The extinction of the Passenger Pigeon had two major causes: commercial exploitation of pigeon meat on a massive scale and loss of habitat.
Large flocks and communal breeding made the species highly vulnerable to hunting. As the flocks dwindled in size, populations decreased below the threshold necessary to propagate the species. Naturalist Paul R. Ehrlich wrote that its extinction "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.