Regarding the recent range expansion of the barred owl - there is still
considerable debate as to whether the barred owl expanded its range
"naturally" or was facilitated by ecological changes caused by human
settlement.

The following is the Abstract of a paper written in 2009, the citation
follows below that if you want to read the whole paper.

ABSTRACT.?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.

Livezey, K.B. 2009. Range expansion of barred owls, Part II:
Facilitating ecological changes. American Midland Naturalist,
161(2):323-349. Available online at http://www.bioone.org/toc/amid/161/2
(August 8, 2010).

The citation for the previous paper is below:
Livezey, K.B. 2009b. Range expansion of barred owls, Part I: Chronology
and distribution. American Midland Naturalist, 161(1):49-56. Available
online at http://www.bioone.org/toc/amid/161/1 (August 8, 2010).

Janet Millard
Wildlife Bio Tech, USFS
Leavenworth, WA

> On the other hand, there appears to be no evidence of the Cattle
> Egret being brought to the U.S. by humans; therefore the Cattle
> Egret would be a "self-introduced" bird, a phenomenon which I
> consider to be a "natural" occurrence. The same could be said of the
> Barred Owl in the Pacific Northwest.