Effects of stream size on bird community structure in coastal temperate forests of the Pacific Northwest, USA

Vegetation and spring breeding bird populations associated with riparian fo rests on high and low order rivers of the Olympic peninsula, Washington (US A), were censussed in 1990 and 1991. The objectives were to (1) provide a b aseline description of bird communities associated with unmanaged riparian forests, (2) determine whether there is a difference between bird communiti es along large and small rivers, and (3) identify what biophysical aspects of the fluvial corridor are most important in determining bird community st ructure. The approach was to link riparian bird community structure to rive r order and discuss relationships to vegetation, geomorphology and associat ed hydrologic disturbance regimes. Large rivers tended to have more deciduous vegetation than small rivers (P = 0.07). Deciduous vegetation and patch types appeared to be linked to rive r size through stream valley shape and disturbance history. Bird species ri chness and abundance were significantly higher (P<0.05) on larger rivers (6 7-140 m width) than on small rivers(12-21 m width) and were positively corr elated with the percentage of deciduous trees present at a site. The data s uggest that migrant birds, such as the black-throated grey warbler (Dendroi ca nigrescens) and warbling vireo (Vireo gilvus), require large tracts of c ontiguous hardwoods for successful breeding. Additionally, birds such as th e Hammond's flycatcher (Empidonax hammondii) and hairy woodpecker (Picoides villosus) appear to depend on the juxtaposition of hardwoods and conifers during the breeding season. We conclude that bird communities on large rive rs differ from those on small rivers and that these differences occur predi ctably with stream order.