The passenger pigeon may have been the most abundant bird since archaeopteryx fluttered its first feather back in the late Jurassic. John James Audubon rode the 55 miles from Henderson, Kentucky, to Louisville one day in autumn 1813, and through the whole long day, he rode under a sky darkened from horizon to horizon by a cloud of passenger pigeons. He estimated that more than a billion birds had passed over him. In 1866, a cloud of birds passed into southern Ontario. It was a mile wide, 300 miles long, and took 14 hours to pass a single point. Latter-day estimates suggest something in excess of 3.5 billion birds in that flock. The continental population may have been as high as 6 billion, a number that could represent anywhere from 25 to 40 percent of all the birds in North America 350 years ago.