The Passenger Pigeon was no mere bird, he was a biological storm. He was the lightning that played between two biotic poles of intolerable intensity: the fat of the land and his own zest for living. Yearly the feathered tempest roared up, down, and across the continent, sucking up the laden fruits of forest and prairie, burning them in a travelling blast of life. Like any other chain reaction, the pigeon could survive no diminution of his own furious intensity. Once the pigeoners had subtracted from his numbers, and once the settlers had chopped gaps in the continuity of his fuel, his flame guttered out with hardly a sputter or even a wisp of smoke.
Aldo Leopold, 1947, On a Monument to the Pigeon
Legendary among ornithologists and laypeople alike as a symbol of staggering abundance on the one hand and of human greed and indifference on the other, the Passenger Pigeon is arguably North America's best known extinct species. Historical accounts of its huge flocks appear beyond belief were they not so consistent among independent observers for more than 2 centuries. “But the most remarkable characteristic of these birds is their associating together, both in their migrations, and also during the period of incubation, in such prodigious numbers, as almost to surpass belief; and which has no parallel among any of the other feathered tribes on earth, with which naturalists are acquainted” ( Wilson 1812 : 102–103). It is estimated that the Passenger Pigeon was once the most abundant land bird in North America, comprising an estimated 3 billion to 5 billion individuals, perhaps a quarter of the continent's avifauna ( Schorger 1955 ). The species occurred only in North America, primarily east of the Rocky Mountains, and bred almost exclusively in the eastern deciduous forest. Despite its vast flocks, this pigeon was extinct in the wild by the end of the nineteenth century. Its last representative, the fabled Martha, died in 1914 in the Cincinnati Zoo.
The wild Passenger Pigeon was never the subject of ornithological research, although certain aspects of its biology, including behavior, were studied in captive birds. Nevertheless, numerous historical accounts from newspapers, magazines, and personal journals allow one to reconstruct many aspects of its ecology and speculate intelligently about the causes of its extinction. Much of this information has been summarized in volumes by Mershon ( Mershon 1907 ), French ( French 1919 ), Mitchell ( Mitchell 1935 ), and Schorger ( Schorger 1955 ; as well as his unpublished notebooks that were invaluable to this account), and previous species summaries by Bendire ( Bendire 1892a ), Forbush ( Forbush 1927b ), and Townsend ( Townsend 1932b ).
The key to the Passenger Pigeon's abundance was its nomadic flocking behavior, which allowed it to exploit seasonally superabundant crops of mast (beechnuts [Fagus grandifolia] and acorns [Quercus spp.]) that were unpredictable in space and time. Passenger Pigeons bred almost exclusively in huge colonies of at least hundreds of thousands of pairs. Aggregating in such immense numbers allowed the species to satiate any potential predators, until they attracted the ultimate predator—humans armed with nineteenth-century technology. The dramatic decline to extinction, which occurred over a period of 30 years in the latter half of the nineteenth century, was driven by a combination of habitat loss and human disruption of essentially every nesting colony. During this period, there were no uninterrupted and completely successful mass nestings, which were necessary to sustain the population.