North America’s Forests: Page 1 Endangered Species Handbook

Forest

North America’s Forests: Page 1

Migratory songbirds of eastern forests have declined by at least 45 percent in the past 50 years. These dazzling migrants--wood warblers, vireos, tanagers, orioles, grosbeaks and thrushes--suffer from a variety of threats. Deforestation of tropical forests where many of these birds spend the winter has been a major factor. Species requiring mature, undisturbed forest have declined to the greatest degree, some losing up to 90 percent of their populations. Clearcut logging and fragmentation of their breeding forests by housing, malls, roads and other development have consumed millions of acres, displacing these songbirds. Many birds returning from tropical wintering grounds find their breeding forests destroyed or too small for successful breeding. A small patch of woods large enough for only one male songbird, surrounded by agriculture or development, will be abandoned if the male is unable to hear the songs of neighboring males (Line 1995). Many birds establish territories and maintain them by singing and challenging other males.

Another effect of forest fragmentation has been the proliferation of forest edge species that prey on songbirds and their eggs (Terborgh 1992). The Brown‑headed Cowbird (Molothus ater), a parasitic species, was once seen only in grasslands. In the past, when huge herds of American Bison roamed the prairies of the Midwest, cowbirds accompanied them, laying their eggs in the nests of grassland birds, who raised them as their own. With the disappearance of the Bison, cowbirds entered the forests bordering agricultural and pasture land, laying their eggs in songbird nests. Some cowbirds now remain close to domestic cattle and fly into the nearest woodland to lay their eggs. Their eggs usually outnumber those of the host species, and the large chicks instinctively nudge the small songbird nestlings out of the nest. Because of the increase in forest openings, the net result of these losses has been a 90 percent decline in the number of songbirds recorded in the 1940s in many eastern forests (Terborgh 1992).

A campaign to declare a large section of the forests in northern New England a reserve began in the mid-1990s. To add urgency to the need for a national park in Maine, New Hampshire and Vermont, a new species of thrush, native only to this region, has been discovered. Once considered a subspecies of the wide-ranging Gray-cheeked Thrush (Catharus minimus), Bicknell's Thrush (Catharus aliciae) was found to be a totally distinct species in 1995, smaller and with a unique song. A bird of high altitude forests, it has already disappeared from its only nesting area in Massachusetts, Mt. Greylock. The major breeding area for Bicknell's Thrushes is now Vermont, whose forests are under pressure from ski developments and proposed wind turbines. Additional populations of fewer than 3,000 birds inhabit neighboring southern Canada, but the traditional practice there of thinning forests renders these forests unsuitable. Its habitat of spruce-fir forests is dying out from acid rain and global warming (BI 2000). This threatened bird is also losing habitat in its West Indian wintering grounds, especially Hispaniola, where forests are being cleared (BI 2000).

Another member of the thrush family is declining toward extinction. Known for its melodious, fluted song, which many find more intricate and beautiful than the Nightingale's, the Wood Thrush (Hylocichla mustelina) has declined precipitously in recent years. Native to undisturbed hardwood and mixed forests in eastern North America, the Wood Thrush winters in Central America, from southern Mexico to southern Panama (Rappole et al. 1983). In August, these birds begin their southerly migration, often gathering in groups where the chorus of 50 or more Wood Thrushes may echo like a cantata in the tall trees. One research project studied these birds in one of their tropical wintering areas in the Tuxtla Mountain region of southern Veracruz, Mexico, on the Gulf of Mexico. Studies of this population of birds began in the mid-1970s when the wintering habitat consisted of primary rainforest. (Rappole et al. 1989). This forest has declined by 85 percent, a far higher percentage than elsewhere in Central America, leaving mere fragments of the once rich tropical rainforest (Rappole et al. 1989). In 1980, the two researchers returned to find that their 12-acre rainforest study plot had been converted to a patchwork of fields and forest fragments; less than 5 acres remained of the original forest (Terborgh 1989). Using radio‑tracking and mist netting, researchers discovered that Wood Thrushes do not adapt to forests without tall trees, even as a wintering area, preferring undisturbed rainforest. Year after year the same individual birds returned to the same sites. In rainforest, they tended to survive the winter, while Wood Thrushes forced to winter in neighboring second-growth or disturbed forest had a high mortality rate from predators and other causes (Rappole et al. 1989).

Throughout their winter range, the forests that once harbored tens or perhaps hundreds of thousands of Wood Thrushes in Mexico and Central America have been turned to pastureland or tilled for agriculture. Between 50 and 70 percent have been destroyed (Collins 1990). Fewer and fewer Wood Thrushes return to nest in Eastern forests, even when they have not been fragmented or cleared (Terborgh 1989). Their nests have also been vulnerable to parasitism by Brown-headed Cowbirds (Terborgh 1989). In Illinois, for example, where Brown‑headed Cowbird populations have skyrocketed in past decades, one study found that 29 of 30 Wood Thrush nests had been parasitized by Brown-headed Cowbirds, who laid eggs that crowded out those laid by the thrushes; the conclusion of the study: "Wood Thrushes are doing nothing but raising cowbirds" (Terborgh 1989). In the few areas where forests are unbroken and extensive, Wood Thrushes have been successful breeders (Robinson 1996).

Unfortunately, the combined effects of habitat destruction in both its breeding and wintering grounds, and parasitism of its nests by cowbirds, may result in the extinction of the Wood Thrush. A recent study in several Midwestern states found that birds from large stretches of forest that are holding their own may be having young that disperse to neighboring forest fragments, only to be crowded out by cowbirds (Robinson 1996). The melodic Wood Thrushes are disappearing throughout their range, along with many other beautiful songbirds.^*

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^*See Video section. “On a Wing and a Song,” a Canadian Broadcasting Co. film produced in 1994 for The Nature of Things series, explores the decline in songbirds. Another excellent film on the same subject with a similar title, “On a Wing and a Prayer,” produced in 1995 by Kurtis Productions and WTTW for the PBS series New Explorers, concentrates on the decline in breeding songbirds of Illinois and their migrations.

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Other changes in environments also contribute to songbirds' declines. Recent research has found that high levels of noise from highways, airports and other man-made activities have interfered with the ability of birds to sing breeding songs and breed successfully. Birds that migrate to the Caribbean and Latin America suffer high mortality from collisions with television and cellphone towers and tall buildings. For many years, one scientist at Chicago's Field Museum has collected migrating songbirds that died from flying into a single building near the museum. His collection now totals 20,000 birds. He estimates that total mortality from building collisions may be 60 million songbirds a year, an enormous loss.

A great concentration of breeding songbirds is found in Belt Woods, a 500-acre-forest near Bowie, Maryland. Trees that were growing when Columbus discovered America survive here. Perhaps the largest tract of old-growth forest in the region, it has a variety of habitats from swamp forest to dry hardwood stands. More than 40 species of migrant birds nest here, including Wood Thrushes, Rose-breasted Grosbeaks, Baltimore (Northern) Orioles, Scarlet Tanagers, Prairie Warblers and Ovenbirds. These are species that have disappeared from woodlands that have become fragmented, or contain few tall trees. This wealth of breeding birds underlines the importance of protecting old-growth forests. Privately owned, Belt Woods was willed by the late owner, William Seton Belts, to the Episcopal Church to be kept intact in perpetuity. The church, however, went to court in the 1990s to change the will to allow sale to developers, claiming that it had more important works to accomplish with the funds than saving the forest.

After conservationists had decided it was a hopeless cause, a local resident, Pam Cooper, decided to spearhead a rescue of these precious woodlands. By conducting publicity campaigns and fund-raising efforts, she and the Western Shore Conservancy raised money. A Maryland author and illustrator of children's books, Lynn Cherry, organized The Friends of the Woods to enlist children to help influence their parents and public opinion to place the preservation of this threatened woodland above that of sheer materialism and to champion the need to take on local conservation problems as a community. Her children's book, Flute's Journey, the Life of a Wood Thrush, chronicled the life of a Wood Thrush born in Belt Woods, its migration to Costa Rica and its uncertain future. The plight of Belt Woods was given national attention in April 1996 on a CBS program, "Sunday Morning," which may have been a crucial turning point in the preservation of this forest. Within months, an arrangement was finalized with conservation organizations that would turn Belt Woods into a reserve, ending the possibility that it would be sold to developers.

At a time when losses in the numbers of North America's beautiful songbirds may have become irreversible, scientific research is revealing that they play a crucial role in controlling insects in forests (Yoon 1994). In a controlled experiment, birds were excluded from a section of forest with netting cages that allowed insects to enter. It was found that trees and plants incurred twice the damage to foliage where birds were excluded (Yoon 1994). The study concludes that songbirds constitute a major and important form of natural pest management, superior to pesticides (Yoon 1994). Keeping songbirds from nesting in a stand of White Oaks, other researchers found that caterpillars consumed so much foliage that the trees grew significantly more slowly than White Oaks with nesting songbirds (Robinson 1996).