for National Geographic News
During June, as the nesting season of birds shifts into high gear across much of the United States and Canada, a small army of birders hits the road to conduct the North American Breeding Bird Survey.
Beginning half an hour before sunrise, to catch the day's most intense period of bird song, they drive assigned 24.5-mile (40-kilometer) routes along secondary roads. At half-mile intervals, they stop and count the birds they see or hear within a radius of a quarter mile.
Each of some 3,000 survey routes covered each year requires 50 stops and takes about five hours to complete.
The U.S. Geological Survey's Patuxent Wildlife Research Center oversees the Breeding Bird Survey from its headquarters in Laurel, Maryland.
The methodology has its limitationsroadside observations do not adequately measure population trends in endangered, nocturnal, and colonial-nesting birds. Moreover, sections of road that pass natural areasincluding the strips of vegetation often left as buffers between the road and cleared landcan give false impressions.
Species may be common along these sections but absent in nearby areas where habitat has been destroyed.
Despite the "roadside bias" issue, which Patuxent officials are addressing, the more than 30 years of BBS data collectively provide valuable insight into state, regional, and continental population trends of about 500 species of North American birds.
BBS results have helped focus public attention on worrisome changes in bird populations, such as the decline of the cerulean warbler, the eastern towhee, the red-headed woodpecker, and the grassland birds east of the Mississippi River.
Bluebirds Are Back
The news isn't all bad. The number of Eastern bluebirds, for example, increased more than 3 percent a year between 1980 and 2001, according to the BBS. The decline of this species, which nests in old woodpecker holes and in tree cavities formed by natural decay, was noted long before the first multi-state BBS in 1966.
Eastern bluebirds thrive in sparsely wooded habitats: forest clearings, beaver ponds, meadows, and farmland. To feed, they fly from perches and catch insects on or near the ground. Their nesting cavities are often in dead, isolated trees.
Early in the 20th century, the open habitats the bluebird requires were reverting to forest. Meanwhile, commercial and residential developers, and managers of timberlands, were removing the bluebird's existing nest sites. Dead trees were regarded as unsightly, and their wildlife value wasn't appreciated.
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