Nest Reuse by Northern Spotted Owls on the East Slope of the Cascade Range, Washington

摘要

During a long-term demography study of Northern Spotted Owls (Strix occidentalis caurina) in the eastern Cascade Range of Washington State in 1989 to 2008, we documented 276 nests of Northern Spotted Owls at 73 different territories. Of these nests, 90.2% were on platforms, mostly in clumps of deformed limbs caused by dwarf mistletoe (primarily Arceuthobium douglasii), and 9.8% were in cavities in trees. Of the nests associated with dwarf mistletoe, 8.4% were nests built by other raptors and 91.6% were either natural accumulations of debris or debris accumulated by other birds or mammals. Owls switched nests between nesting attempts 81.2% of the time. The presence of a new male or female at a territory did not affect the odds of switching nests between nesting attempts. The odds an owl would reuse a nest were 6 times greater for owls that were successful in the previous nesting attempt compared to owls that were unsuccessful, given the same type of nest structure. The odds an owl would reuse a cavity nest were 4.7 times greater than the odds an owl would reuse a platform nest, given the same level of nest success the previous year. The estimated mean annual survival rate (φ) of nest structures was 0.98 (SE 5 0.006), suggesting that mean life expectancy of nests was 42 y. However, nests on dwarf mistletoe platforms may be more ephemeral than cavity nests or the nest trees themselves, and management for viable nest areas for Spotted Owls should include multiple trees with mistletoe brooms suitable for alternate nests. Our results, and results from other studies, indicate that Douglas-fir (Pseudotsuga menziesii) trees infected with dwarf mistletoe are an important habitat component for Spotted Owls and many other species of birds and arboreal mammals on the east slope of the Cascade Range in Washington.