The Status of Bird Species - What Are The Major Threats To Birds?
Habitat Loss and Environmental Decline
The driving force behind current declines in many bird species is the destruction, degradation, and fragmentation of habitat due to increasing human population size and the wasteful consumption of resources. The leading cause of habitat destruction in the United States is agricultural development. Large corporate farms cause environmental damage by clearing out native plant species, planting only one or a few crops, and draining wetlands. Natural habitats are also lost to urban sprawl, logging, mining, and road building.
Tropical bird species are threatened by large-scale deforestation worldwide. In Asia, for example, a 2001 study by BirdLife International suggested that one in four bird species is threatened, the majority due to loss of forest habitat. Populations have declined especially sharply in the last two decades, coincident with what BirdLife International calls "habitat loss or degradation resulting from unsustainable and often illegal logging, and land or wetland clearance for agriculture or exotic timber plantations." Large species, such as the Philippine eagle, are most quickly harmed by deforestation—these require large areas of undisturbed forest to hunt and breed.
Many Arctic bird species are threatened by habitat loss due to global warming. In April 2000 the World Wildlife Fund released a report indicating that a world climate change as small as 1.7 degrees centigrade (about 3.3 degrees Fahrenheit) would significantly reduce tundra habitat—the frozen arctic plain that serves as a breeding ground for many bird species. Among the tundra species already threatened are the red-breasted goose, the tundra bean goose, and the spoon-billed sandpiper.
Island species are also particularly vulnerable to habitat destruction because their ranges are usually very small to begin with. In addition, because many island birds evolved in the absence of predators, there are a large number of flightless species—these are highly vulnerable to hunting or predation by introduced species, including humans, cats, dogs, and rats. At one time some 75 percent of all bird extinctions occurred on islands. It is estimated that two-thirds of Hawaii's original bird fauna is already extinct. Of the remaining one-third, a large majority are imperiled. Habitat destruction in Hawaii has been so extensive that all the lowland species now present are non-native species introduced by humans.
Pesticides
During the latter half of the twentieth century, pesticides and other toxic chemicals were recognized as a major cause of avian mortality and a primary factor in the endangerment of several species, including the bald eagle and peregrine falcon. While the U.S. Environmental Protection Agency regulates the manufacture and use of toxic chemicals nationwide, the Fish and Wildlife Service (under the Federal Insecticide, Fungicide, and Rodenticide Act [Amended 1988]) is responsible for preventing and punishing the misuse of chemicals that affect wildlife.
Many chemicals harmful to birds, such as DDT and toxaphene, have been banned. Other chemicals, such as endrin, the most toxic of the chlorinated hydrocarbon pesticides, are still legal for some uses. Endrin was responsible for the disappearance of the brown pelican from Louisiana, a population that once numbered 50,000 individuals.
Oil Spills
Oil spills constitute a major threat to birds. (See Figure 8.1.) One of the worst and most infamous spills in history occurred on March 24, 1989, when the Exxon Valdez tanker released 11 million tons of crude oil into Alaska's Prince William Sound. To many Americans, it still exemplifies the disastrous effects oil spills have on wildlife. Thousands of birds died immediately after coming in contact with the oil, either from losing the insulation of their feathers or by ingesting lethal amounts of oil when they tried to clean themselves. Exxon personnel burned untold piles of birds; others were saved in cold storage under orders from the Fish and Wildlife Service. A complete count was never obtained, but Wildlife Service biologists estimated that between 250,000 and 400,000 sea birds died.
Approximately 40 percent of the region's entire population of common murres—estimated at 91,000—was eliminated. The yellow-billed loon population was also seriously depleted, as was the population of Kittlitz's murrelet, a species found almost exclusively in Prince William Sound. Other affected bird species included the bald eagle, black oystercatcher, common loon, harlequin duck, marbled murrelet, pigeon guillemot, and the pelagic, red-faced, and double-crested cormorants. Of these, the common loon, the harlequin duck, the pigeon guillemot, and the three species of cormorants have not increased in population size since the spill and were still considered "not recovered" in 2002. In addition, the Kittlitz's murrelet appears to be suffering from continued population decline, and its future prospects appear bleak.
The detergents used to clean up oil spills can also be deadly to waterfowl—detergents destroy feathers, which leads to fatal chills or trauma. Research has shown that even after careful rehabilitation, birds that have been returned to nature after a spill often die in a matter of months. In 1996 Dr. Daniel Anderson, a biologist at the University of California at Davis, found that only 12 to 15 percent of rehabilitated pelicans survived for two years, compared to the 80 to 90 percent of pelicans not exposed to oil. For many ornithologists, these dismal results raise the issue of whether avian rescue efforts are worthwhile. Could money spent on rehabilitation be better used for spill prevention and habitat restoration? Oregon ornithologist Dr. Brian Sharp argues that the cleanup effort might ease the conscience of the public and of politicians, but in reality, does very little to benefit birds. However, new methods of treating oiled birds and of controlling spills have increased the bird survival rate from 5 percent to between 60 and 80 percent for some species. Under the Clean Water Act, the oil industry pays a tax that helps fund cleanups after spills.
Domestic Cats
Studies in the United States and Britain have shown that house cats kill millions of small birds and mammals every year, a death toll that contributes to declines of rare species in some areas. The University of Wisconsin reported that in that state alone, cats killed 19 million songbirds and 140,000 game birds in a single year. The British study reported that Britain's 5 million house cats account for an annual prey toll of some 70 million animals, 20 million of which are birds. The study also found that cats were responsible for a third to half of all the sparrow deaths in England. Both studies determined that factors such as whether a cat was well-fed at home, wore a bell collar, or was declawed made no difference to its
FIGURE 8.1
A bird is cleaned of oil after the disastrous Exxon Valdez spill in Prince William Sound, Alaska, in 1989. An estimated 250,000–400,000 sea birds died in the spill. (
Trade in Exotic Birds
Birds are among the most popular pets in American homes. An estimated 6–10 percent of American households own pet birds. Many of these are common finches, canaries, or parakeets, all of which are raised in captivity in the United States. However, wild birds are owned and traded as well, including numerous species of passerines (song birds) and psittacines (parrots and their relatives).
Passerines include any of the approximately 4,800 species of song birds. The most commonly traded passerines include warblers, buntings, weavers, finches, starlings, flycatchers, and sparrows. Passerines are regarded as low-value birds, and few passerines are endangered due to trade.
The 333 species of psittacines, however, are generally rarer, and thus much more valuable, than passerines. They comprise about 15 percent of the pet bird market in the United States. The most commonly traded psittacines are macaws, Amazons, cockatoos, lovebirds, lories, and parakeets. In addition to their vivid colors and pleasant songs, many of these birds possess the ability to "talk," which makes them particularly appealing to some owners. Bird dealers have created demand for an ever-increasing variety of birds, including parrots, macaws, cockatoos, parakeets, mynahs, toucans, tanagers, and other tropical species.
Laws in Mexico, Guatemala, and Honduras ban trade in parrots, and U.S. law bars importation of birds taken illegally from other countries. Some countries still allow exports, however, and there is also a great deal of smuggling. Legislation passed in 1992 to halt the legal importation of parrots is, ironically, believed to have increased smuggling. In 1998 customs officials announced the arrest of more than forty people for smuggling hundreds of rare parrots and other wildlife across the Mexican border. The animals seized were believed to be worth hundreds of thousands of dollars in the pet trade, although some were considered nearly priceless because of their rarity in the wild.
The illegal bird trade has severely harmed many threatened species. New York Zoological Society bird curator Don Bruning believes that species such as the scarlet macaw are now practically extinct throughout Central America due to illegal trade. Over the past twenty years, smuggling has reduced red crown parrot populations by 80 percent and yellow-headed parrots by 90 percent in Mexico. In 2000 the World Wildlife Fund (WWF) identified the horned parakeet as one of the ten species most threatened by illegal trade.
Invasive Species—The Case of Guam
Invasive species have damaged bird populations in some parts of the world, particularly those that occupy islands. Guam's unique bird fauna has been all but wiped out by the brown tree snake, an invasive species. The brown tree snake was probably introduced from New Guinea via ship cargo in the 1950s, and had spread throughout the island by 1968. The snakes have no natural enemies on the island and plentiful prey in the form of forest birds. There are now believed to be as many as 13,000 snakes in a single square mile in some forest habitats. Twelve bird species have already gone extinct on Guam, including the Guam flycatcher, the Rufus fantail, the white-throated ground dove, and the cardinal honey-eater. Several other Guam bird species are close to extinction. Many of these birds are or were unique to Guam. Measures have been implemented to try to keep this destructive snake from invading other islands, including careful inspection of all cargo arriving from Guam. The removal of the brown tree snake in select habitat areas on Guam (which is a high effort project, requiring the constant trapping of snakes) allowed the reintroduction of one bird, the flightless Guam rail, in 1998. The Guam rail had gone extinct in the wild, but a population is maintained in captivity.
Other particularly destructive invasive species include several associated with humans, including cats, dogs, and rats, which often prey on birds and their eggs. In fact, the World Conservation Union (IUCN) reports that invasive species represent the single most frequent cause of bird extinctions since 1800. Invasive species are currently estimated to affect 350, or 30 percent, of all IUCN-listed threatened birds.
Salton Sea Deaths
The Salton Sea, located 150 miles southeast of Los Angeles in the Sonora Desert, is a 35-mile-long expanse of salt marsh and open water encompassing 35,484 acres and situated 227 feet below sea level. The sea formed from a salt-covered depression known as the Salton Sink in 1905, when a levee on the Colorado River broke, filling the depression with water. Subsequently, the area has received additional water, primarily from agricultural runoff. Because the Salton Sea has no outlet, water is lost only through evaporation, leaving dissolved salts behind. The salinity (the amount of salt in the water) in the Salton Sea has increased gradually over time, and is estimated at 25 percent greater than the ocean in 2004. The Salton Sea serves as habitat for migrating birds and provides winter habitat for waterfowl. The area is second only to the Texas coastline in the number of bird species sighted, and nearly 400 species had been reported by 2004. The Salton Sea National Wildlife Refuge was established in 1930 by presidential proclamation.
In the last decades of the twentieth century the Salton Sea entered a rapid and initially inexplicable decline that resulted in the deaths of countless birds and fish. The first unusual avian deaths were reported in 1987, and a task force was created by the California Department of Fish and Game in 1988 to study the problem. In 1992 a massive die-off occurred in which over 150,000 birds died. Some of the fatalities were attributed to avian cholera, but experts remained baffled by the majority of casualties. At that point, the Department of the Interior initiated a $10 million Salton Sea project aimed at combating rising salinity and other environmental problems. The Salton Sea Authority was established in 1993 to coordinate activity. In 1996 there was another mass epidemic in which over 20,000 birds from 64 species died, including 1,200 brown pelicans, an endangered species. This time, the cause was identified as avian botulism. The same year, thousands of tilapia fish were also killed by botulism. Authorities tentatively attributed the avian deaths to botulism from consuming tainted fish. Throughout 1997 a variety of initiatives were proposed in an effort to combat high salinity and other problems at the Salton Sea. Nonetheless, in May of that year another 10,000 birds from 51 species died, along with thousands of tilapia. Causes of death included avian botulism, Newcastle disease, avian cholera, and poisoning by toxic algae.
Scientists have so far failed to establish a precise link between water quality and bird die-offs, but suspect a combination of natural and man-made contaminants. Evaporation and agricultural runoff have increased the salinity of the Salton Sea to levels 25 percent higher than in the Pacific Ocean. Experts fear that high salt levels increase the susceptibility of fish to disease, and that birds are impacted when they consume affected fish. Another suspected cause of environmental deterioration is the defunct Salton Sea Test Base (SSTB), which served as a center for arms testing and weapons research during World War II. The U.S. Army Corps of Engineers initiated a clean-up project to decontaminate the SSTB, which occupied over 20,000 acres of land and water in the southwest corner of the Salton Sea. Environmentalists also believe that agricultural runoff from California's Imperial Valley, one of the most productive farming areas in the United States, encourages algae blooms that are deadly to fish. The Salton Sea is also polluted by additional agricultural runoff from Mexico and by untreated sewage from rivers that flow across the Mexican-U.S. border. Contamination from DDT, DDE (a byproduct of DDT), and selenium also were documented as contributing to the decline.
The Salton Sea Task Force, the U.S. Bureau of Reclamation, and the California State Legislature have combined forces to restore the Salton Sea to health. However, another outbreak of botulism was reported in 2000. The endangered brown pelican suffered greatly in this outbreak, with 717 individuals dying. Another six hundred brown pelicans were rehabilitated and released in December 2000. Avian botulism is not fatal if treatment is begun early, but birds do not show symptoms until the disease has progressed. In addition to the brown pelican, thirty-five other species were affected in the 2000 outbreak. All affected species eat tilapia. Fish and Wildlife Service employees, along with California Department of Fish and Game, helped to round up sick birds and transport them to an open-air bird hospital built in 1997 from funds raised by volunteers. Recovered birds were released near the Tijuana Slough and Seal Beach national wildlife refuges, both located in Southern California.
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