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.
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) 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.
TABLE 9.3 Major international conventions and U.S. legislation devoted to migratory bird conservation, 1990–2001
| TABLE 9.3 | |
|---|---|
| Major international conventions and U.S. legislation devoted to migratory bird conservation, 1990–2001 | |
| Year | Authority |
| SOURCE: "Appendix 3. Primary International Conventions and Major Domestic Legislation for the Conservation of Migratory Birds and Their Habitats in the United States," in A Blueprint for the Future of Migratory Birds: Migratory Bird Program Strategic Plan 2004–2014, U.S. Department of the Interior, U.S. Fish and Wildlife Service, 2004, http://www.fws.gov/migratorybirds/mbstratplan/MBStratPlanTOC.htm (accessed March 9, 2006) | |
| 1900 | Lacey Act (amended 1981) |
| 1913 | Weeks-McLean Law (Migratory Bird Conservation Act 1913) |
| 1916 | Convention for the Protection of Migratory Birds (Canada) |
| 1918 | Migratory Bird Treaty Act |
| 1929 | Migratory Bird Conservation Act |
| 1934 | Migratory Bird Hunting and Conservation Stamp Act (Duck Stamp Act) |
| 1936 | Migratory Bird Convention with Mexico (amended 1972) |
| 1940 | Pan American (or Western Hemisphere) Convention |
| 1940 | Bald Eagle Protection Act |
| 1956 | Waterfowl Depredations Prevention Act |
| 1961 | Wetlands Loan Act of 1961 (amended 1969, 1976) |
| 1972 | Migratory Bird Convention with Japan |
| 1972 | Convention on Wetlands of International Importance especially as waterfowl habitats (RAMSAR) |
| 1973 | Endangered Species Act (ESA) |
| 1973 | Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) |
| 1976 | Migratory Bird Convention with the Union of Soviet Socialist Republics |
| 1978 | Antarctic Conservation Act |
| 1980 | Fish and Wildlife Conservation Act (amended 1988, 1989) |
| 1982 | Convention on Conservation of Antarctic Living Marine Resources |
| 1986 | Emergency Wetlands Resources Act |
| 1987 | Driftnet Impact Monitoring, Assessment, and Control Act of 1987 |
| 1989 | North American Wetlands Conservation Act (NAWCA) |
| 1990 | Coastal Wetlands Planning, Protection and Restoration Act |
| 1992 | Wild Bird Conservation Act |
| 2000 | Neotropical Migratory Bird Conservation Act |
| 2001 | Responsibilities of Federal Agencies to Protect Migratory Birds (executive order 13186) |
Oil Spills
Oil spills constitute a major threat to birds. (See Figure 9.8.) One of the worst and most infamous spills in history occurred on March 24, 1989, when the Exxon Valdez tanker released eleven 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 U.S. Fish and Wildlife Service. A complete count was never obtained, but FWS biologists estimated that between 250,000 and 400,000 sea birds died as a result of the accident.
Approximately 40% 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.
FIGURE 9.8 A bird is cleaned of oil after the disastrous Exxon Valadez spill in Prince William Sound, Alaska, in 1989.
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% of rehabilitated pelicans survived for two years, compared to the 80% to 90% of pelicans not exposed to oil (Verne G. Kopytoff, "Birds Rescued in Spills Do Poorly, Study Finds," New York Times, November 12, 1996). 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 argued in the same New York Times article 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% to between 60% and 80% 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. Many cat victims are plentiful urban species, but studies by the U.S. Fish and Wildlife Service have shown that cats also kill hundreds of millions of migratory songbirds annually (http://www.fws.gov/birds/mortality-fact-sheet.pdf). In addition, cats have devastated bird fauna on some islands and are believed to have contributed to the declines of several grassland species in the United States.
Trade in Exotic Birds
Birds are among the most popular pets in American homes. According to the American Pet Products Manufacturers Association in 2005/2006 National Pet Owners Survey (http://www.appma.org/press_industrytrends.asp), more than 16.6 million birds are kept as pets in the United States. 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).
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. 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.
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. According to Earl William Campbell III in "Brown Treesnake Fact Sheet" (U.S. Fish and Wildlife Service, May 3, 2004), the brown tree snake was probably introduced from New Guinea via ship cargo in the late 1940s. The snake population thrived on the island because of the absence of natural enemies and the presence of plentiful prey in the form of forest birds. There are now believed to be as many as fourteen thousand snakes in a single square mile in some forest habitats. Nine 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.
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