The Passenger Pigeon, once probably the most numerous bird on the planet, made its home in the billion or so acres of primary forest that once covered North America east of the Rocky Mountains. Their flocks, a mile wide and up to 300 miles long, were so dense that they darkened the sky for hours and days as the flock passed overhead. Population estimates from the 19th century ranged from 1 billion to close to 4 billion individuals. Total populations may have reached 5 billion individuals and comprised up to 40% of the total number of birds in North America (Schorger 1995). This may be the only species for which the exact time of extinction is known.
Its food consisted of the acorn of the numerous species of oak, the seeds of beech, chestnut, maple, elm, and other hardwoods, of pine and hemlock. They did also eat the fruits and berries of bushes and shrubs. Angleworms, snails, caterpillars, and soft-bodied insects such as grasshoppers helped to vary the vegetarian diet. From the frequent mineral springs and licks the bird gratified its craving for salt, a condiment eagerly sought by all grain feeders.
Passenger Pigeons bred in large colonies, with up to 100 nests in a single tree. Nesting colonies could cover from 30 up to 850 square miles of forest. The nest was loosely made of small twigs. Generally, one egg was laid and incubated by both parents. Both parents tended the chick, and after about 2 weeks, the chick, still unable to fly, would be abandoned. The entire flock would depart, and the chicks would drop to the ground. After a few days, the chicks would begin to fly and to care for themselves (Fuller, 1987).
The extinction of the Passenger Pigeon was caused by extensive hunting, and disappearance of their habitat, when the forests were cleared and converted to farmland. The proximate causes were Newcastle disease and the breakdown of social facilitation. It had suggested that the Passenger Pigeon somehow depended on large numbers for its survival. As the flocks dwindled in size, it was doomed to disappear.