both reduction of total habitat area and increased isolation of habitat remnants, has strong negative effects on forest-dwelling long-range migrants that need forest habitat, while often favoring short-range migrants and residents that do not have such strict habitat requirements.
Ecologist David Wilcove of the Wilderness Society has tested the nest predation hypothesis experimentally by putting quail eggs in straw-colored wicker baskets either on or above the ground, and placing large numbers of such pseudonests in forest patches of various sizes. He also constructed some artificial cavity nests to compare with the artificial cup nests. Wilcove found that predation was heavier in suburban woodlots (70 percent) than in rural woodlots (48 percent), and much lower in large continuous forests than in smaller fragments. In the Great Smoky Mountains, eggs in only 2 percent of the experimental nests were destroyed. Several of the species that are most sensitive to forest fragmentation, such as the Ovenbird and Black-and-white Warbler, nest on or near the ground, and most of the migrants make cup nests rather than nesting in cavities. Predation on cup nests was much higher than on the cavity nests, and more ground nests were destroyed than those placed above ground level.
The pseudonests were more conspicuous than normal nests, so Wilcove's experiments cannot be used to determine actual predation rates, but they do strongly indicate that higher levels of nest predation are at least a partial explanation for the decline of migrant songbirds in forest fragments. Again, the increased proportion of forest edge in fragments is implicated; many important nest predators, especially the Blue Jay, American Crow, and Common Grackle, are most common along woodland borders. In addition jays and crows have benefited greatly from other human-induced changes in the landscape, such as increased suburbanization. High losses of songbird eggs in suburban areas are doubtless due to the abundance there not just of nest-robbing birds, but of dogs, cats, rats, raccoons, and gray squirrels as well.
So it looks as if many factors may be contributing to the decline of songbirds. Thus, sadly, the prognosis is grim. Ornithologists think that cowbirds are likely to continue to increase, and the now-thriving nest predators are unlikely to decline. The loss of songbirds might be halted if conservation depended entirely on temperate zone events, because habitat fragmentation in the United States and Canada could be stopped or its effects controlled. But the inexorable destruction of tropical rain forests shows no sign of abating. If it continues at current rates for another few decades, it seems likely that many of our passerine species will become much rarer or even disappear.
North American migrants that overwinter in mature tropical forest are listed here (this tabulation is based on the work of Princeton ecologist John Terborgh with additions by David Wilcove). These species are ones that tend to shun disturbance, but may do well in second-growth tropical forest, edges, or woodlots. Unfortunately, however, deforestation in the tropics rarely leads to such habitats, but rather to vast expanses of overgrazed pastures, canefields, and the like. Thus the following should be the North American birds most at risk as the destruction of tropical forests continues. If possible, you might want to do long-term censuses of breeding populations of one or more of these species should they occur in your area. And should your birding take you to the tropics at the right season, watch for our" migrants. We have much to learn about their wintering ecology.
Helping to Conserve Birds -- National Level;
Conservation of Kirtland's Warbler.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.