In 1813, ornithologist John James Audubon was riding across the state of Kentucky when the sky was darkened by an enormous flock of passenger pigeons. The cloud of birds continued past all day. He estimated that there were as many as 1 billion pigeons in the flock; other scientists have calculated that the species once constituted 25 to 40 percent of all birds in the U.S.

Just over a century later, on September 1, 1914 at 1 p.m., Martha, the world’s last passenger pigeon, died at the Cincinnati Zoo. For the last 97 years, her body has been at Smithsonian’s Museum of Natural History, a reminder of the fragility of natural ecosystems and the looming threat of species extinction.

“Before the 1840s, they were one of the most numerous species of birds in North America,” says James Dean, collections manager in the division of birds at the museum. “They occurred over much of the United States, from the central plains all the way over to the east.” About twice the size of common pigeons, they ate mostly seeds and nuts, and typically lived in giant, dense flocks with a tight-knit social structure.

But over the latter half of the 19th century, their numbers dropped steadily. “By the 1870s and 80s, they were really starting to decline,” Dean says. “A species like this, once their populations start declining far enough, they’re just not able to sustain the colonies. They don’t reproduce enough, and the flocks get smaller and smaller.”

The initial cause was the cutting down of forests to build houses and clear farmland. “This disrupted their life cycle,” says Dean. “They were in these huge flocks, and they needed vast tracts of forests for roosting and feeding ground.”

As pigeon meat began to be sold in stores as a cheap source of protein, the threat from hunters became even more significant than that of lost habitat. The pigeons’ intensely social nature, once a strength, became a liability. “Commercial hunters would get word that a flock had showed up at some locality, and the hunters would go and set off nets or just fire repeatedly with their shotguns,” Dean says. “The flock was such a tight-knit group that even as individuals were falling and dying, the rest of the flock wouldn’t leave.” Other methods of killing were crueler, with some hunters soaking grain in alcohol to make them easier to kill.

As scientists began to realize the danger that the species might actually die out, there were some last ditch efforts to save the passenger pigeons. “The Cincinnati Zoo had a standing offer of $1,000 for a mate for Martha that had been put in place about 15 years before she died,” Dean says. But the slaughter of passenger pigeons continued regardless. “That was a period of time when conservation was just getting started,” he says. “There were really no laws to protect the birds at all.” The last confirmed report of a specimen in the wild was in 1900.

Because the birds had evolved to breed in enormous colonies, all attempts at breeding small groups in captivity failed. As Martha aged, researchers realized the species was doomed. When she finally died, it was widely known that she was the last of her kind. “There was a lot of sadness. This was an early recognition of species extinction,” says Dean. “The zoo had roped off the area around her cage and instituted a quiet zone.”

Afterward, the zoo donated Martha’s body to the Natural History Museum. “They froze her up in a 300 pound block of ice and shipped her to the scientists at the Smithsonian to study and preserve,” Dean says. “It came here and she was prepared as a taxidermy mount, and also parts of her internal organs were saved here in our fluid collection.”

Nearly a century later, the story of the passenger pigeon remains a troubling portent for those concerned about the environment. “There are other species of birds, like the Carolina parakeet, that the last known individual died,” says Dean. “But we still get more phone calls and inquiries about Martha than any other. It seems like she has become an icon of the conservation movement for saving species.”