The Citizen's Guide to Migratory Bird Conservation
Obstacles in Temperate North America
Obstacles in Temperate North America
Migrant birds face tremendous threats on their breeding grounds. Only 250 years ago, the forests of North America provided ideal habitat for many migrants. By 1920, however, much of the landscape had been deforested. In recent decades, many of the forests cut in the 19th and 20th centuries have regrown or been replanted, especially in the Northeast. But problems for forest-dwelling birds remain.
Why? Much of the forest that does remain has been fragmented, that is, parceled into small blocks by urbanization, agriculture, timber harvesting, and other human activities. Such fragmentation seems to be a serious problem for some neotropical migrants. Unlike large forest tracts, small, scattered woodlands present numerous edges -- boundaries created by roads, fields, housing developments, and possibly clearcuts. these edges can allow open-land predators such as jays and crows, which feed on songbirds and their nestlings, to intrude into the forest. Creeping urbanization has also allowed increases in predators that live and thrive around humans, such as raccoons, opossums, and cats.
Fragmentation is a particular problem for neotropical migrants because of their nesting habits. Most neotropical migrants build open, cup-shaped nests that are relatively easy for predators to spot. They also tend to lay only a few eggs each year. Many nest on the ground, making them susceptible to predation.
Edge habitat and open-cup nests also cause birds to be susceptible to cowbird parasitism. Unlike most other birds, Brown-headed Cowbirds do not build their own nests; instead, they lay their eggs in the nests of other birds, sometimes destroying the eggs of the unwitting host. And even if the host eggs are not destroyed the cowbird eggs generally hatch first and the large, aggressive cowbird checks crowd out the host young, killing them directly, or killing them indirectly by eating all the food brought by the parents.
At the time of European settlement, Brown-headed Cowbirds lived in the Great Plains of North America. in the past 150 years, however, as forest has been cleared for agriculture, cowbirds have expanded their range dramatically. The number of cowbirds has skyrocketed, and so has the number of bird species they are known to parasitize -- now over 200. In central Illinois, where very little forest remains, cowbirds parasitize 75 percent of the nests of some species of migratory birds, such as the Wood Thrush.
Forest fragmentation seems to be less of a problem in western North America, where migrants face a different set of challenges. The West is made up of diverse habitats -- montane forest, riparian (streamside) habitat, desert, grassland, and shrubsteppe, to name a few. Neotropical migrants live in all of these habitats at some time of year, but most species nest in montane forests and riparian areas, which make up just a small percentage of western lands. Consequently, populations of western migratory birds may be smaller than those of eastern species.
As in the East, habitat degradation is a key factor in the declines of some western species. For example, disruption of riparian habitat by cattle grazing and agriculture has enabled cowbirds to take advantage of several species of migrants that nest in these areas. As much as 95 percent of the riparian habitat has been lost in many western states. As a result, populations of species that depend on riparian habitat, such as the southwestern Willow Flycatcher and Yellow Warbler, are in trouble. Riparian woodlands in California have lost most of their bird species.
Declines of grassland species, such as Grasshopper and Lark Sparrows, are also cause for concern. According to Breeding Bird Survey data, grassland bird species are showing steeper and more consistent declines over the past 25 years than are other birds. While these trends are not entirely understood, biologists suspect that changes such as the disappearance of the great bison herds from the Great Plans and habitat loss due to agricultural activity are factors in teh declines. In many states grassland habitat has nearly vanished. In others, grasslands have become fragmented, a process analogous to forest fragmentation, so bird species that require large areas of this habitat are unable to nest successfully. the growing body of scientific research will shed more light on grassland birds and their populations changes in the years to come.
Russell Greenberg and Susan Lumpkin, Smithsonian Migratory Bird Center, National Zoological Park, Washington, DC 20008
Arthur Weissman, Green Seal, 1250 23rd St. NW, Washington, DC 20037
This section has been excerpted in its entirety. Author information and article text current as of 1995.
Greenberg, R., S. Lumpkin, and A. Weissman. 1995. Citizen's Guide to Migratory Bird Conservation, Bonney, R., S. Carlson, and M. Fischer, eds. Cornell Laboratory of Ornithology, Ithaca, NY, USA.
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