The Passenger Pigeon (Ectopistes migratorius) was once so abundant that migrating flocks passing overhead could darken the sky for several days. One 19th-century account estimated more than 2.2 billion birds in a single flock; another calculated 136 million birds in a Wisconsin nesting area. In 1813, John James Audubon reported a migrating flock in Kentucky that passed undiminished for three days overhead: “the light of noon-day was obscured as by an eclipse.”
Yet in the second half of the 19th century, pressed by overhunting and deforestation, Passenger Pigeon populations began to decline. Within a few decades, they became scarce. The species disappeared altogether 100 years ago with the death of Martha,the last known Passenger Pigeon, who died in the Cincinnati Zoo on September 1, 1914.