Wikipedia: Passenger pigeon

Passenger pigeon (Ectopistes migratorius). The Passenger pigeon was once probably the most common bird in the world. It is estimated that there were as many as five billion passenger pigeons in the US. They lived in enormous flocks, the largest probably consisting of 2 billion birds. A flock like that would be a mile wide and 300 long, taking several days to pass. During the summer, the passenger pigeon lived throughout the part of Northern America that is east of the Rocky Mountains. In the winter, they lived in the southern US.

The passenger pigeon was a very social bird. It would live in colonies with up to a hundred nests in a single tree, and colonies stretching over hundreds of square miles.

It was hunted for food, and in the mid-1800s it was noticable that numbers were dropping. The passenger pigeon only laid one egg, so once numbers started to decline it would have taked time for them to start rising again. Almost all of the remaining quarter million passenger pigeons were killed in 1896 by sports hunters. The last known passenger pigeon died in the Cincinnati Zoo in 1914.