Of all the animals that man has caused to become extinct, the story of the passenger pigeon makes me saddest.

From Wikipedia:

The passenger pigeon (Ectopistes migratorius) or wild pigeon was a species of pigeon that was once the most common bird in North America. It lived in enormous flocks. During migration, flocks could be 1 mile (1.6 km) wide and 300 miles (500 km) long, taking several hours to pass and containing up to two billion birds.^[1][2]

Some estimate that there were three billion to five billion passenger pigeons in the United States when Europeans arrived in North America.^[3] Others argue that the species had not been common in the Pre-Columbian period, but their numbers grew when devastation of the American Indianpopulation by European diseases led to reduced competition for food.^[4]

During the 19th century, the species went from being one of the most abundant birds in the world to extinction.^[5] At the time, passenger pigeons had one of the largest groups or flocks of any animal, second only to the Rocky Mountain locust.