Over the past half-century, North America has lost more than a quarter of its entire bird population, or around 3 billion birds.
That's according to a new estimate published in the journal Science by researchers who brought together a variety of information that has been collected on 529 bird species since 1970.
"We saw this tremendous net loss across the entire bird community," says Ken Rosenberg, an applied conservation scientist at the Cornell Lab of Ornithology in Ithaca, N.Y. "By our estimates, it's a 30% loss in the total number of breeding birds."
Rosenberg and his colleagues already knew that a number of bird populations had been decreasing.
"But we also knew that other bird populations were increasing," he says. "And what we didn't know is whether there was a net change." Scientists thought there might simply be a shift in the total bird population toward more generalist birds adapted to living around humans.
To find out, the researchers collected data from long-running surveys conducted with the help of volunteer bird spotters, such as the North American Breeding Bird Survey and the Audubon Christmas Bird Count. They combined that data with a decade's worth of data on migrating bird flocks detected by 143 weather radar installations.
Their results show that more than 90% of the loss can be attributed to just a dozen bird families, including sparrows, warblers, blackbirds and finches.
Common birds with decreasing populations include meadowlarks, dark-eyed juncos, horned larks and red-winged blackbirds, says Rosenberg. Grassland birds have suffered a 53% decrease in their numbers, and more than a third of the shorebird population has been lost.
Bird populations that have increased include raptors, like the bald eagle, and waterfowl.
"The numbers of ducks and geese are larger than they've ever been, and that's not an accident," says Rosenberg. "It's because hunters who primarily want to see healthy waterfowl populations for recreational hunting have raised their voices."
Applied ecologist Ted Simons of North Carolina State University says that trying to enumerate bird populations and tracking them over time is a daunting task with a lot of uncertainty.
"People are doing a wonderful effort to try and understand our bird populations, but the actual systems that we have in place to try and answer really tough questions like this are really far short of what we need," says Simons. "We're certainly far from having the tools and having the resources to have real high confidence in our estimates of these populations."
Still, he says, "I think it is very likely that we are seeing substantial declines in our bird populations, particularly migratory birds."
Other researchers say this continentwide decrease in bird numbers is about what they expected.
"I think that I buy the magnitude of loss," says Kristen Ruegg, a biologist at Colorado State University in Fort Collins. "Overall, the conclusions weren't necessarily surprising. I mean, they were depressing but not surprising,"
Ruegg says there have been hints that the loss was this large from a variety of sources over the past few decades. But in most cases, these were species-specific accounts of local extinctions or models of projected losses resulting from things like climate change.
This study, she says, "really sort of wakes people up to the idea that this is happening."
Elise Zipkin, a quantitative ecologist at Michigan State University, says the loss of individuals can be a big problem.
"Just because a species hasn't gone extinct or isn't even necessarily close to extinction, it might still be in trouble," she says. "We need to be thinking about conservation efforts for that."
The researchers cite a variety of potential causes for the loss of birds, including habitat degradation, urbanization and the use of toxic pesticides, notes Zipkin.
"And so I think this kind of lays the gauntlet," she says, "for people to be thinking about 'All right, how can we estimate maybe the relative contributions of these things to individual populations and their declines.' "
An earlier version of this story misspelled Kristen Ruegg's first name as Kristin.
AUDIE CORNISH, HOST:
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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