The last passenger pigeon on Earth died just more than 100 years ago. Housed at the Cincinnati Zoo and named "Martha," she was the final holdout of a species that went from one of the planet's most abundant birds to one of its highest-profile extinctions. And it all happened within a few decades, an early stage of what many scientists now agree is Earth's sixth mass extinction event.

Martha was found dead in the bottom of her cage on Sept. 1, 1914, at the age of 29. She had been born in captivity at the Cincinnati Zoo in 1885, and scientists had tried frantically to breed her once the plight of her species became clear.

But it was too late, and Sept. 1 now marks the extinction of passenger pigeons, which had been one of eastern North America's most iconic animals. In 2010, the conservation group WildEarth Guardians declared Sept. 1 "Passenger Pigeon Day" in honor of Martha's death.

Passenger pigeons once accounted for up to 40 percent of the total bird population in the U.S., according to the Smithsonian Institution, with an estimated 3 billion to 5 billion of them occupying North America when European explorers first arrived. Many of those explorers reported seeing "countless numbers" and "infinite multitudes" of passenger pigeons flying overhead, with flocks said to be so large and dense that they sometimes blocked the sun for hours.

martha last passenger pigeon Martha, seen in 1914, wasn't helped by the pleas of conservationists. (Photo: Enno Meyer [Public domain]/Wikimedia Commons)

Yet by the early 1900s, the species had all but disappeared. Virtually no wild passenger pigeons could be found. Suddenly, Martha seemed to be the last of her kind.

Martha's relatives had fallen victim to a familiar duo of threats that still haunt endangered species today: overhunting and habitat loss. Because passenger pigeons flew in such big, dense flocks, it was easy for colonists and settlers to shoot them. Professional hunters began killing and netting them en masse in the early 19th century, selling their meat and feathers in city markets. At the same time, the vast Eastern forests where passenger pigeons nested were being rapidly cleared for new farms and cities, further decimating the birds. Still, no conservation laws existed to protect them.

Wild passenger pigeons had grown scarce by the 1890s, spurring government officials to finally heed long-ignored warnings of conservationists. One of the last large nesting colonies was found in Petoskey, Michigan, and the Michigan Legislature passed a ban on netting passenger pigeons within two miles of a nesting area. But according to Encyclopedia Smithsonian, the law was weakly enforced and led to few arrests. The state then passed a 10-year ban on all hunting of the birds in 1897, but by then hunters couldn't find many to shoot anyway.

From 1909 to 1912, the American Ornithologists' Union offered $1,500 to anyone who could find a nest or colony of passenger pigeons. No one ever did, and Martha died two years later, foreshadowing an extinction crisis that kept snowballing over the next century. The U.S. endangered species list now includes more than 2,000 total listings, and the International Union for Conservation of Nature lists 9,741 species as "endangered" worldwide and 6,127 are "critically endangered."

All five of Earth's previous mass extinctions occurred long before humans evolved, but scientists say we're seeing one now — and we may also be causing it. The passenger pigeon, along with other early casualties like the dodo and the thylacine, is now seen as a canary in the coal mine for this crisis. It's too late to save Martha and her kind, but it's not too late to make sure their deaths weren't in vain.

In a timely sign of hope, the Smithsonian National Zoo announced today that one of the most endangered animals in the U.S. is now enjoying a "record-breaking" year of recovery, with 50 offspring born in 2011. The black-footed ferret was once thought to be extinct in the wild, but this month marks the anniversary of the discovery of a small remaining group in Wyoming. And now, thanks to conservation efforts informed by Martha's cautionary tale, black-footed ferrets are making a comeback.

Below is a musical tribute to Martha by the late John Herald, a New York folk and bluegrass musician from the same ilk as Bob Dylan, Pete Seeger and Joan Baez:

Martha has long served as a symbol for the threat of extinction, but her profile is likely to grow even more. That's because, as Project Passenger Pigeon points out, Sept. 1, 2014, marked the 100-year anniversary of Martha's death — as well as a full century of lessons learned and implemented in her memory.

Editor's note: This story has been updated since it was published in September 2011.

Ode to Martha, the last passenger pigeon
One of eastern North America's most iconic animals vanished forever on Sept. 1, 1914. Now, 97 years later, the passenger pigeon has become an icon for something