Habitat use and densities of cavity-nesting birds in the Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3n204383p

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  • I studied abundance patterns and habitat use of eight cavitynesting bird species in the Coast Ranges of Western Oregon during the spring and summer of 1985 and 1986. Three age classes of unmanaged Douglas-fir (Pseudotsuga menziesii) stands were selected for study: young (40-80 yrs), mature (80-200 yrs), and old-growth (>200 yrs). In general, densities of all cavity-nesting birds were higher in mature and old-growth forests than in young forests; five species in 1985 and 6 species in 1986 were significantly more abundant in old-growth forests compared with young forests. No species had an exclusive association with old-growth forests, however, densities of pileated woodpeckers (Dryocopus pileatus) and chestnut-backed chickadees (Parus rufescens) were consistently (both years) higher in forests >80 years in age. Brown creepers (Certhia americana) and chestnut-backed chickadees were the most abundant species in all stand types. Abundance of most cavity-nesters was correlated consistently with stand age and several vegetative characteristics. Densities of pileated woodpeckers and red-breasted sapsuckers (Sphyrapicus ruber) increased with abundance of large conifers, while the former species and chestnut-backed chickadees were correlated positively with densities of large snags. These habitat features were more common in older-aged (>80 yrs) forests. In contrast, most cavity-nesting birds were associated negatively with densities of small snags and trees, which are most common in young forests. Geographic variables also influenced bird abundance patterns. Densities of northern pygmy owls (Glaucidium gnoma) were associated with numbers of big leaf maple (Acer macrophyllum) trees, which were generally more abundant in the northern Coast Range. Abundance of red-breasted nuthatches (Sitta canadensis) increased with distance from the coast. Other correlations with vegetative and geographic characteristics were inconsistent between years and bird species. In addition, regression analyses accounted for low amounts of variation in species abundance. I found 277 active nests of 9 cavity-nesting bird species, and nests were relatively evenly distributed among stand types. No differences were found among species in their nest-tree characteristics. In general, all species preferred to nest in Douglas-fir snags >50 cm in diameter and >21 m in height. Mean nest-tree diameter ranged from 54 cm for northern pygmy owls to 113 cm for red-breasted sapsuckers; the mean dbh for all species combined was 94 cm. Large, tall snags were used significantly (P < 0.01) more than available in all stand types. Cavity-nesting birds also selected harder snags with more bark and branches compared with available snags. Nest-site characteristics were found to be similar among species. In all stand types, cavity-nesting birds nested in areas with high densities of medium (20-49 cm dbh) and large (>50 cm dbh) snags, dense midstory and understory cover, and a few supercanopy trees. I recommended managing for mean tree (snags and live tree replacements) sizes >94 cm in diameter (dbh) and >31 m tall. Relatively hard (decay class 2) Douglas-fir and grand fir snags with >78% bark and >1 limb should be retained. Creation or retention of snags in clear-cuts will not provide nesting habitat for forest interior species. Management for snags and live tree replacements in all ages of managed forests, and in snag habitat islands and modified shelterwoods, was recommended.
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