Martha, the last passenger pigeon

Smithsonian magazine this month features Marta, a passenger pigeon who died in 1914, having lived alone at the Cincinnati Zoo despite a standing reward of $1,000, back when that was real money, for anyone who could bring in a live male. This article says the last wild sighting was in 1900: there are a few later ones mentioned in zoological or cryptozoological literature, but there's no doubt the species is now extinct. I've seen Martha, and there is kind of a pall of sadness surrounding her exhibit. People stop talking when they get close.
Audubon once saw a flock pass over his head that he estimated contained a billion individuals. A billion. At least a quarter of all birds in the U.S. at one time were passenger pigeons. If we can exterminate abundance like that, we can exterminate anything. Food for thought.