Before the passenger pigeon's abrupt extinction, tales of the birds were epic. Their gargantuan flocks comprised billions of birds. When flying overhead, they obscured the sun. And the sound of their flapping wings rivaled the roar of a locomotive.

But in September 1914—100 years ago this month—the last living specimen died in captivity at the Cincinnati Zoo. The extinction of any one animal can upset an entire ecosystem. Birds, for example, provide insect control, pollination, and seed dispersal for plants. Protecting them, and other species, protects the environment, and the first step is knowing the size of individual populations.

Accounts of the passenger pigeon's numbers sound mythical, but the late A.W. Schorger, who published the definitive history of the bird in 1955, concluded that 3 to 5 billion once inhabited North America. And despite conducting his work years after the last passenger pigeon died, he is considered to have produced the best estimate of their peak population.

"No one was keeping track back then," said Stanley Temple, a retired professor of conservation at the University of Wisconsin-Madison who has digitized Mr. Schorger's research for Project Passenger Pigeon. "No one was doing anything we would think of today as a wildlife census."

Until Mr. Schorger published his book "The Passenger Pigeon: Its History and Extinction," only isolated descriptions of the birds had been available, including descriptions by the naturalists John James Audubon, Alexander Wilson and William Ross King, who reported seeing flocks at different times ranging from 1.1 billion to 3.7 billion pigeons.