Audubon estimated that an average flock of these pigeons contained a billion and a quarter of birds, which consumed more than eight and a half million bushels of mast in a day's feeding. They were slain by millions during the middle of the last century, and from one region in Michigan in one year three million Passenger Pigeons were killed for market, while in that roost alone as many more perished because of the barbarous methods of hunting them. They supplied a means of living for thousands of hunters, who devastated their flocks with nets and guns, and even with fire. Yet so vast were their numbers that after thirty years of observation Audubon was able to say that "even in the face of such dreadful havoc nothing but the diminution of our forests can accomplish their decrease." Many theories have been advanced to account for the disappearance of the wild pigeons, among them that their migration may have been overwhelmed by some cyclonic disturbance of the atmosphere which destroyed their myriads at one blow. The big "nesting" of 1878 in Michigan was undoubtedly the last large migration, but the pigeons continued to nest infrequently in Michigan and the North for several years after that, and until as late as 1886 they were trapped for market or for trap-shooting. Therefore the pigeons did not become extinct in a day; nor did one tremendous catastrophe wipe them from the face of the earth. They gradually became fewer and existed for twenty years or more after the date set as that of the final extermination. At one time the wild pigeons covered the entire north from the Gaspé Peninsula to the Red River of the North. Separate nestings and flights were of regular yearly occurrence over this vast eastern and northern expanse. Gradually civilization, molestation and warfare drove them from the Atlantic seaboard west, until Michigan was their last grand rendezvous, in which region their mighty hosts congregated for the final grand nesting in 1878. As late as 1845 they were quite numerous on the Gaspé Peninsula of Quebec, but disappeared from there about that time. The habits of the birds were such that they could not thrive singly nor in small bodies, but were dependent upon one another, and vast communities were necessary to their very existence, while an enormous quantity of food was necessary for their sustenance. The cutting off of the forests and food supply interfered with their plan of existence and drove them into new localities, and the ever increasing slaughter could not help but lessen their once vast numbers.