Ecological relationships of birds in forests of western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1j92gb06g

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  • The avifaunal composition of ten western Oregon forest stands located at the eastern base of the Coast Range was examined on a seasonal basis. The stands were dominated by Oregon white oak, Douglas fir or western hemlock, Avian populations were sampled monthly from January 1968 to January 1970, using permanent transacts. In order to determine seasonal changes in bird species composition and diversity, variations in the ecological roles of the bird species, their patterns of habitat utilization, and the importance of habitat components in determining the abundance of species, information was gathered on the behavior and activity patterns, morphological variation and dietary habits of the bird species, and on the vegetative structure of the stands. Intensive studies were centered on seven permanent resident species: Black-capped Chickadee, Chestnut-backed Chickadee, White-breasted Nuthatch, Red-breasted Nuthatch, Brown Creeper, Rufous-sided Towhee, and Oregon Junco. Oak-dominated stands had the highest bird species diversity in all seasons. This is in contrast to the expected increase in diversity with each successional sere, In most cases the actual species diversity was at least 90 percent of the maximum possible diversity. All fir and hemlock stands shared a large number of species and supported roughly similar total populations, in the ecotonal areas diversity was slightly higher than in the surrounding pure stands of either deciduous or coniferous vegetation. More individuals and species were found in western Oregon forests than reported for forests in eastern United States. Further, a large proportion of these birds were permanent residents. Because of this large number of permanent residents, the percentage of migratory birds was lower than in eastern forests. These differences may in part stem from the milder winter climate characteristic of western Oregon. More than 50 percent of the birds present during all seasons in all vegetative types belonged to either the foliage-insect or foliage-seed eating ecological roles. Insect activity had a major influence on avifaunal structure, as at any time of the year 60 to 80 percent of the species recorded belonged to the insect-eating roles. When the effects of vegetative structural features on avian abundance were compared, little difference was found between the importance of variables in the fir and hemlock areas. For the bird species inhabitating all vegetative seres analysed in this study, the same set of structural components affected each species' abundance throughout its ecological distribution, The avifauna of these western Oregon forests thus does not fit into any recognised plant community classification. The birds move between areas within their range of ecological tolerance, providing an energy link between the immobile vegetation.
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