July 28, 2003 Songbirds are in trouble throughout the eastern U.S. and new research suggests that raccoons are a major part of the problem. Raccoons love eggs, and the study shows that populations of birds with accessible nests have been dropping since raccoon populations began rising in the early 1980s in Illinois.
"Declines in vulnerable, low-nesting songbird species in Illinois have paralleled increases in raccoon populations," says Kenneth Schmidt of Texas Tech University in Lubbock in the August issue of Conservation Biology.
Previous studies have shown that Illinois may be losing more songbirds than it produces. Low-nesting birds are doing particularly poorly, and artificial nest experiments suggest that raccoons are among the main predators of eggs and chicks of the state's ground-nesting birds. Raccoons have increased greatly in Illinois in the last 20 years, with surveys spotting three times as many of the nocturnal carnivores in recent years as in the early 1980s.
To see if the state-wide decline in low-nesting songbirds is linked to the state-wide increase in raccoons, Schmidt used existing data to track the population trends of 40 bird species along 41 roadside routes in Illinois from 1966 to 2001. There were 18 low-nesting species (with nests less than 8 feet above the ground) and 22 high-nesting species (with nests more than 8 feet above the ground). The data Schmidt used came from the Breeding Bird Survey, which samples birds along more than 3,000 25-mile roadside routes in the U.S. and Canada.
Schmidt found that the decline in low-nesting birds did coincide with the early 1980s rise in raccoons. Before 1980, the population trends were about the same for low- and high-nesting birds. But after 1980, more low-nesting species declined: more than 70% of the low-nesting species declined while only half of the high-nesting species declined along the routes studied between 1980 and 2001. The other half of the high-nesting species increased.
Moreover, Schmidt found that after the raccoon population began rising, the diversity of low-nesting birds decreased while the diversity of high-nesting birds increased: the number of low-nesting species dropped about 10% while the number of high-nesting species rose about 15% between 1980 and 2001 (from about 10 to 9 species, and from about 9 to 10.5 species per route, respectively).
These findings may apply throughout the eastern U.S. The raccoon increase is driven by the eradication of top carnivores, and by habitat fragmentation and conversion to agriculture. In Illinois, more than 70% of forest gone, and row-crops cover about half of the land. "Habitat conversion and the loss of top carnivores have allowed...raccoons to flourish, in turn creating a hostile landscape for songbirds," says Schmidt. This situation is exacerbated by the fact that the Illinois raccoon harvest decreased from nearly 400,000 in 1979 to about 70,000 in 1990.
What's bad for the raccoons is likely to be good for the birds, and Schmidt predicts that low-nesting songbirds in the eastern U.S. could rebound as raccoon rabies spreads from its origin along the Virginia/West Virginia border.
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