The passenger pigeon, whose name derived from the French word passager, meaning “passing by” due to its migratory habits was a bird similar to the mourning dove that was endemic to North America.
The adult male had grey upperparts tinged with olive brown and a darker slate grey lower back and rump. The wings were pale grey with irregular black spots and dark brown primaries and secondaries with a narrow white edge. The two central tail feathers were brown, while the rest of the tail was white. On the underparts, the breast was pink, and the belly was white.
The head, nape, and hindneck were blue-grey and on the sides of the neck and upper mantle were display feathers that were bronze, violet, or gold, depending on the light. The bill was black, the eyes were red with a purple eye-ring, and the legs and feet were a bright coral colour.
Female passenger pigeons were duller and slightly smaller than the male. The upperparts were brown, and the lower throat and breast were greyish-brown fading to white on the belly. The wings had more spots than the male and the outer edges of the primaries were edged with buff.
On the head, the forehead, crown, and nape were greyish-brown and the feathers on the side of the neck were less iridescent than the male. The eye was orange with a grey-blue orbital ring, and the legs and feet were a paler red.
The passenger pigeon was found across the northern and eastern parts of North America where it lived in deciduous forests and constantly migrated within its range in search of food and shelter. Out of breeding season it travelled south, spending winters in large swamps or forested areas across the southern United States as well as Cuba and Mexico in particularly severe winters.
At the height of its population, it was estimated that numbers of passenger pigeons reached 5 billion, accounting for up to 40 percent of the total bird population of America. Colonies of breeding passenger pigeons were so large they were referred to as cities which could measure thousands of hectares in size. They travelled in huge flocks sometimes over a kilometre wide, in narrow columns that twisted and turned, similar to the murmurations of starlings.
Marvelling at one of these spectacles, Audubon wrote:
In the autumn of 1813, I left my house at Henderson, on the banks of the Ohio, on my way to Louisville. In passing over the Barrens a few miles beyond Hardensburgh, I observed the Pigeons flying from north-east to south-west, in greater numbers than I thought I had ever seen them before, and feeling an inclination to count the flocks that might pass within the reach of my eye in one hour, I dismounted, seated myself on an eminence, and began to mark with my pencil, making a dot for every flock that passed. In a short time finding the task which I had undertaken impracticable, as the birds poured in in countless multitudes, I rose, and counting the dots then put down, found that 163 had been made in twenty-one minutes. I travelled on, and still met more the farther I proceeded. The air was literally filled with Pigeons; the light of noon-day was obscured as by an eclipse, the dung fell in spots, not unlike melting flakes of snow; and the continued buzz of wings had a tendency to lull my senses to repose.
Deforestation and hunting contributed to the species’ rapid decline and in less than a century the breeding population had shrunk to such a size that it could no longer propagate. The last wild bird was shot in 1901, and Martha, the final bird kept in captivity, died on the 1st September 1914, in Cincinnati Zoo.