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Master of Science (MS) in Biology
To sustain forest health, increase species diversity and reduce wildfire events in the ponderosa pine (Pinus ponderosa) forests of at Turnbull National Wildlife Refuge (TNWR) managers have implemented a variety of approaches including prescribed burning and non-commercial thinning. The impacts of thinning on owl species diversity and habitat occupancy have not been studied in these ponderosa pine forests. My study had the following objectives 1) compare forest stand metrics between treatment plots, 2) compare owl species richness between treatment plots, 3) examine if occupancy and detection patterns of Great Horned Owls varied with forest condition and season, respectively, and 4) determine how the frequency of calls and frequency of detection of calls for Great Horned Owls varied between treatments and seasons. I established three stations within three sites for each of three treatments: control (no management activity; thinned 5 years and thinned 11 years prior to study. I measured 13 habitat metrics associated with trees and ground cover at each station. I used SongMeter SM2+ digital recorders to collect owl calls in nightly sessions from 30 June 2014 to 10 August 2015. I identified 112,025 territory hoots (calls) between 27 sampling stations over 1,107 sessions using Raven Pro 1.5 spectrographs. Although there were more live trees and snags in the Control plots, the diameter at breast height did not differ between treatments. There were more stumps in the 5 year since thinning treatment. I identified only Great Horned Owl calls, although Northern Pygmy Owls, Glaucidium gnoma, were observed in areas adjacent to my treatment sites. Occupancy and detectability analysis conducted using Program Presence revealed no differences in occupancy between the control and thinned plots but detectability was least during fall and early winter. Chi square analysis of the frequency of call detections and of calls also indicated a decrease in the number of detections during fall and early winter as well as an increase in call frequency in late winter and lasting until spring. These results indicate that Great Horned Owls dominate the ponderosa pine forests of TNWR and that thinning has little impact on their use of the forests or calling behavior. Monitoring of owls in all refuge habitats over multiple years is recommended too understand species richness of owls across the entire refuge.
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Brady, Christopher P., "Effects of non-commercial thinning on Great Horned Owls on Turnbull National Wildlife Refuge" (2020). EWU Masters Thesis Collection. 639.