A hundred years ago on Monday, a once-mighty species became extinct. At the Cincinnati Zoo, a passenger pigeon named Martha died at the age of 29.
People coming to the zoo to see the last passenger pigeon were disappointed by the bird, which barely budged off its perch. As Joel Greenberg writes in his recent book A Feathered River Across the Sky, some threw sand into its cage to try to force it to walk around. But on that first day of September a century ago, Martha no longer had to put up with such humiliations.
It was a quiet end to a noisy species. As recently as the mid-1800s, deafening flocks of billions of passenger pigeons swarmed across the eastern half of the United States. But they proved no match for humans, whose rapidly advancing technology drove the birds to extinction in a matter of decades.
Other species were also spiraling toward extinction in the late 1800s, but the destruction of the passenger pigeon happened on full public display. "It became the icon of extinction," says Mark Barrow, a historian at Virginia Tech and the author of Nature's Ghosts.
A hundred years later, the passenger pigeon remains iconic and is inspiring extravagant new technological feats. One team of scientists is even trying to bring the species back from extinction, using genetic engineering and cloning. Others are analyzing bits of passenger pigeon DNA to reconstruct its lost ways of life. (Read "Bringing Them Back to Life" in National Geographic magazine.)
And whether scientists are able to bring passenger pigeons back or not, the birds may still offer lessons about keeping other species from following it into oblivion.
Animation by Donald E. Hurlbert, Smithsonian Institution
A Technological Coup de Grâce
It was hard for early naturalists to imagine that the passenger pigeon could ever become extinct. But they didn't realize that a technological revolution was about to hit.
"The telegraph allowed word to go out: 'The pigeons are here,'" says David Blockstein, a senior scientist at the National Council for Science and the Environment and a founder of Project Passenger Pigeon. Thousands of hunters would then jump on newly built trains to ride out to wherever the pigeons had settled and start slaughtering them.
The hunters weren't just killing the birds to feed their families, however. Pigeons would be stuffed into barrels and loaded back onto the trains, which would deliver them to distant cities, where they'd be sold everywhere from open air markets to fine restaurants. "Technology enabled the market," says Blockstein.
Soon this technology-driven slaughter was decimating the passenger pigeon. Its decline was so worrisome that Congress passed the Lacey Act, one of the first laws to protect wildlife in the United States. The Lacey Act would eventually help protect many species, but for the passenger pigeon it came too late.
In 1900, the year in which the act was made into law, naturalists spotted a single wild passenger pigeon in Ohio. They never saw another one in the wild again.
For the next 14 years, the species clung to existence in a few zoos. But the birds proved to be poor breeders in captivity. Martha, the last of her kind, was barren.
DNA and De-Extinction
While technology spelled the doom of passenger pigeons, some scientists believe they can use technology to bring the species back. When Martha died, biologists didn't even know that genes are encoded in DNA. Now they have the technology to extract DNA from preserved passenger pigeons in museum collections.
In 2012, a group of scientists launched a project now dubbed the Great Passenger Pigeon Comeback to create cloned passenger pigeons—or at least birds genetically engineered to have passenger pigeon traits.
Two years later, on the hundredth anniversary of the species' extinction, project scientists are still hard at work. But they can't say when—or even if—the bird will fly again.
Can the red-breasted American passenger pigeon, hunted to extinction a century ago, be revived from museum specimens? Yes, say geneticist George Church of Harvard University and his colleagues.
"It's all moving forward at the speed of science," jokes Ben Novak, a project member at the University of California at Santa Cruz.
Novak and his colleagues can't extract an intact passenger pigeon genome from museum specimens. So they're hoping they can do the next best thing: retool the genome of a living bird species so that it gives rise to a passenger pigeon.
The closest living relatives of passenger pigeons are band-tailed pigeons, which live in the western United States. The project scientists hope that their museum DNA fragments will include some unique sequences that play important roles in producing passenger pigeons—whether they help build the bird's distinctive wedge-shaped tail, its red breast, or its ultra-social behavior. It may also be possible to look at living bird species with some of these traits to pinpoint their genetic basis.
Once the scientists have created a passenger pigeon-like genome, they will insert this altered DNA into reproductive cells in band-tailed pigeon embryos. The birds will mature, mate, and lay eggs. And out of those eggs will emerge passenger pigeons—or at least birds that are a lot like the way passenger pigeons used to be.
Reverse-Engineering a Species
Just about every step in the plan for de-extinction will take the project scientists into uncharted scientific territory. The scientists can't start editing the band-tailed pigeon genome until they have a map showing the location and sequence of all its genes. But no such map exists, so Novak and his colleagues are building one.
Nor has anyone ever cloned a bird. Next year, the project researchers hope to take some steps in that direction with preliminary experiments on reproductive cells from band-tailed pigeons.
Even if the scientists end up with a brood of passenger pigeon chicks in a few years, they will still be a long way from successfully reestablishing the species. That's because the naturalists who had the opportunity to observe passenger pigeons left a lot of open questions about the natural history of the birds.
No one knows how much of the bird's social behavior is instinctive from birth, for example, and how much they must learn from older birds within an established flock. Nor does anyone know how big a passenger pigeon population has to be to sustain itself, or what range of ecosystems can support it.
Novak and his colleagues are investigating these questions, too. "We are interested in figuring out when passenger pigeons reached their highest numbers," he says. Seeing how much passenger pigeon DNA varied among individuals over time can give him and his colleagues some clues to the size of the pigeon population over the past few thousand years. It's possible that the giant flocks that early naturalists wrote about were a peak in a long-term cycle of giant booms and busts.
It's also possible that the flocks were an unprecedented explosion brought on when Europeans pushed Native Americans out of the bird's range. Understanding the pigeon's past could help Novak and his colleagues give it a future.
Preventive Measures
Even if we never resurrect the passenger pigeon, however, Blockstein sees many lessons in its disappearance that apply to protecting threatened species today.
It's a mistake to assume that a species with a big population is immune to extinction, for example. "The endangered species category is really all based on numbers, rather than biology," he explains. Even a species with billions of members may have a biological Achilles' heel that makes it vulnerable to human pressure.
To appreciate a species' true risk, we have to understand not just its biology, but also our own technological advances. In the 1800s, the new technology included the telegraph and trains. Now it includes global positioning systems, cell phones, and huge fishing vessels. "We have factory ships that can vacuum up the ocean," says Blockstein.
A few years ago, Blockstein got a chance to meet Martha. After she died, she was packed in a 300-pound block of ice and shipped to the Smithsonian Institution, where she was dissected, stuffed, and mounted. She was moved around over the years and taken off public display in 1999.
This summer, Smithsonian curators brought her out again for a new exhibit called "Once There Were Billions: Vanished Birds of North America." You can see Martha yourself there; the exhibition will be on view until October 2015.
"You're filled with awe to see the last of its species," says Blockstein. "But there's so much more to the passenger pigeon than this last individual that ended up living out her time in a cage."
Follow Carl Zimmer on Twitter and read his science blog The Loom on NationalGeographic.com.
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- "Bringing Them Back to Life"
- "Your De-Extinction Questions Answered"
- "The Promise and Pitfalls of Resurrection Ecology"
- "What We've Lost: Species Extinction Time Line"
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