Neotropical migrants

Birds that migrate each year between the American tropics and higher latitudes, especially in North America, are known as neotropical migrants. So called because they migrate from the tropics of the "new world" (the Western hemisphere), neotropical migrants are a topic of concern because many species have been declining in recent years, and because they are vulnerable to habitat destruction and other hazards in both winter and summer ranges. One survey of 62 forest-dwelling migrant bird species found that 44 declined significantly from 1978–1987. Migrants' dependence on nesting and feeding habitat in multiple regions, usually in different countries, makes conservation difficult, since habitat protection often requires cooperation between multiple countries. In addition, it is often unclear whose fault it is when populations fall. Most of the outcry over neotropical migrants has been raised in the United States and Canada, where summer bird populations have thinned noticeably in recent decades. Conservationists in northern latitudes tend to attribute bird disappearances to the destruction of tropical forests and other winter habitat. In response, governments in Mexico, Central America, and South America argue that loss of suitable summer nesting habitat, as well as summer feeding and shelter requirements, are responsible for the decline of migratory birds.

About 250 species of birds breed in North America and winter in the south. In the southern and western United States 50-60% of breeding birds are migrants, a number that rises to 80% in southern Canada and to 90% in the Canadian sub-arctic. Among the familiar migrants are species of orioles, hummingbirds, sandpipers and other wading birds, herons and bitterns, flycatchers, swallows, and almost all warblers. Half of these winter in Mexico, the Bahamas, and the Greater Antilles islands. Some 30 species winter as far south as the western Amazon Basin and the foothills of the Andes Mountains, where forest clearance poses a significant threat. Species most vulnerable to habitat loss may be those that require large areas of continuous habitat, especially the small woodland birds, and aquatic birds whose wetland habitats are being drained, filled, or contaminated in both summer and winter ranges.

In addition to habitat loss, neotropical migrants suffer from agricultural pesticides, which poison both food sources and the birds themselves. The use of pesticides, including such persistent chemicals as dichlorodiphenyl-trichloro-ethane (DDT), has risen in Latin America during the same period that logging has decimated forest habitat. Furthermore, some ecologists argue that migrants are especially vulnerable in their winter habitat because winter ranges tend to be geographically more restricted than summer ranges. Sometimes an entire population winters in just a few islands, swamps, or bays. Any habitat damage, pesticide use, or predation could impact a large part of the population if the birds concentrate in a small area.

Population declines are by no means limited to loss of wintering grounds. An estimated 40% of North America's eastern deciduous forests, the primary breeding grounds of many migrants, have either been cleared or fragmented. Habitat fragmentation is the term used to describe breaking up a patch of habitat into small, dispersed units, or dissecting a patch of habitat with roads or suburban developments. This problem is especially severe on the outskirts of urban areas, where suburbs continue to cut into the surrounding countryside. For reasons not fully understood, neotropical migrants appear to be more susceptible to habitat fragmentation than short-distance migrants or species that remain in residence year round. Hazards of fragmentation include nesting failure due to predation (often by raccoons, snakes, crows, or jays), nesting failure due to competition with human-adapted species such as starlings and English sparrows, and mortality due to domestic house cats. (An Australian study of house cat predation estimated that 500,000 cats in the state of Victoria had killed 13 million small birds and mammals, including 67 different native bird species.) In addition to hazards in their summer and winter ranges, threats along migration routes can impact migratory bird populations. Loss of stop-over wetlands , forests, and grasslands can reduce food sources and protective cover along migration routes. Sometimes birds are even forced to find alternative migration paths.

European-African migrants suffer similar fates and risks to those of American migrants. In addition to wetland drainage , habitat fragmentation, and increased pesticide use, Europeans and Africans also continue to hunt their migratory birds, a hazard that probably impacts American migrants but that is little documented here. It is estimated that in Italy alone, some 50 million songbirds are killed each year as epicurean delicacies. Hunting is also practiced in Spain and France, as well as in African countries where people truly may be short of food.

One of the principal sources of data on bird population changes is the North American Breeding Bird Survey, an annual survey conducted by volunteers who traverse a total of 3,000 established transects each year and report the number of breeding birds observed. While this record is by no means complete, it provides the best available approximation of general trends. Not all birds are disappearing. Some have even increased slightly, as a consequence of reduced exposure to DDT, a pesticide outlawed in the United States because it poisons birds, and because of habitat restoration efforts. However the BBS has documented significant and troubling declines in dozens of species.

Declines in migrant bird populations alarm many people because birds are often seen as indicators of more general ecosystem health . Birds are highly visible and their disappearance is noticeable, but they may indicate the simultaneous declines of insects, amphibians, fish, and other less visible groups using the same habitat areas.

[Mary Ann Cunningham Ph.D. ]

RESOURCES

BOOKS

Terbogh, J. Where Have All the Birds Gone? Princeton, NJ: Princeton University Press, 1989.

PERIODICALS

Friesen, L., P. F. Eagles, and R. J. Mackay. "Effects of Residential Development on Forest-Dwelling Neotropical Migrant Songbirds," Conservation Biology 9, no. 6 (1995): 1408–l4.

Keast, A. "The Nearctic-Neotropical Bird Migration System." Israel Journal of Zoology 4, no. 1 (1995): 455–70.

"Neotropical Migratory Birds." Conservation Biology 7, no. 3 (1993): 501–09.

Youth, H. "Flying Into Trouble" Worldwatch 7 (1994): 110–17.