A hundred years ago this month, the passenger pigeon went extinct. The last passenger pigeon was named Martha, and she died in a zoo. She was the last of billions; the passenger pigeon had an unbelievably large population in its heyday, accounting for roughly 25% of birds in North America at one time.
In this video, the Field Museum's Emily Graslie walks us through the history of this long-lost bird. Saddest/weirdest part: the use of "Judas Pigeons," discussed around 2:40.
For an in-depth look at the passenger pigeon and its demise, check out Joel Greenberg's book A Feathered River Across the Sky. For a quicker read, I suggest 10 Facts About the Extinct Passenger Pigeon.