The Decline of Eastern Songbirds
http://www.stanford.edu/group/stanfordbirds/text/essays/Eastern_Songbirds.html
there are many reasons for songbirds' decline
Since 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 (perching birds) 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.
North American Migrants that Overwinter in Mature Tropical
Forest
Mississippi Kite
Chuck-will's Widow
Black-billed Cuckoo
Yellow-bellied Flycatcher
Western Wood-Pewee
Gray-cheeked Thrush
Black-capped Vireo
Red-eyed Vireo
Black-and-white Warbler, Prothonotary Warbler
Swainson's Warbler
Blue-winged Warbler
Bachman's Warbler (extinct?)
Magnolia Warbler
Black-throated Green Warbler
Grace's Warbler
Bay-breasted Warbler
Ovenbird
Kentucky Warbler
American Redstart
Scarlet Tanager