As railroads penetrated the upper Middle West after the Civil War, many millions of pigeons were shipped to cities along the Atlantic seaboard, since, by then, clearing of oak and beech forests and hunting had already exterminated the birds on the East Coast. Extinction of the Passenger Pigeon came with stunning rapidity. Michigan was its last stronghold; about three million birds were shipped east from there by a single hunter in 1878. Eleven years later, 1889, the species was extinct in that state. Although small groups of pigeons were held in various places in captivity, efforts to maintain those flocks failed. The last known individual of the species, a female named Martha, died in 1914 in the Cincinnati Zoo and is now on display in the U.S. National Museum of Natural History.
Of course, market hunting ended as soon as harvesting the birds was no longer economically profitable. That point was reached when tens of thousands of the birds still flew within large stretches of suitable habitat. Much of that habitat still exists today, although many of the largest nut-producing trees that were common in the heyday of the pigeon were logged. Why, then, did the birds go extinct? No one knows for sure, but it appears that to survive they needed to nest in vast colonies. Perhaps this permitted them to "swamp" predators with their enormous numbers, so that the relatively few predators in the area of a roost were unable to make a significant dent in the huge breeding colonies. And since these colonies dispersed as soon as breeding was over, predators were prevented from building up their populations on the basis of such an ephemeral resource. In any case, the fate of the Passenger Pigeon illustrates a very important principle of conservation biology: it is not always necessary to kill the last pair of a species to force it to extinction.
Sad to say, the lesson of the Passenger Pigeon has not been learned. At the present time the White-crowned Pigeon is threatened by the horrendous slaughter of nesting birds on its Caribbean breeding grounds.