Recovery Plan for the Northern Spotted Owl - DRAFT

also use a variety of nest sites in younger-aged stands. But one critical piece ofinformation should be assessed before this hypothesis can be tested. That is,the relative nesting success of birds using cavities and broken-top sites shouldbe compared to that of birds using the presumably structurally less stabledebris platforms. In any event, artificial nest sites probably could be providedfor these birds (Madison and Woodbridge pers. comm.). European owls in thegenus Strix readily use nest boxes (Southern 1970, Saurola 1989). If spottedowls behave in a similar fashion to other Strix owls, the availability of nest sitesprobably is not a critical management problem. However, a critical study ofnest-box acceptance by spotted owls has not been conducted.Several studies have been conducted on the structure of spotted owl nestinghabitat (LaHaye 1988, Buchanan 1991, Self and Nelson 1991 pers. comm.). Inthe two studies that compared nest sites with available habitat, one inunmanaged forest and the other in managed forest (LaHaye 1988 andBuchanan 1991, respectively), owls nested in forests that differed from whatwas available to them, suggesting selection by the owls. In general, owlspreferentially used forests with greater complexity and structure. Nestinghabitat structure reported by Self and Nelson in managed forests (1991 pers.comm.) was strikingly similar to the habitat structure used by foraging spottedowls in unmanaged stands within the same province (Solis 1983).Roosting habitat: Northern spotted owl roosting habitat has been describedby Forsman (1976), Barrows and Barrows (1978), Forsman (1980), Solis(1983), Forsman et al. (1984), Chavez-Leon (1989), Sisco (1990), and Blakesleyet al. (In Press). Roost sites are typically areas of relatively dense vegetation(high canopy closure dominated by large-diameter trees). During the summerthese sites are usually cool, shady spots near streams or are on the lower thirdof slopes (possibly a simple correlation with stream position; Forsman 1976,Solis 1983, Blakesley et al. In Press). Spotted owls respond to variation intemperature and exposure by moving within the canopy to find favorablemicroclimate conditions (Forsman 1976, Barrows and Barrows 1978, Forsman1980, Barrows 1981, Solis 1983, Forsman et al. 1984). The multistoried standstructure of roost sites facilitates this movement. Because of this observedbehavioral response to variation in temperature, it has been hypothesized thatold-growth forests are necessary to spotted owls for them to avoid heat stress(Barrows and Barrows 1978). However, Gutierrez (1985) pointed out that thereare other plausible hypotheses to explain the associated owls with old-growth.Foraging habitat: Of the major spotted owl habitat categories, feedinghabitat appears to be the most variable (summarized in Thomas et al. 1990).This is predictable given the highly variable distribution and abundancepatterns of the owl's primary prey (Ward 1990). Within a given geographicprovince, foraging habitat may be more variable than either nesting or roostinghabitat. Nevertheless, spotted owl foraging habitat is characterized by highcanopy closure and complex structure. Comparisons of habitat amongunmanaged stands used by foraging owls and managed stands occupied bynesting owls shows a surprising concordance of structural habitat features inCalifornia (Appendix B).Solis and Gutierrez (1990) presented evidence that male and female spottedowls may segregate their foraging habitat. The smaller males appeared to beusing stands that had higher tree density than were the larger females, whichforaged in less dense habitats. Earhart and Johnson (1970) suggested thatdifferential habitat use by male and female owls may occur because the highwing loading of the females would make them less maneuverable than males.However, this probably would be a consequence rather than a cause of reversedsexual dimorphism (Muller 1986, Solis and Gutierrez 1990).20