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DeHart, Karl M. – Great Horned Owl distribution and habitat use relative to post-release survival of endangered Attwater’s Prairie Chickens. 2003.

Chapter One

I conducted a nighttime survey with a handheld spotlight to locate great horned owls (Bubo virginianus) for research of habitat use and to evaluate use of perch manipulated areas on the Attwater Prairie Chicken National Wildlife Refuge in 1999.  During 12 survey periods, I identified 13 great horned owls within a 300-m survey radius of the established survey points (n = 228 stops).  I identified 12 great horned owls beyond 300-m from the survey point.  The farthest distance I positively identified a great horned owl from my survey point was approximately 600-m.  This survey method provided useful results, and I believe that with further development it can be broadly applied in open habitats.

Chapter Two

I used point count surveys, radio telemetry, and incidental observations to determine great horned owl (Bubo virginianus) use of the Attwater Prairie Chicken National Wildlife Refuge in 1998 and 1999. This information is important for managers who are attempting to reduce predation of the endangered Attwater’s prairie chicken (Tympanuchus cupido attwateri). I conducted point count surveys (n = 14) on the refuge, and determined there was a 25% probability of detecting an owl at any station and I estimated that 93% of the area around survey stations was occupied. From July -December the average great horned owl home range was 720 ha (minimum convex polygon). Based on combined data from the Area Occupied estimate, range estimates based on telemetry, habitat analysis, and incidental observations, I concluded the entire refuge was used by great horned owls, including use of the refuge by nonresident owls. All but one owl had all habitat types (off refuge, woodlots, wood-prairie edge, and prairie interior) within their home range. Results from pooled telemetry data from seven owls showed the owls were not using the habitats in proportion to their availability. A relative selection index (proportion used/proportion available) showed that two or more of the radio-marked great horned owls selected each of the four habitats. Thus, all of the Attwater Prairie Chicken National Wildlife Refuge provides habitat for great horned owls.



Perch-Manipulation: An experiment to Reduce Great Horned Owl Predation of Released Attwater’s Prairie Chickens – Chapter Three


I studied habitat use by great horned owls (Bubo virginianus) on the Attwater Prairie Chicken National Wildlife Refuge. I manipulated perch availability for owls and examined the effects of this manipulation on owl use of the area and the frequency of mortality of endangered captive-reared Attwater’s prairie chickens (Tympanuchus cupido attwateri). I removed or placed perching deterrents on potential owl perches in sections of the Attwater Prairie Chicken National Wildlife Refuge. Perch-manipulation took place just prior to the 1999 release of captive-reared Attwater’ 5 prairie chicken. I monitored great horned owl use of manipulated areas by tracking eight radio-marked owls, surveying for owls, and I located APC mortalities. Two of five owls with sufficient numbers of telemetry relocations used the manipulated areas significantly less than expected. Three owls used manipulated habitat in proportion to its presence in their range, but use of the prairie by two of these was primarily limited to areas < 100-m from unmanipulated perches. Of 1 1 1 raptor sightings on the refuge during diurnal surveys and nocturnal spotlight surveys, only one was in manipulated habitat. I attributed no Attwater’s prairie chicken mortality to great horned owls in perch manipulated areas in 1999. I recommend management of the perches in the prairie by removal or by placement of deterrents. I also recommend the removal of all wooded areas on the Attwater Prairie Chicken National Wildlife Refuge.

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