Bird’s Eye View of Passenger Pigeons Nesting, by Lewis Lumen Cross, features a now-extinct species of birds, the Passenger Pigeons. The birds migrated in flocks of millions throughout midwestern North America and east of the Rocky Mountains during the 1800s. An observer’s report described their flight as ​“a mile wide, taking up to several hours to pass overhead and sounding like thunder as they approached.” The birds were hunted out of existence in the early 1900s, when their extinction became a significant marker in wildlife conservation awareness and laws.