Today is the anniversary of noteworthy event. Exactly 100 years ago — on September 1, 1914 — the passenger pigeon was driven to extinction.

they were probably the most numerous bird species on the planet

You may not have heard of this species. But there were once incomprehensibly huge numbers of these birds. When Europeans first arrived to North America, there were somewhere between three and five billion of them in existence. (In comparison, there are now around 10.8 million common pigeons on the continent today.) At the time, they were probably the most numerous bird species on the planet.

Over the years, however, the passenger pigeons were decimated by hunting, deforestation, and natural population variation. By 1914, there was exactly one bird left: Martha, a resident of the Cincinnati Zoo.

At 1 pm on September 1, Martha died, likely of old age. Her body was stuffed and preserved, and is now on display at the Smithsonian Museum of Natural History — a potent symbol of our ability to drive any species to extinction.

How a species went from billion to zero

The same characteristics that made the passenger pigeon so numerous made the species especially vulnerable to human activities.

The pigeons had evolved to live in a few vast, nomadic flocks, which would migrate from forest to forest in search of ripening crops of seeds and nuts. Nowadays, it's hard for us to comprehend just how huge these flocks were: the biggest ones were hundreds of miles long, and for an observer on the ground, they'd literally darken the sky for hours as they passed.

The flocks migrated from Quebec to Texas to Florida, devouring each area's stock of seeds for about a month at a time before moving on, and not returning to a given area for a decade or longer, giving it time to recover. It was an effective strategy, but it required something that would soon disappear: a continent's worth of rich, unbroken forest, so at any given time, one spot could provide plentiful amounts of food for millions of birds.

Soon after Europeans arrived in North America, they set to work clearing these forests and turning them into farmland. By 1872, half of the continent's virgin forest cover had been cut down. Having a smaller pool of forest to subsist on destabilized the passenger pigeons' populations. Deforestation was the first nail in the birds' coffin.

Next came hunting. Starting in the 19th century, passenger pigeons were professionally hunted, packed in barrels, and sold as a cheap protein source in cities.

hunters used nets, guns, fire, and toxic fumes to kill thousands of birds at a time

It was so cheap partly because the birds were so easy to kill. Passenger pigeons had evolved to tolerate a baseline level of death from natural predators like squirrels and hawks: at any given site, the number of carnivores present simply couldn't make a dent in the huge populations, so enduring it made more evolutionary sense than fleeing.

But human hunters could make a dent. They used nets, guns, fire, and even toxic fumes to kill tens of thousands of birds in a single day. Some individual hunters shipped millions of birds each year to East Coast cities, selling them for as little as a penny per bird. They were most commonly eaten in pigeon pie.

There's also new evidence that natural variation in the size of the pigeon flocks may have played a role in their disappearance. In a paper published in July, geneticists sequenced the DNA from a three preserved passenger pigeon specimens kept in museums.

The birds' level of genetic diversity suggests that even without humans around, their population size fluctuated wildly — going from mere millions to billions and back over time — due to natural factors such as acorn production. It's possible that the species' numbers were already declining when humans began to clear their forests and hunt them in earnest.

Due to all of these factors, the huge flocks were depleted over the course of decades. Because each bird laid just a single egg annually, they were unable to build their numbers back up. By the 1860s, people began to notice that the passenger pigeons seemed to be disappearing.

The very last passenger pigeon

Some conservationists tried to save the birds, and a few state legislatures even passed laws restricting the hunting of birds in nesting areas. But they were weakly enforced — and though it was in the hunters' own long-term self-interest to let some birds reproduce, they didn't follow the rules.

Most importantly, the restrictions came too late. The birds were unable to live in smaller flocks — because that made them vulnerable to natural predators — and there was enough momentum built into their decline that it couldn't be stopped.

the last verified sighting of a bird in the wild came in 1900

Some scientists tried to breed the birds in captivity. As the number of wild birds declined, University of Chicago zoologist Charles Otis Whitman bred about a dozen passenger pigeons, and in 1902, he sent one — named Martha Washington — to the Cincinnati Zoo, which also began attempting a breeding program.

Soon, though, Whitman's other birds died out, and the zoo was unable to find a mate for Martha. The last verified sighting of a bird in the wild came in 1900, when a boy in Ohio killed one with a BB gun.

For years, Martha survived as perhaps the only member of her species left in the world. On September 1, 1914, she died of natural causes, at an estimated age of 29. She was quickly frozen, and sent by train to the Smithsonian, where curators stuffed her and put her on display.

In the years since, Martha has become something of a symbol of two contradictory human qualities: the ease with which we drive other species extinct and our occasional desire to do something about it. The Smithsonian argues that her death helped catalyze the movement for conservation laws, and one group is now even trying to use modern science to bring the species back to life, though it may not be possible.

We've driven lots of species extinct, and the problem is only getting worse. What makes this story unique is that, usually, when we do so, we have no idea where or when the last individual died. But we know the very last passenger pigeon to fly the earth — and we have her body on display in a museum.

Further reading: The world is on the brink of a mass extinction. Here's how to avoid that.