The passenger pigeon (Ectopistes migratorius), perhaps the world's most abundant bird species at one time, became extinct due directly to human activity. In the mid-1800s passenger pigeons travelled in flocks of astounding numbers. Alexander Wilson, the father of American ornithology, noted a flock he estimated to contain two billion birds. The artist and naturalist John James Audubon once observed a flock over a three-day period and estimated the birds were flying overhead at a rate of 300 million per hour.

The species became extinct within a span of 50 years, several factors having led to its rapid demise. The passenger pigeon was considered an agricultural pest, thus providing ample reason to kill large numbers of the birds. It was also in demand as food, largely due to the fact that nesting flocks were easily accessible. Young squabs were easy prey for hunters who knocked them from their...