Doing some reading on the passenger pigeon last night... some pretty amazing accounts. Scientists during the 1800s estimated that these were numerous enough to form one-quarter of total bird numbers in Eastern North America. Migrating flocks would take over a day to pass overhead with the noise of flying and roosting birds keeping people awake at night. And as YC wrote, habitat loss was as responsible for the extinction as shooting... the loss of natural forest cover that provided passenger pigeon's food source.