In Part I (Livezey, 2009), I presented the chronology and distribution of the range expansion of Barred Owls (Strix varia) from the late 1800s to the present. Here I explore what had prevented Barred Owls from expanding their range westward during recent millennia and what allowed them to do so during the past century. Using strength-of-evidence analysis, I evaluate the plausibility of the five ecological or behavioral changes proposed in the literature to have facilitated the range expansion. From this evaluation, three of these changes appear to be implausible, one appears to be plausible after modifying its location, and one appears to be very plausible. For the very plausible one, I score seven ecological changes that may have affected it using five strength-of-evidence criteria. Overall, it appears 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 apparently provided Barred Owls with sufficient foraging habitat, protection from the weather, and, possibly, concealment from avian predators to allow Barred Owls to move westward. Decades later, increases in forests in the northern Great Plains allowed Barred Owls to connect their eastern and western distributions across southern Canada. These increases in forests evidently were caused by European settlers excluding fires historically set by Native Americans, suppressing fires and planting trees. They apparently were caused, to lesser degrees, by European settlers extirpating bison (Bison bison), overhunting elk (Cervus elaphus) and deer (Odocoileus spp.) and, in some areas, extirpating beaver (Castor canadensis) and replacing native ungulates with livestock. Accordingly, it appears the range expansion was prohibited for millennia by actions of Native Americans and recently facilitated by actions of European settlers.
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Vol. 161 • No. 2