The passenger pigeon, often thought more closely related to the mourning doves than to other pigeons, was the most abundant bird in the world, maybe in all of history. Native Americans and, on a much larger scale, European settlers harvested this seemingly unlimited resource. If a flock took three days to pass overhead, what harm could nets, arrows, or even shotguns really do?
In a tragic lesson of conservation, we learned that no species is immune to overpredation. I've personally visited Martha, last of her species, now a sad, stuffed exhibit in the Smithsonian.
Long after the 1914 extinction (there are scattered reports indicating Martha was not the last, but there's no question the bird is gone for good), scientists have studied the species' DNA and found where it really belonged - with the other New World pigeons, but not very close to them. Ornithologist Kevin Johnson says, "This bird is pretty diverged from its nearest relatives, meaning it had a unique place in the world. It represented a unique lineage that's now gone."