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Habitat Use and Selection by California Spotted Owls in a Postfire Landscape. Monica L. Bond, M.S. Derek E. Lee, M.S. Rodney B. Siegel, Ph.D. James P. Ward, Ph.D. Spotted Owls Strix occidentalis. One of the rarest bird species to breed in mainland U.S.
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Monica L. Bond, M.S. Derek E. Lee, M.S.Rodney B. Siegel, Ph.D.James P. Ward, Ph.D.
Managers have presumed that high-severity fires that burn most trees in a stand and reduces canopy cover pose the greatest risk to owl habitat. This presumption has led to proposals for timber harvests to lower fire risk, and for post-fire salvage logging.
However, all the previous research on California spotted owl (Strix occidentalis occidentalis) habitat associations has been conducted in unburned forests where the primary disturbance was logging, not fire. The California spotted owl evolved in a heterogeneous forest landscape shaped in large part by fires that created a mosaic of patches of forest burned at varying severities. Fire creates habitat conditions that can benefit spotted owl prey species. Indeed, spotted owls have continued to live and reproduce in burned forests - but what we didn’t know was how owls specifically use post-fire landscapes.
We conducted a study of habitat use and selection by California spotted owls occupying a burned landscape. We radio-tracked 7 owls in a 4-year old burn in the southern Sierra Nevada. We found that when given a choice of different burn severities near their core-use areas, our sample of spotted owls preferred to forage in burned habitat over unburned, especially severely burned. They roosted in more traditional owl habitat with high canopy. These results show that, depending on how it burns, fire may not be a threat to spotted owls as previously thought, and in fact may be beneficial to this species. Our results also show that logging does not equate to fire, as they are very different disturbances with different effects on spotted owls.
Quantify probability owls would select roost and foraging sites according to burn severity.
Contrast owl habitat structure among burn-severity classes.Study Objectives
Greenhorn Mountains and Kern Plateau, Sequoia National Forest. Vegetation dominated by Sierra Mixed Conifer at mid elevations and red fir at higher elevations
We located and determined reproductive status of owls starting in May 2006.
We captured and applied back-pack style transmitters specially designed to minimize contact with the owl’s back to reduce heat and feather loss.
We radio tracked each owl nightly or every other night between 9:00 pm and 4:30 am from May through August. We collected a representative sample of foraging locations throughout the night.
We located each owl during the day every 7-10 days to locate roosts and estimate our telemetry error. We also collected regurgitated pellets for diet analysis.
Finally, we measured habitat structure at a subsample of 33 roost sites and at 20 randomly located sites in each of the 4 burn-severity categories within owl foraging ranges. At each site, we measured tree canopy cover, tree size by condition, herb and shrub cover, and size and number of fallen logs, and we assessed vegetation burn severity.
← example of foraging range
After obtaining all of our telemetry locations, we approximated a foraging range for each owl by creating a circle with radius that extended from a nest or roost center to the furthest documented foraging location for that owl. Within each circle, we quantified used habitat as our telemetry locations and available habitat from a systematic grid of points placed at equal spacing throughout the foraging range.
Resource selection ratios (ŵ):
Proportion of roost sites in a burn-severity class / proportion of that class available within foraging ranges (Manly et al. 2002).
Used 95% confidence limits to denote selection of a burn-severity class if the selection ratio > 1, use-equal-to-availability if the selection ratio = 1, or avoidance if the selection ratio < 1.
Combined roost sites of all 7 owls in the sample.
Used habitat = telemetry-error circle around each foraging location.
Available habitat = systematic sample of all telemetry-error circles within each foraging range.
Estimate resource selection probability functions.
Model selection as a function of burn severity, distance from nest/roost centers, owl, slope, and edge.
Describe differences in habitat structure among burn-severity categories.
Model probability of a site being one of 4 burn-severity categories as a function of independent variables of habitat measurements, using multinomial logistic regression.
Example of highly burned habitat
Example of unburned habitat
Example of moderately burned habitat
Spotted owl pairs in all 4 territories attempted to nest in spring 2006 as determined by behavior and evidence of brood patches on females.
Sampled roost sites had an average tree canopy cover similar to unburned forests, and a high basal area of large trees.
Twelve different vegetation types were used by foraging owls, but 78% were in Sierran Mixed Conifer vegetation and 12% in Montane Hardwood and Hardwood-Conifer vegetation.
All burn-severity classes were used for foraging, and proportions of availability for the 4 burn-severity classes were not highly variable among individual owl foraging ranges.
The mean Resource Selection Probability Function generated by combining RSPFs from individual owls shows a curvilinear reduction in probability of use with distance from the center of foraging range that varied with severity of burn.
Overall, the probability that any of these 7 owls would use a site for foraging was greatest when the site was burned – especially high-severity – and was located within approximately 1 km of a nest or roost center.
Low-severity burned sites had numerous large trees and our sampled roost sites had relatively high canopy cover, much like roosting habitat in unburned forests. These conditions provide cover for thermoregulation and protection from predators and mobbing by other bird species.
High-severity sites had the greatest herb and shrub cover and the highest basal area of snags. The primary prey taken by these owls was pocket gopher. In the Sierra Nevada, pocket gophers are uncommon in mature and old forests with little or no herbaceous cover, and they feed primarily on herbaceous grasses and forbs. Another favored owl prey item in our study area was dusky-footed woodrats, which are more abundant in plant communities with greater shrub and herbaceous cover and their nests often encircle snags. Forest fire may have enhanced habitat conditions for both pocket gophers and woodrats, and the standing fire-killed trees offered perch sites for foraging spotted owls.
California spotted owls (n = 7) 4 years after McNally Fire:
The California spotted owls we studied used all severities of burned forest for some aspect of their behavior including nesting, roosting, and foraging during the breeding season 4 years after a large forest fire. Additional research is needed to assess how postfire habitat changes over a longer timeframe effect spotted owls.In the meantime, we urge restraint in assuming all fire, including high-severity fire, has a negative impact on old-forest habitat specialists like spotted owls.
Previous radiotelemetry studies in unburned forests found that California spotted owls did not use tree plantations and areas with low canopy for foraging, thus it is important to stress that logging and fire are different types of disturbances that result in different effects on owl habitat selection.
We recommend longer-term studies to quantify vital rates and site occupancy in burned versus unburned landscapes over multiple years in conjunction with prey studies and without the confounding effect of postfire salvage logging.