A preliminary investigation of the spotted owl in Oregon Public Deposited



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  • Between 1970 and 1974, data were collected on the distribution and biology of the spotted owl (Strix occidentalis) in Oregon. One-hundred and sixteen pairs and seven single birds were located. Spotted owls occurred throughout the mountains of western Oregon and on the east slope of the Cascade Range at least as far east as Badger Butte, Hood River County; Abbot Butte, Jefferson County; and Swan Lake Point, Klamath County. The upper elevational limits of the species increased from about 1,350 meters in northern Oregon to 1,770 meters in southern Oregon. Although spotted owls were not uncommon in some areas, evidence indicated that the population was declining as a result of habitat loss. A total of 2,647 prey items were identified from 42 pairs of owls. Prey species included 29 mammals, 20 birds, 2 reptiles, a crayfish, a terrestrial snail, and 26 genera of insects. Mammalian prey comprised over 90 percent of the biomass consumed. The flying squirrel (Glaucomys sabrinus) was the principal prey species (13-48 percent of total biomass consumed), except in dry forest areas, where wood rats (Neotoma fuscipes and N. cinerea) became most important (7-78 percent of total biomass). Other important prey included snowshoe hares (Lepus americanus), red tree voles (Phenacomys longicaudus), deer mice (Peromyscus maniculatus), western red-backed voles (Clethrionomys occidentalis), Mazama pocket gophers (Thomomys mazama), pikas (Ochotona princeps), and small birds. Predation on showshoe hares, gophers, moles, and insects was heaviest during the late spring and summer months. Spotted owls foraged primarily at night, and often captured arboreal animals (squirrels, wood rats, and tree voles) by grabbing them from limbs or tree trunks. Eighteen spotted owl nests were located, including 13 in cavities in living old-growth conifers, three in clumps of deformed limbs caused by dwarfmistletoes (Arceuthobium spp. ), and two in platform nests constructed by other species. Nest height above ground ranged from 19. 8-55.2 meters. Owls added no materials to their nests except small amounts of molted down. The mean date of clutch initiation for 15 nests was 29 March (range 9 March - 19 April). As egg-laying neared, adult activities (vocalization, copulation, courtship feeding, roosting) became increasingly centered around nest trees, and several days before they laid eggs, females began to roost inside their nest cavities. Incubation, which lasted approximately 30 days, was performed entirely by females. Males fed females during this period. Owlets fledged at 34-36 days of age (between early May and mid -June), and were fed by their parents until late September. At fledging, owlets were weak fliers, and often fell from their nests to the ground. When this occurred, they regained safe perches in trees or low bushes by climbing. Forty-eight nesting attempts were observed (38 successful). Total number of young fledged was 63. Mean number of young fledged per successful nest was 1.61 (range 1-3). The percentage of pairs attempting to nest ranged from 89 percent in 1972, to 16 percent in 1973, and 46 percent in 1974. I suspected, but did not verify, that the low numbers of breeding pairs in some years reflected a decline in prey numbers. Percent of nesting pairs which fledged young was 92 in 1972, 40 in 1973, and 72 in 1974. A principal cause of nest failure in all years was nest desertion during early stages of nesting. In 1972, juvenile mortality between fledging and the end of August was 35 percent. Predation was suspected as the principal cause of mortality, but several young were killed when the fell from nests, and two young died in their nest. Most broods did not move far from their nests until they began dispersal in September or October. Of 14 broods checked in late August, 5 were within 160 meters of their nests, 5 were 170-250 meters from their nests, and 2 had moved 487 and 670 meters, respectively. Two broods could not be relocated, but had moved 1, 050 and 365 meters, respectively, when last seen in July. Owlets underwent two molts during their first summer; the white natal down was replaced by the downy mesoptile plumage before owlets fledged. Replacement of the mesoptile plumage by the first winter plumage then occurred over a 4-month period and was complete by the end of September or early October. In the latter plumage owlets were nearly indistinguishable from adults. Of 123 sites where spotted owls were located, 117 (95 percent) where characterized by unharvested old-growth conifer forests. Two pairs occupied old-growth forests which had been partially logged about 30 years previous, and three occupied second-growth forests which contained minor old -growth components. The multilayered structure of old-growth stands provided owls with large trees for nests and winter roosts, small shaded summer roost trees, and a closed canopy (canopy closure ranged from 53-86 percent at nest sites). Owls occurred in most coniferous associations found in western Oregon and the Cascades, with the exception of subalpine forests, open ponderosa pine (Pinus ponderosa) forests and lodgepole pine (Pinus contorta) forests. Owls showed a slight preference (X2 = P < . 01) for nests located on north or east exposures, possibly because trees there were usually larger and forests were denser than on south or west exposures. Fifteen of 18 nests were within 400 meters of permanent water (range = 15-1,417 meters). Sources of water utilized by all 18 pairs consisted of small perennial streams or springs. Timber harvest occurred or was scheduled in 52 percent of the owl habitats located during the study. In most cases, timber harvest within an occupied habitat did not drive the owls completely out of the area, because only small portions of extensive forest areas were harvested. When portions of small forest areas (less than about 80 hectares) were harvested, however, owls often disappeared from these areas. Two pairs located in old-growth forests which had been subjected to very light over story removal indicated that, under some circumstances, owls could tolerate this type of harvest activity. Clear-cut harvest, however, eliminated roosting, nesting and most foraging in the affected areas.
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