Effects of habitat fragmentation on birds in the coastal coniferous forests of the Pacific Northwest

Few studies have been done in the Pacific Northwest on the effects of habitat fragmentation on birds. Comparisons among studies is difficult because of different study designs and possible regional variation in bird response. Timber harvesting and human settlements have greatly fragmented the once vast amounts of old-growth forests. Forest patches of the Pacific Northwest are typically surrounded by forests of different ages rather than agricultural lands, as is found in much of eastern North America. In Washington, one three-year study showed that overall bird species richness and abundance varied little in a managed coniferous forest despite differing degrees of fragmentation. Some individual species, however, increased or decreased with the amount of clearcut area and other landscape variables. Species associated with open habitats or edges increased, while those associated with forests having a well-developed canopy decreased. There is substantial variation in avian response to landscape variables that characterize watersheds. At the stand level, canopy dwellers and cavity nesting species show the most negative response to increasing levels of canopy reduction, whereas species associated with the ground or shrub layer are least affected. Cowbird parasitism is negligible in the mountains of the Pacific Northwest, but apparently is more widespread in the large valleys such as the Puget Sound lowlands and Oregon's Willamette Valley where more farmland and urban, non-forest environments exist. More studies are needed on fragmentation effects on birds and cowbird parasitism in the region.