Within-stand nest site selection by spotted owls in the eastern Washington Cascades

We describe 83 nest sites (0.2-ha areas) of northern spotted owls (Strix occidentalis caurina) in mixed conifer forests on the eastern slope of the Cascade Mountains, Washington. Approximately 74% of the nest sites were in forests in intermediate stages of succession, and 27% were in old-growth forests (median = 122 yr, range 54-700 yr). Most sites were naturally regenerated after fire, but 23% of the nest sites had been partially harvested ≥40 years ago. We tested the hypothesis that habitat structure does not influence nest site selection within forested stands, because such knowledge would aid conservation strategies that may include silvicultural prescriptions for creating future habitat. We compared habitat characteristics at 62 nest sites with those at 62 random sites within the same forest stands. Compared with random sites, spotted owl nest sites had canopies of dominant and/or codominant and intermediate trees that were farther aboveground (P = 0.02 and 0.07, respectively), more 35-60-cm-dbh (diam at breast height) Douglas-fir (Pseudotsuga menziesii) trees (P = 0.03), greater basal area of Douglas-fir trees (P = 0.02), more 61-84-cm-dbh ponderosa pine (Pinus ponderosa) trees (P = 0.03), greater live tree basal area (P = 0.09), greater basal area of Class IV snags (broken snags with no branches and little bark; P < 0.001), less basal area of a group of relatively uncommon conifer species (P = 0.02), fewer 10-34-cm-dbh uncommon conifer species (P = 0.08), and less basal area of Class I and II snags (intact or nearly intact snags with branches and most bark remaining; P = 0.08 and 0.095, respectively). Volume of coarse woody debris (P > 0.13 in all decay classes) and percent canopy closure (P = 0.45) did not differ between nest and random sites. Data support the hypothesis that nest sites are selected as part of an antipredator strategy.