Northern spotted owl and barred owl home range size and habitat selection in coastal northwestern California

Graduation Date

2015

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Natural Resources: Wildlife, 2015

Committee Chair Name

T. Luke George

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

Northern spotted owl, California, Humboldt State University -- Theses -- Wildlife, Strix occidentalis caurina, Home range, Barred owl, Habitat selection, Strix varia

Abstract

The federally threatened northern spotted owl (Strix occidentalis caurina) continues to experience range-wide population declines despite two-plus decades of intensive conservation and recovery efforts. Management of the owl has been a highly contentious issue, focused primarily on setting aside critical habitat and preserving old-growth and mature forests in the Pacific Northwest. Recent evidence however, indicates that competition from the invading and closely-related barred owl (Strix varia) is the primary threat to northern spotted owl populations. While agonistic interactions are widely documented, there remains a lack of information on how these two species compete for ecological resources. The redwood region of coastal northwestern California, where spotted owl populations remain relatively high, offered a unique opportunity to study resource competition and partitioning between northern spotted owls and barred owls. I used radio-telemetry to locate both species at night with the objectives of (1) determining home range size of northern spotted owls and barred owls (2) determining foraging habitat selection and resource partitioning of sympatric northern spotted owls and barred owls. Average home range size for northern spotted owls and barred owls during the breeding season was 391 ha and 303 ha, respectively, and average home range size during the nonbreeding season was 560 ha and 442 ha, respectively. I did not find a significant difference in home range size between the two species in either season. Spotted owls occupied smaller home ranges in my study area than in the northern portion of their range and both species exhibited relatively little home range size variation between seasons. My top four models (ΔAICc2) indicated that both species exhibited habitat selection for percent understory vegetation, hardwood trees (percent and basal area), and distance to nest. However, variables in my top models included the interactive effects of the owl species, providing evidence that some degree of resource partitioning may exist between northern spotted owls and barred owls in coastal northwestern California.

https://scholarworks.calstate.edu/concern/theses/th83m195z

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