Graduate Thesis Or Dissertation
 

The range expansion of the northern barred owl : an evaluation of the impact on spotted owls

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  • Since their range expansion into the Pacific Northwest, anecdotal evidence suggests that northern barred owls (Strix varia varia) may be displacing northern spotted owls (Strix occidentalis caurina). My objectives were to characterize barred owl distribution and population increase in Oregon, investigate spotted owl territory performance before and after barred owl detection, and document cases of hybridization between barred owls and spotted owls. Between 1974-1998, 706 unique barred owl territories were reported in Oregon. At five spotted owl demographic study areas in Washington and Oregon, barred owl detections increased rapidly between 1987-1999. After barred owls were detected within 0.80 km of the spotted owl territory center, occupancy of spotted owls declined, and there was a significant likelihood of spotted owl displacement when barred owls were either currently or previously present. When barred owls were detected between 0.81-2.40 km from the spotted owl territory center, there was no difference in mean occupancy of spotted owls after barred owls were detected, and spotted owl detection rates when barred owls were currently or previously present were not significantly different from territories without barred owls. There was no effect on spotted owl reproductive performance after barred owls were detected within 0.80 km or between 0.8 1-2.40 km of the territory center. When barred owls were detected within 0.80 km of a spotted owl territory center, 46% of the spotted owls moved > 0.80 km and 39% were never found again. In comparison, at territories without barred owls, only 21% of spotted owls moved > 0.80 km, and only 11% disappeared completely. When barred owls were detected between 0.81-2.40 km from the territory center there was no difference in rates of movement or disappearance of spotted owls between territories with and without barred owls. Reports of hybridization between spotted owls and barred owls are uncommon. Between 1974 and 1999, 24 adult and 26 juvenile hybrids were confirmed in Washington and Oregon. Data from this study suggest that barred owls pose a threat to spotted owls but it is too soon to predict whether trends observed in this study will continue, or will spread to other areas.
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