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Lane, W.H. 1997. Distribution and ecology of boreal owls in Northeast Minnesota. M.S. Thesis, University of Minnesota. 89 pp.


Chapter 1: Distribution, Abundance, and Habitat Use of Territorial Male Boreal Owls in Northeast Minnesota

Nocturnal surveys were conducted in northeast Minnesota from 1987 to 1992 (inclusive), to locate singing territorial male boreal owls (Aegolius funereus). Vocalizing owls were detected on 234 occasions, with 171 of the detections (73.1 %) categorized as unique (i.e., individual owls), and 63 detections (26.9%) categorized as owls previously detected (heard during > 1 survey effort). The rate of encountering singing owls ranged from a low of 0.028 owls heard/km surveyed (all detections) in 1991, to 0.091 owls/km surveyed in 1989. Indices for the abundance of individual owls located per route length ranged from 0.056 in 1987, to 0.219 owls/linear km of survey rote in 1989. Boreal owl singing activity increased towards 15 April, and decreased, thereafter. Territorial boreal owls used pole sized trees in upland mixed forest stands greater than expected and open/brush/regenerative stands significantly less than expected for courtship activities. Stands supporting vocalizing male owls were generally located in mature, mixed forest tracts, containing sawtimber sized quaking aspen (Populus tremuloides).

Chapter 2: Habitat Use and Movements of Male Boreal Owls in Northeast Minnesota

I monitored 10 radio equipped male boreal owls in northeast Minnesota from 1990-1992; four in 1990, four in 1991, and two in 1992. Owls were captured at singing perch or cavity site locations and monitored while they remained within the study area or until transmitters were removed following the breeding season. Owls most often roosted in lowland conifer forests, even though these forest types represented only 8.3% of the study area. Black spruce (Picea mariana) was identified as the roost tree on 94 (81.7%) of 115 observations. Only three marked owls remained in the study area into the summer: six owls presumably left. the study area and one was killed by an avian predator, most likely a, great horned owl, (Bubo virginianus). All monitoring efforts of owls that ended in loss of signal were males that were either unpaired, or had experienced nest failure shortly after egg laying. Minimum Convex Polygon (MCP) home range estimates for four male owls in 1991 averaged 1202, ha (range=7421,444 ha), but when limited to movements prior to nest failure, the average MCP estimate was 425 ha (range=203 74 ha). Fifty percent Harmonic Mean Transformation Activity Areas (HMT AA) averaged 141 ha (n=4, range=101 208 ha), also indicating that boreal owls restricted their movements to relatively small areas during the breeding season.

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