A preliminary investigation of the spotted owl in Oregon

Between 1970 and 1974, data were collected on the distributionand biology of the spotted owl (Strix occidentalis) in Oregon. One-hundred and sixteen pairs and seven single birds were located. Spottedowls occurred throughout the mountains of western Oregon and on theeast slope of the Cascade Range at least as far east as Badger Butte,Hood River County; Abbot Butte, Jefferson County; and Swan LakePoint, Klamath County. The upper elevational limits of the speciesincreased from about 1,350 meters in northern Oregon to 1,770 metersin southern Oregon. Although spotted owls were not uncommon insome areas, evidence indicated that the population was declining as aresult of habitat loss.
A total of 2,647 prey items were identified from 42 pairs ofowls. Prey species included 29 mammals, 20 birds, 2 reptiles, acrayfish, a terrestrial snail, and 26 genera of insects. Mammalianprey comprised over 90 percent of the biomass consumed. The flyingsquirrel (Glaucomvs sabrinus) was the principal prey species (13-48

percent of total biomass consumed), except in dry forest areas, wherewood rats (Neotoma fuscipes and N. cinerea became most important(7-78 percent of total biomass). Other important prey included snow-shoe hares (Lepus americanus), red tree voles (Phenacomvs longi-caudus), deer mice (Peromyscus maniculatus), western red-backedvoles (Clethrionomys occidentalis), Mazama pocket gophers(Thomomvs mazama), pikas (Ochotona princeps), and small birds.Predation on showshoe hares, gophers, moles, and insects washeaviest during the late spring and summer months. Spotted owlsforaged primarily at night, and often captured arboreal animals(squirrels, wood rats, and tree voles) by grabbing them from limbsor tree trunks.
Eighteen spotted owl nests were located, including 13 in cavitiesin living old-growth conifers, three in clumps of deformed limbscaused by dwarfmistletoes (Arceuthobium spp.), and two in platformnests constructed by other species. Nest height above ground rangedfrom 19.8-55.2 meters. Owls added no materials to their nestsexcept small amounts of molted down. The mean date of clutchinitiation for 15 nests was 29 March (range 9 March - 19 April). Asegg-laying neared, adult activities (vocalization, copulation, court-ship feeding, roosting) became increasingly centered around nesttrees, and several days before they laid eggs, females began to roostinside their nest cavities. Incubation, which lasted approximately
30 days, was performed entirely by females. Males fed femalesduring this period. Owlets fledged at 34-36 days of age (betweenearly May and mid-June), and were fed by their parents until lateSeptember. At fledging, owlets were weak fliers, and often fell fromtheir nests to the ground. When this occurred, they regained safeperches in trees or low bushes by climbing. Forty-eight nestingattempts were observed (38 successful). Total number of youngfledged was 63. Mean number of young fledged per successful nestwas 1. 61 (range 1-3). The percentage of pairs attempting to nestranged from 89 percent in 1972, to 16 percent in 1973, and 46 percentin 1974. I suspected, but did not verify, that the low numbers ofbreeding pairs in some years reflected a decline in prey numbers.Percent of nesting pairs which fledged young was 92 in 1972, 40 in1973, and 72 in 1974. A principal cause of nest failure in all yearswas nest desertion during early stages of nesting. In 1972, juvenilemortality between fledging and the end of August was 35 percent.Predation was suspected as the principal cause of mortality, but sev-eral young were killed when the fell from nests, and two young died intheir nest.
Most broods did not move far from their nests until they begandispersal in September or October. Of 14 broods checked in lateAugust, 5 were within 160 meters of their nests, 5 were 170-250 metersfrom their nests, and 2 had moved 487 and 670 meters, respectively.
Two broods could not be relocated, but had moved 1,050 and 365meters, respectively, when last seen in July.
Owlets underwent two molts during their first summer; thewhite natal down was replaced by the downy mesoptile plumage beforeowlets fledged. Replacement of the mesoptile plumage by the firstwinter plumage then occurred over a 4-month period and was completeby the end of September or early October. In the latter plumageowlets were nearly indistinguishable from adults.
Of 123 sites where spotted owls were located, 117 (95 percent)where characterized by unharvested old-growth conifer forests. Twopairs occupied old-growth forests which had been partially loggedabout 30 years previous, and three occupied second-growth forestswhich contained minor old-growth components. The multilayeredstructure of old-growth stands provided owls with large trees for nestsand winter roosts, small shaded summer roost trees, and a closedcanopy (canopy closure ranged from 53-86 percent at nest sites).Owls occurred in most coniferous associations found in western Ore-gon and the Cascades, with the exception of subalpine forests, openponderosa pine (Pinus ponderosa) forests and lodgepole pine (Pinus contorta) forests.
Owls showed a slight preference (x2 = P exposures. Fifteen of 18 nests were within 400 meters of permanentwater (range = 15-1,417 meters). Sources of water utilized by all 18pairs consisted of small perennial streams or springs.
Timber harvest occurred or was scheduled in 52 percent of theowl habitats located during the study. In most cases, timber harvestwithin an occupied habitat did not drive the owls completely out of thearea, because only small portions of extensive forest areas wereharvested. When portions of small forest areas (less than about 80hectares) were harvested, however, owls often disappeared from theseareas. Two pairs located in old-growth forests which had been sub-jected to very light overstory removal indicated that, under somecircumstances, owls could tolerate this type of harvest activity.Clear-cut harvest, however, eliminated roosting, nesting and mostforaging in the affected areas.