The passenger pigeon was a bird species that once filled North American skies in flocks so enormous they darkened the sun for hours as they passed overhead. A single flock might contain a billion birds, stretching a mile wide and 300 miles long. When they roosted in forests, their combined weight broke thick tree branches.

These pigeons were about as large as mourning doves, with blue-gray backs and reddish-orange breasts. They nested in huge colonies, sometimes covering forty square miles of forest. Native Americans and early settlers hunted them for food, but the birds were so abundant that no one imagined they could disappear.

Then came industrial-scale hunting in the 1800s. People shipped trainloads of dead pigeons to city markets. Hunters cut down nesting trees and destroyed their forest homes. Within decades, billions became millions, then thousands, then hundreds. Martha, the last passenger pigeon, died in the Cincinnati Zoo in 1914.

The passenger pigeon's extinction showed how even the most abundant species can disappear when hunted without limits and robbed of the habitat they need to survive.