The historical lack of trees in the Great Plains acted as a barrier to the range expansion and recent increases in forests broke down this barrier. Increases in forest distribution along the Missouri River and its tributaries provided barred owls with sufficient foraging habitat, protection from the weather, and concealment from avian predators to allow barred owls to move westward. Decades later, increases in forests in the northern Great Plains allowed them to connect their eastern and western distributions across southern Canada. These increases in forests were caused by European-American settlers via their exclusion of fires historically set by Native Americans, by their suppression of accidental fires and by increased tree-planting; to a lesser degree this regional net forest increase was also caused by these settlers’ extirpating bison (Bison bison) and by overhunting elk (Cervus canadensis), deer (Odocoileus spp. ) and, in some areas, by extirpating beaver (Castor canadensis) and replacing native ungulates with livestock. Increase in trees in the Great Plains from fire suppression or tree planting is considered a main cause of the range expansions of many other species of birds including western kingbird (Tyrannus verticalis), eastern phoebe (Sayornis phoebe), eastern bluebird (Sialia sialis), blue jay (Cyanocitta cristata), brown thrasher (Toxostoma rufum), red-tailed hawk (Buteo jamaicensis), and American crow (Corvus brachyrhynchus).