The Passenger Pigeon, Ectopistes migratorius, was once probably the most common bird in the world. It is estimated that there were as many as 5 billion pigeons of this species in the United States. They lived in flocks as large as a mile wide and 300 miles long and containing two billion birds. It lived in colonies with up to a hundred nests in a single tree. They resided throughout much of North America east of the Rocky Mountains. They wintered in the southern U.S.
They were hunted into extinction by humans. It was hunted for food, hog feed, as live targets for trap shooting and as agricultural fertilizer. Slaves and servants in 18th and 19th century America often only ate pigeon meat as there was often no other for them to eat. Most restaurants in the Eastern United States also served Passenger Pigeon to customers.
In the mid-1800s, it was noticeable that their numbers were dropping. The passenger pigeon only laid one egg at a time, so once numbers started to decline it would have taken time for them to start rising again. Almost all of the remaining quarter-million Passenger Pigeons were killed in a single day in 1896 by sport hunters who knew they were shooting the last wild flock. The last wild Passenger Pigeon was shot by a 14-year-old boy in Ohio in March of 1900.
There were other significant reasons for its extinction as well.
Deforestation was harmful as the birds relied on acorn and beech mast for breeding.