Graduate Thesis Defense
|Title:||Diet, breeding success, and detectability of Great Horned Owls (Bubo virginianus) in the Arctic|
|Date:||Wednesday, 12 December 2018|
|Location:||Murie Bldg, rm 103|
|Major Professor:||Knut Kielland|
I studied the diet, breeding success, detectability, and density of great horned owls (Bubo virginianus) in the Middle Fork of the Koyukuk River Valley in Arctic Alaska. The study site extended from the southern slopes of the Brooks Range to latitudinal tree line, the northern breeding limit of the species, and included what are likely to be the northernmost great horned owl nests on record (up to 68.01 degrees north). I completed the study during the great horned owl breeding seasons of 2017 and 2018, during years of high snowshoe hare (Lepus americanus) abundance. The focus of this study was to gain an understanding of how high snowshoe hare abundance influences the recruitment, diet, and distribution of this apex generalist predator, and to determine best methods of detecting owls for similar studies in the future. I used motion sensor cameras on nests as well as pellet analysis for diet and breeding studies, and call surveys for information on detectability and density. Great horned owl diet consisted mostly of snowshoe hares (mean 80%, range 65-99%), with an average prey size of 714 grams (95% CI ± 34.26). Nestlings received an average of 531 grams (95% CI ± 84.41) of prey per chick per day, and the proportion of hares in their diet positively correlated with fledging success (R2 = 0.673, p = 0.0006). During call surveys, length of playback was the most important factor in detecting owls throughout 12 minute surveys, reaching 23% (95% CI = ± 6.4) at 3 minutes, and up to 80% (95% CI = ± 6.1) at 9 minutes. Inclusion of silent listening periods was not necessary in detecting owls during playback surveys (Pa – Pb = -0.0831, and z = -0.892, two-tail = .3724, 95% CI = ±1.96). Call surveys gave an estimate of 4.15 owls per square kilometer (z = 4.302, 95% CI = ± 2.63). This was the northernmost study of North America’s most widespread year-round bird of prey, and the first density estimate at their northern breeding limit.