As a boy, Joel Sartore read about passenger pigeons, a once common North American bird that likely numbered in the billions, and had a difficult time understanding why people allowed the pigeon to be hunted to the point of extinction. More than five decades later, Sartore, a photographer for National Geographic and founder of the Photo Ark, is even farther from any understanding. Satore will speak about his career and the Photo Ark work, which aims to photograph all species living in zoos and wildlife sanctuaries, at 6 p.m. Thursday in the Sun Room in the Memorial Union.