Recovery Plan for the Northern Spotted Owl - DRAFT
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Recovery Plan for the Northern Spotted Owl - DRAFT

Recovery Plan for the Northern Spotted Owl - DRAFT Recovery Plan for the Northern Spotted Owl - DRAFT

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suitable habitat for that region (more than 21 inches dbh, more than 20percent dominant canopy cover, and more than 70 percent total canopy cover).A use-availability analysis showed that 69 percent of the owls showed significantpreference for suitable habitat during the breeding season and 39 percentdid so during the nonbreeding season. The authors concluded that habitatpreferences were exhibited for stands with large trees and high canopy cover,but that the preferences were weaker in this study than in other studies.An example in which owls may not have avoided more open areas is providedby Kerns (1989) who studied radio-transmittered birds in managed stands inthe California Coast province. He monitored three birds during the breedingseason, obtaining 94, 95, and 151 locations. Habitat in the area used by eachTable B.12. Relative use by northern spotted owls of stands with different over- andunder-story canopy coveragea-Canopy cover No. of No. Obsns.observa- No. of per 100Over- Under tions acres acresstory -story41-70% 41-70% 314 1568 20.00.40% 41-70% 88 1363 6.50-40% 0-40% 89 3191 2.8--Data from Solis (1983).I/11 I -...... 111.111,11, .......... - --- - -- - - , I ....... 1.111,111, .............. ......... I..Table B.13. Habitat selection by spotted owls in the Klamath and Six Rivers NationalI Forests (source: Zabel et al. 1991).Klamath Nat'l Forest(Ukonum District)aSix Rivers Nat'l Forest(Mad River District)b%of %of %of %ofVariable Value Study area Locations Study area LocationsMean dbh 36" 24 23 31 39Canopy

Discussionbird varied considerably in canopy closure. The density of locations for onebird ("Bill") was higher in areas with higher canopy closure; one bird ("Luke")did not appear to discriminate on the basis of canopy cover; and the third bird("Betsy") used thinned, and thus more open, stands more than unthinned,more closed stands. Kerns (1989) reported that brush rabbits comprised 33percent of the prey (by biomass) of the owls he studied. (By contrast, thisspecies comprised less than 1 percent of the biomass in the diet of the birdsstudied by Solis 1983.) Possibly, birds feeding heavily on brush rabbits do notshow a strong preference for stands with dense canopies, but more data areneeded to resolve this question.A few studies have investigated preferences for other habitat features.Blakesley et al. (1992) found that roosting occurred more often at elevations of1,000 to 3,000 feet than would be expected if sites were distributed randomlywith respect to elevation. In contrast, Zabel et al. (1991) found that lowelevations were preferred at both of the forests they studied during bothseasons. Blakesley et al. (1992) found that owls avoided gentle slopes butother slopes were used in proportion to their availability. Zabel et al. (1991)found no preferences for slope. Blakesley et al. (1992) did not detect anypreferences by owls for a particular aspect, and detected a preference forroosting on the lower third of slopes. Thus, in general, preferences for thesefeatures were either weak or inconsistent among sites.Recent results indicate that the pattern of habitat selection in California ismore complex than that in Oregon and Washington north of the Klamathprovince and west of the Cascade crest. The 11 to 21 -inch diameter categoryused in these stands corresponds roughly to the "young" category in Thomas etal. (1990). In the studies Thomas et al. (1990) reviewed, this size class wasavoided by 55 percent of 130 birds and only preferred by 3 percent of them.Solis and Gutierrez (1990) found similar patterns, but Blakesley et al. (1992)and Zabel et al. (1991) found little or no tendency to avoid this size class. Thework of Sisco (1990) is particularly interesting. The birds he studied showedfew preferences for habitat defined by mean diameter, but they selected sitesfor roosting and foraging that were similar to sites in old-growth stands.The analyses employing canopy cover as well as tree diameter suggested thatboth of these variables may be important in determining preferred habitat inCalifornia. Results from the work of Solis (1983) summarized in Table B. 12suggested that both overstory and understory canopy cover were positivelycorrelated with use by owls, as does the report by Zabel et al. (1991) in whichowls generally preferred habitats meeting the Forest Service's definition ofsuitable habitat.Additional use-availability analyses employing different habitat definitions, andfunctional studies of the specific resources provided by these habitats, shouldimprove our ability to discern highly preferred and avoided habitats in California.Studies from the California Coast province, the California Cascades, andthe eastside of the Cascades are also needed.Abundance of Owls in Different HabitatsThomas et al. (1990) concluded that in most parts of the range density wasextremely low in landscapes dominated by stands less than 80 years old andlacking old-growth, and that density increased with the amount of old-growthpresent in a landscape or study plot. Thomas et al. (1990) also noted thatrecent studies in California "strongly suggest that suitable and even superior307

suitable habitat <strong>for</strong> that region (more than 21 inches dbh, more than 20percent dominant canopy cover, and more than 70 percent total canopy cover).A use-availability analysis showed that 69 percent of <strong>the</strong> owls showed significantpreference <strong>for</strong> suitable habitat during <strong>the</strong> breeding season and 39 percentdid so during <strong>the</strong> nonbreeding season. The authors concluded that habitatpreferences were exhibited <strong>for</strong> stands with large trees and high canopy cover,but that <strong>the</strong> preferences were weaker in this study than in o<strong>the</strong>r studies.An example in which owls may not have avoided more open areas is providedby Kerns (1989) who studied radio-transmittered birds in managed stands in<strong>the</strong> Cali<strong>for</strong>nia Coast province. He monitored three birds during <strong>the</strong> breedingseason, obtaining 94, 95, and 151 locations. Habitat in <strong>the</strong> area used by eachTable B.12. Relative use by nor<strong>the</strong>rn spotted owls of stands with different over- andunder-story canopy coveragea-Canopy cover No. of No. Obsns.observa- No. of per 100Over- Under tions acres acresstory -story41-70% 41-70% 314 1568 20.00.40% 41-70% 88 1363 6.50-40% 0-40% 89 3191 2.8--Data from Solis (1983).I/11 I -...... 111.111,11, .......... - --- - -- - - , I ....... 1.111,111, .............. ......... I..Table B.13. Habitat selection by spotted owls in <strong>the</strong> Klamath and Six Rivers NationalI Forests (source: Zabel et al. 1991).Klamath Nat'l Forest(Ukonum District)aSix Rivers Nat'l Forest(Mad River District)b%of %of %of %ofVariable Value Study area Locations Study area LocationsMean dbh 36" 24 23 31 39Canopy

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