Environmental changes are allowing -- indeed pushing -- eastern birds into the Northwest, according to a University of Idaho zoologist. Since 1950, such species as barred owls, blue jays, vireos and several kinds of warblers have been turning up with increasing frequency in Idaho, Oregon and Washington. The reasons, theorizes Prof. Earl J. Larrison, are divided "about 50-50" between destructive alterations in the birds' natural habitats and human development that has allowed them to cross natural barriers. Many eastern birds have been prevented from moving west by the evergreen forests of the north or the vast treeless plains of the mid-continent. But "though the plantings of orchards and windbreaks along the region where the northern prairies meet the coniferous forests," Larrison says, "man has created a natural bridge over which the birds can move."

The birds have been able to cross the Rockies because the "bridge" on the prairies feeds directly into a series of low-lying valleys through which they can easily move, he says.

Some of the larger immigrants, such as the barred owl and the blue jay, have taken permanent residence in the Northwest, while smaller types appear to be summer visitors.

"These eastern species and subspecies may be wandering and exploring more because of ecological disruption in their original homes," Larrison says.