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The skies over North America were once filled with passenger pigeons. In the 19th century, birdwatchers described clouds of pigeons so big they blocked the sun. About a hundred years ago, the species went extinct. NPR's Nell Greenfieldboyce reports that scientists have been trying to learn more about how the species got killed off by looking at the birds' DNA.
NELL GREENFIELDBOYCE, BYLINE: Passenger pigeons are still around. They're just dead, like Martha. She's perched on a branch in a glass box at the Smithsonian's National Museum of Natural History. Compared to a modern city pigeon, Martha looks thin and delicate.
HELEN JAMES: She's somber looking. She's not very brightly colored. She has a long, tapering tale, more like a mourning dove.
GREENFIELDBOYCE: Helen James is the curator for birds here. She says Martha lived at the Cincinnati Zoo and was believed to be the last remaining member of her species until September 1, 1914, when she was found lifeless at the bottom of her cage.
JAMES: It was really a wake-up call. It had been the most abundant species of bird in North America.
GREENFIELDBOYCE: And yet hunting and a changing landscape wiped it out in just a few decades. That fascinates Beth Shapiro at the University of California, Santa Cruz.
BETH SHAPIRO: Why did little, tiny populations of this bird not somehow survive in some refugial forest somewhere? Why did they just go from billions to none?
GREENFIELDBOYCE: To try to find out, she and some colleagues have been extracting DNA from scores of passenger pigeons preserved in museum collections.
SHAPIRO: Curators were kind enough to let us chop off a tiny little piece of skin from the bottom of one the toes, a little toe pad.
GREENFIELDBOYCE: They were able to get the entire DNA sequence from several of these birds and compare it to the DNA of their closest living relative, the band-tailed pigeon. Now, a few years ago, there was a previous genetic study. It suggested that the passenger pigeons' population size might have fluctuated wildly through history, possibly making them especially vulnerable to extinction.
This new study didn't find that. Instead, it suggests an enormous, stable population that existed for a really long time, meaning that these birds likely evolved to depend on each other. Gemma Murray is one of the researchers at the University of California, Santa Cruz.
GEMMA MURRAY: So it's known that they collaborated in finding food, and they also collaborated in rearing young.
GREENFIELDBOYCE: When their numbers plummeted in the 19th century, the big flocks they'd always known were suddenly gone.
MURRAY: They weren't well-suited to then - trying to live in smaller and most disparate populations.
GREENFIELDBOYCE: The work appears in the journal Science, and it impressed Helen James, the curator at the Smithsonian.
JAMES: Well, I think it's hyper-cool.
GREENFIELDBOYCE: She shows me a tray filled with more than a dozen passenger pigeons laid out in rows.
JAMES: You know, we can learn things that we just never imagined 10 years ago that we would be able to learn about the history of these birds.
GREENFIELDBOYCE: To her, these birds are a reminder that even though conservation efforts often focus on small groups of rare animals, something really common can be at risk, too. Nell Greenfieldboyce, NPR News.
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