Analysis of great horned owl pellets with rhinoceros auklet remains

Compared to most avian predators, owls inflict minimal damage on bones regurgitated as pellets. Analysis of these bones is useful to ornithologists characterizing owl dietary habits (Dexter 1978, Kirkpatrick and Conway 1947, Stegeman 1957), to ecologists assessing compositions of small-mammal communities (Phillips 1951, Long and Kerfoot 1963), and to paleobiologists reconstructing ancient communities (Mayhew 1977, Dodson and Wexlar 1979, Hoffman 1988). The extent and nature of the damage inflicted on the bones may allow for identification of the type of owl forming the pellets (Dodson and Wexlar 1979, Hoffman 1988). Great Horned Owls (Bubo virginianus) usually prey on a variety of smallto medium-sized mammals and birds, with mammals forming the bulk of their diets (Dexter 1978, Kirkpatrick and Conway 1947, Stegeman 1957). However, Great Horned Owls living on Protection Island, Jefferson Co., Washington, are noteworthy in that their regurgitated pellets contain bird remains exclusively. Here we characterize these unusual pellets and their contents. Great Horned Owl pellets (87 in 1987; 42 in 1988) were collected at a roost site from June to August. The collection area, 25 m in diameter, was at the edge of a coniferous stand at the northeastern part of the island. Observations suggest that the pellets were produced by a single family of owls (two adults and three juveniles in 1987). After softening each pellet in water, bones were extracted and identified by type and, when possible, source species. Each bone was characterized as complete or fragmented. A fragmented long bone was listed as retaining either the proximal or distal end, or only the midshaft; size was estimated to the nearest 25% of the original unbroken element (after Hoffman 1988). To characterize whole pellets, lengths and widths of 30 pellets collected in 1988 (all those from which complete data could be obtained) were determined to the nearest millimeter. The pellets were dried to constant mass and the bones extracted. The extracted bones were then dried to constant mass and a bone/ pellet mass ratio was determined for each pellet. The pellets were cylindrical with rounded ends, with bones embedded in a matrix of feather material