Great Horned Owl Bubo virginianus Gmelin       Field Guide IDs: NG-248; G-174; PE-172; PW-120; AE-pl 282; AW-pl 288; AM(II)-162 Nest Location Nest Type Eggs & Mating System Dev. & Parental Care Primary & 2ndary Diet.. Foraging Strategy ABANDONED NEST F-M I: 26-35 DAYS SEMIALTRICIAL 2 30 feet - 50 feet (15 feet - 70 feet) CLIFF F? 2-3 (1-6+) MONOG? F: 35 DAYS MF BIRDS SM VERTS INSECTS BREEDING: Conif or decid forest and woodland, swamp, orchard, park, riparian forest, semidesert. 1 brood. DISPLAYS: Male performs noisy aerial courtship display; ritually feeds female. Pair bill, bob, call, and click. NEST: In abandoned tree nest of raptor, corvid, occ squirrel; also tree cavity, cave, crevice, stump, and on ground in log, among rocks; of sticks, moss, hair, shredded bark, rootlets, etc., lined slightly with feathers and down. Perennial. EGGS: Dull white. 2.2" (55 mm). DIET: Esp rabbits and rodents, pheasants, quail, passerines, occ fish, amphibians, reptiles, scorpions. Mainly nocturnal, but also hunts crepuscularly. Ejects pellets. CONSERVATION: Winter resident. Occ uses nest box. NOTES: Incubating bird often snow-covered in n; early eggs may freeze. Population density in n tracks snowshoe hare density; disperse when hare numbers crash. Young hatch asynchronously; rapidly develop ability to regulate body temperature, fly at 63-70 days, fed for several months. Adults perform distraction display. Daytime roost in dense conif near trunk. Most do not breed before second year. Arctic populations eat more birds. Cache prey; defrost frozen cache by "incubating" it ("prey thawing"). STANFORD. NOTES: Several pairs are resident on campus. Nests in tall trees (such as eucalyptus), often in old Red-tailed and Red-shouldered hawk nests. A pair usually nests in the arboretum. ESSAYS: Irruptions; Pellets; How Owls Hunt in the Dark; Mobbing; Brood Reduction; Breeding Season REFERENCES: Adamcik and Keith, 1978; Marti, 1974; Turner and McClanahan, 1981.