Distribution and biology of the spotted owl in Oregon

We studied the distribution, habitat, home range characteristics, reproductive biology, diet, vocal-izations. activity patterns, and social behavior of the spotted owl (Sift occidentalis) in Oregon from 1969through 1980. Spotted owls were located at 636 sites, including 591 (93%) on federal lands. The rangeincluded western Oregon and the east slope of the Cascade Range. Most pairs (97.6%) were found in unloggedold-growth forests or in mixed forests of old-growth and mature timber. No owls were found in forestsyounger than 36 years old. Paired individuals tended to occupy the same areas year after year and use thesame nests more than once. Mean nearest neighbor distances were 2.6 km west of the crest of the CascadeMountains and 3.3 km on the east slope of the Cascades. From 1969 to 1978, the population declined at anaverage annual rate of 0.8%. The principal cause of site abandonment was timber harvest.
Home range areas ranged from 549 to 3,380 ha. Seasonal home ranges averaged largest during fall andwinter. Home range overlap averaged 68% for paired individuals and 12% for individuals occupying adjacentterritories. Old-growth forests were consistently preferred for foraging and roosting by all owls studied. Inmost cases, use of younger forests was either less than or not significantly different than expected. There wasno apparent tendency to concentrate foraging activity in "edge" areas such as along clear-cut boundaries orroadsides. Owls roosted significantly higher in the forest canopy during cold, wet weather than during warmor hot weather.
Of 47 nests located. 30 were in cavities in old-growth conifers and 17 were on platforms of sticks or otherdebris in mature or old-growth conifers. None of the owls built its own nest. Average nest height was 27.3m (range 10.0-55.3 m, SE .¡ 1.67). Forty-two nests were in unmanaged old-growth forests. Canopy closureexceeded 70% in most nest areas.
On the average, 62% of the pairs checked each year attempted to nest; 81% of all nesting attempts weresuccessful. Egg laying began between 9 March and 19 April (f .. 2 April). Average clutch size was 2.Incubation lasted 30 } 2 days. Eggs hatched between 8 April and 20 May. Females alone incubated andbrooded the young. Males provided all food for the female and young until the young were approximately2 weeks old. The young left the nest at 34-36 days of age. After leaving the nest, owlets were fed by theadults until late August or September. The young became independent and began dispersal in October.Owlet mortality during the first summer of life was 35%. Before they were killed by predators in midwinter,2 radio-tagged juveniles traveled maximum straight line distances of 10.1 and 16.4 km from their respectivenatal sites.
The diet varied seasonally and included a variety of mammals, birds, and insects. Mammals comprised92% of all prey taken. Foraging activity usually began shortly after sunset and ceased shortly before sunrise.Calls given by spotted owls were classified into 13 types. Juveniles did not give adult vocalizations until lateSeptember or October of their first year.