Recovery Plan for the Northern Spotted Owl - DRAFT

ResultsDiscussionspotted owl habitat can develop faster in coastal forest of redwood and mixedredwood and Douglas-fir" (p. 185) and that "the full range of suitable habitatsfor spotted owls in California has not yet been determined" (p. 166). Reportsappearing since Thomas et al. (1990) are described in the following section.Carey et al. (1991) found spotted owls only in old-growth stands during a studyin the southern Oregon Coast Range. Three owls were recorded in 1985, andsix were recorded in 1986. The results showed a statistically significanttendency for abundance to increase with stand age, but sample sizes wereinadequate to test the null hypothesis that abundance is equal in mature andold-growth stands.Blakesley et al. (1992) found 79 nest sites in stands of 11 to 21 inches or morethan 21 inches and no nests in other habitats. Nest densities per acre werevirtually identical in the two habitats. Sites also were characterized by elevation,position, aspect, and slope. Elevations less than 1,000 feet were avoidedand nests were concentrated in the lower third of slopes. No preferences werefound for aspect or slope.Meyer et al. (1992) reported that the amount of mature and old-growth forestin circles at owl sites was higher than the amount in randomly placed sites, afinding that is consistent with previous work. However, when individualsuccessional stages were analyzed, only the amount of old-growth habitat wassignificantly greater at owl sites than at randomly placed sites.We analyzed the data provided by Self and Brown (pers. comm.) to investigaterelative densities of nests in different habitats. We defined the study area as allthe land within 1.3 miles of the nests and recorded the amounts of differenthabitats in this area, and the number of nests per square mile of each habitattype. This type of analysis should not be used to estimate density of nests (orowls) in a given habitat across the entire landscape. For example, if a habitatwere highly preferred for nesting and occurred only in scattered patches, thenthe process discussed previously would substantially underestimate truedensity. The analysis probably does provide a valid indication of relativepreference, however, especially when substantial differences in density occurbetween habitats. In the data from Self and Brown (pers. comm.) nests wereconcentrated in the stands with the largest trees and densest canopy (TableB. 14). Density was next highest in stands with smaller trees but dense canopies,and was lower in stands with more open canopies and either large ormedium size trees. The study also suggested that montane forest types werenot as preferred as Douglas-fir or mixed conifer types. Solis (1983:33, 41) alsocommented that the owls he studied seemed to avoid montane forest types.The suggestion by Thomas et al. (1990) that more work was needed on habitatsuitability in California is supported by the recent results reported earlier.Findings by Blakesley et al. (1992) and Self and Brown (pers. comm.) thatnests were almost equally common in stands with mean tree diameters of 11 to21 inches and more than 21 inches provides a sharp contrast to results fromOregon and Washington. It is also noteworthy that in the study by Self andBrown (pers. comm.) density varied much more with canopy cover than it didwith tree diameter (Table B. 14). The structural features of nest sites in California(Table B.2) show a remarkable similarity to the structure of old-growthsites and nest sites elsewhere in the range. This suggests that owls in Californiaare selecting sites with large trees even when the mean diameter in the308