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."
Another significant reason for its extinction was deforestation. The birds traveled and reproduced in prodigious numbers, satiating predators before any substantial negative impact was made in the bird's habitat. It is unclear the extent to which deforestation impacted important habitats for the species, since large swaths of forested area continued to be present throughout the habitat of the Passenger Pigeon well into the latter half of the 19th century.