The volunteers wake at dawn during the height of the North American bird breeding season and drive a pre-determined 24.5 mile (39.4 kilometer) back-road route with stops at 0.5 mile (0.8 kilometer) intervals. At each stop the volunteers count all the birds they can see or hear.
The survey was designed to give researchers an annual index of abundances for each species, said Keith Pardieck, national coordinator for the breeding bird survey at the Patuxent Wildlife Research Center.
"You have indices that can be compared through time, for example to see where robins are increasing or decreasing," he said. This data, he added, allows researchers to devise conservation strategies for specific species.
Doherty and colleagues took the breeding bird survey data and looked at in a way that allowed them to ask questions about bird populations at the community level.
The researchers analyzed the bird species within six orders that are active during the day and thus readily detected by the survey. Each individual species was placed either into the community of having males and females of different color (dichromatic) or males and females of the same colored feathers (monochromatic).
According to the researchers' analysis of this data, bird species whose males and females differ in color have a 23 percent higher local extinction rate than do species whose males and females do not differ in color.
Despite the local extinction pressure, the total number of dichromatic species did not decrease over the 21-year study period. Rather, the researchers found that the local extinction rates were matched by local species turnover rates.
"There is a pretty good correlation. Where extinction rates are highest, turnover rates are highest," said Doherty.
This finding suggests that as a local population of a dichromatic species begins to decline another population moves in and takes over the habitat, keeping the species population stable.
Based on these results, Doherty and colleagues suggest that human activities that restrict the movement of bird populations from one habitat to another, such as logging or suburban sprawl, may put global populations of dichromatic bird species at risk of extinction.
"Colonization is dependent on movement and dispersal, so anything that would disrupt the free flow of individual birds from one local community to another could have deleterious effects," said Doherty.
Nichols said that one of the next steps in the research is to devise a methodology that permits researchers to "ask questions at a species-specific level" rather than the community level and thus get an idea of what species might be at risk.
Another interesting point raised by the study that Stenseth and Saetre say is worthy of further investigation is the regional differences in extinction and turnover rates across North America.
For example Southern Texas and the prairie region of the north-central part of the continent have much more stable community structures than other regions. There is little difference in extinction and turnover rates between dichromatic and monochromatic species in these regions.
"A major challenge for future studies would be to unravel the underlying ecological processes that determine the relative stability of communities," write Stenseth and Saetre.
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National Geographic Bird Resources:
Bald Eagles: Come Back From the Brink
Experience the Sights and Sounds of Eagles
Snowy OwlsMuscle & Magic
Attwater's Prairie-ChickensDown to a Handful
Recent "Birder's Journal" Stories from Robert Winkler:
Giving Thanks for Wild Turkey Sightings
Birder's Journal: Ghost Town's Curse Haunts New England Forest
Birder's Journal: Looking at a Handy New Guide
Birder's Journal: Learning to Let Birds Come to You
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Nationalgeographic.com Bird-Watching Sites:
Florida Keys Area
Maine's Acadia National Park
New Orleans Area
New York City Area
North Carolina's Outer Banks
Rocky Mountain National Park
Salt Lake City Area
San Francisco Area
Santa Fe Area
South Dakota's Black Hills
Washington's Olympic National Park
Wyoming's Grand Teton National Park
Yellowstone National Park
Yosemite National Park
From the National Geographic Store:
Guide to North American Birds
Portable Birdsong Identifier
Additional Information from Related Web Sites:
American Bird Center
American Bird Conservancy
Fish and Wildlife Service Bird Web Site
National Audubon Society
Environmental Protection Agency: Bird Conservation
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