The most beloved bird in history may very well have been a 29-year-old pigeon by the name of Martha. It was the early 1900s, and Martha was at the height of her fame. Perched on her humble roost at the Cincinnati Zoo, she was an object of fascination to the thousands of visitors who lined up just to catch a glimpse. Martha may not have looked the part of an animal celebrity, but she was hardly average—in fact, she was the very definition of one of a kind. After the death of her companion George in 1910, Martha had become the world's last-living passenger pigeon.

There was a time not long before when her kind accounted for more than a quarter of the birds in North America and may have been the most abundant bird species on the planet. Passenger pigeons used to travel at 60 miles an hour in flocks a mile wide and 300 miles long. Witnesses compared them to a train rumbling through a tunnel. 

Ironically, the passenger pigeons' very abundance may have spelled their doom. An agricultural pest and reliable source of protein, they became easy targets for hunters who killed them in the tens of thousands. In a matter of decades, a bird that once numbered in the billions was reduced to a few, and then, eventually, to one.

Martha, who'd grown up in captivity, had no offspring of her own. At 1 p.m. on September 1, 1914, Martha fell from her perch, never to rise again—one of the rare occasions in which historians could identify the exact moment of a species' extinction.

Of course, the real tragedy was that the loss of the passenger pigeon was neither surprising nor unique. For as long as the Earth has sustained life, it has also seen the permanent disappearance of life forms, the dinosaurs being a particularly extreme example. But Martha's high-profile death trained national attention on an alarming new trend. Close to a thousand animal species alone have died off in the last 500 years, and the trend is only getting worse.