Today marks the hundredth anniversary of the extinction of passenger pigeons, a once-plentiful breed of birds that was hunted to extinction by early settlers who came from Europe to America. Although it is too late to bring back the passenger pigeons, scientists today use the birds as a beacon of hope for a day when species will no longer be driven to extinction by humans, or even for a time when scientists can bring animals back from extinction using cloning technology.

Passenger pigeons were famed for their docility; the birds would stay still even as people approached and shot them, making them easy targets. There were once an estimated nine billion passenger pigeons in America when the Mayflower landed -- more than double the number of all birds in North America now. The pigeons traveled in huge groups; one bystander recalled the pigeons actually darkening the sky. But Americans hunted the pigeons with great relish. One legendary tale, which Bill Bryson recounts in his book "Made in America", says that over a hundred pigeons were killed with one shot from a gun.

The passenger pigeon was not the only animal to go extinct in the late 1800s, but it was one of the sharpest falls, and one of the most widely publicized.

"It became the icon of extinction," said Mark Barrow, who wrote a book about extinction called "Nature's Ghosts".

There is a group of researchers looking for ways to bring the passenger pigeon back by using cloning. Others are studying the pigeon's DNA to learn what they can about the bygone bird. Even if scientists never bring back the passenger pigeon, there is no doubt that humans have learned an important lesson about conservation from the extinction of the bird.

Martha, the last passenger pigeon who died 100 years ago today, is stuffed and on display in the Smithsonian National Museum of Natural History in Washington, DC.

In the early years of European settling in North America, the passenger pigeon was fed to pigs and eaten by humans. Yet many of the birds were killed for sport rather than for food. Hunters attacked the birds with gusto. One of the first acts ever passed in America to protect wildlife, the Lacey Act, was passed in 1900 because the government worried about the fast decline of the passenger pigeon and what might happen to other species. However, the act came too late for this tragic species. After the act was passed a few passenger pigeons continued to live in zoos, but they did not breed well in captivity and the birds soon went completely extinct.