In the early part of the nineteenth century, passenger pigeons were so plentiful in the United States that no one thought they could ever disappear. Flocks of these birds were so large that they darkened the sky as they flew over. In 1810,Alexander Wilson, a naturalist, claimed that he saw a flock in Kentucky with more than 2 billion birds.The flock, he said,was a mile wide and 240 miles long. By the 1890s,however, it was hard to find a passenger pigeon, and by 1914 they had all disappeared. Human settlement had replaced much of their habitat, and the pigeons had been hunted for food. Millions of the birds had been shipped to cities, where they were eaten much the same way as Americans eat chickens today. The passenger pigeon has come to symbolize extinction...