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Scientists worry that bird populations across North America have been decreasing. NPR's Nell Greenfieldboyce reports that researchers now have a rough estimate of how the total number of birds has changed since 1970.
NELL GREENFIELDBOYCE, BYLINE: Ken Rosenberg is 65 years old, and he says, over his lifetime, he's noticed a decline in migrating birds, like evening grosbeaks.
KEN ROSENBERG: When I was a kid, there were years when you could see 50 or 100 at your feeder, and now you're lucky in a big year to see 10.
GREENFIELDBOYCE: Rosenberg works at the Cornell Lab of Ornithology. He knew that some bird populations, like bald eagles, have actually gone up over the last few decades. So he wondered how the total number of birds in the sky might be changing.
ROSENBERG: Are there fewer birds than there were in 1970, or are we seeing a shift and losing some of the rare specialized species and shifting towards more generalist, more common birds, species that are more adapted to humans?
GREENFIELDBOYCE: To find out, he and some colleagues gathered all the data they could on over 500 bird species; a lot of it came from bird surveys done each year by volunteers. They also used data from weather radar installations that can detect flocks of migrating birds. They crunched all the numbers and were stunned by the results.
ROSENBERG: By our estimates, it's a 30% loss in the total number of breeding birds since 1970 - less than 50 years - and that's 3 billion birds.
GREENFIELDBOYCE: Three billion fewer birds. Rosenberg says most of that loss comes from bird families like sparrows, finches, warblers and swallows.
ROSENBERG: Things like meadowlarks, dark-eyed junco, horned lark, red-winged blackbird.
GREENFIELDBOYCE: And the main culprit is probably the loss of their natural habitat, with increased urbanization and more agriculture. This massive bird accounting project is described in the journal Science - it's not exactly as precise as balancing your checkbook. Ted Simons is an ecologist with North Carolina State University. He says trying to count and track birds is a daunting task.
TED SIMONS: We're certainly far from having the tools and having the resources to have real high confidence in our estimates of these populations.
GREENFIELDBOYCE: Still, he thinks it's very likely that the total bird population has substantially declined. And others say the new estimate sounds about right, like migratory bird researcher Kristen Ruegg at Colorado State University.
KRISTEN RUEGG: Overall, the conclusions weren't necessarily surprising. I mean, they were depressing but not surprising.
GREENFIELDBOYCE: She says having this estimate is a way to wake people up to the problem.
Nell Greenfieldboyce, NPR News.
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