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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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habitat use and selection by california spotted owls in a postfire landscape

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.

slide2

Spotted Owls

Strix occidentalis

  • One of the rarest bird species to breed in mainland U.S.
  • Higher survival and better reproductive success in territories with more forest with high canopy cover and large trees.
  • Important Management Indicator Species for the health of old coniferous forests.
slide3

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.

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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.

study objectives
Identify habitat used by spotted owls for nesting, roosting, and foraging in a burned landscape.

Quantify probability owls would select roost and foraging sites according to burn severity.

Contrast owl habitat structure among burn-severity classes.

Study Objectives
study area southern sierra nevada
Study Area: Southern Sierra Nevada

Greenhorn Mountains and Kern Plateau, Sequoia National Forest. Vegetation dominated by Sierra Mixed Conifer at mid elevations and red fir at higher elevations

study area southern sierra nevada1

McNally Fire 22 JULY 2002

Study Area: Southern Sierra Nevada

In July and August of 2002, the McNally Fire burned approximately 61,000 ha in the Sequoia and Inyo National Forests, including 33,700 ha of conifer-dominated forests.

fire in owl habitat
Fire in Owl Habitat
  • 7 owls (4♂; 3♀) in 4 territories. Within foraging ranges:
  • 31% unburned
  • 29% low severity
  • 27% mod. severity
  • 13% high severity
data collection methods
Data Collection Methods

We located and determined reproductive status of owls starting in May 2006.

data collection methods1
Data Collection Methods

We captured and applied back-pack style transmitters specially designed to minimize contact with the owl’s back to reduce heat and feather loss.

data collection methods2
Data Collection Methods

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.

data collection methods3
Data Collection Methods

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.

data collection methods4
Data Collection Methods

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.

sample sizes
Sample Sizes
  • 4 nests; 1 fledgling produced.
  • 60 roost sites; 7–11 per owl.
  • 301 foraging locations; 32–53 per owl.
  • 33 vegetation surveys at roost sites.
  • 80 vegetation surveys at random plots in owl foraging ranges; ~20 plots per burn-severity class.
data analyses
Data Analyses

← 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.

data analyses roosting
Data Analyses – Roosting

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.

data analyses foraging
Data Analyses – Foraging

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.

data analyses habitat
Data Analyses – Habitat

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.

data analyses habitat1
Data Analyses – Habitat

Example of highly burned habitat

Example of unburned habitat

Example of moderately burned habitat

results nesting
Results - Nesting

Spotted owl pairs in all 4 territories attempted to nest in spring 2006 as determined by behavior and evidence of brood patches on females.

  • One nest tree in unburned mixed conifer-hardwood, one in conifer forest burned at low severity, and 2 in conifer forest burned at moderate severity.
  • One pair, nesting in a stand burned at moderate severity, produced the only fledgling of the 4 nesting attempts.
  • All 4 nest trees were large conifers greater than 72 cm, including one apparently killed by the fire within a moderately burned stand
results roosting
Results - Roosting

Sampled roost sites had an average tree canopy cover similar to unburned forests, and a high basal area of large trees.

  • Our sample of spotted owls:
  • selected low severity burned forests for roosting (95% confidence interval of ŵ: 1.31–2.60);
  • used unburned forest (0.36–1.46) for roosting in proportion to its availability; and
  • avoided forest burned at moderate severity (0.02–0.97).
  • Only 1 incidence of roosting in high-severity burned forest.
results foraging
Results - Foraging

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.

probability of use
Probability of Use

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.

probability of use1
Probability of Use

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.

results habitat
Results - Habitat

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.

results habitat1
Results - Habitat

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.

bottom line as the headline
Bottom Line as the Headline

California spotted owls (n = 7) 4 years after McNally Fire:

  • Roosting: Selected low-severity and used unburned forest in proportion to availability;
  • Foraging: Selected high-severity and avoided unburned forest near core-use area.
slide28

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.

slide29

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.

acknowledgements
Acknowledgements
  • Field assistants Keith Bruno and Liz Kehas
  • USDA Forest Service biologists Wendy Rannals and Robin Galloway
  • Funding from Resources Legacy Fund Foundation and Environment Now