BREEDING BIRD COMMUNITY COMPOSITION IN RELATION TO RIPARIAN VEGETATION STRUCTURE IN THE WESTERN UNITED-STATES

Riparian zones, which are under increasing pressure by various users, provide sources of avian diversity in many landscapes of the semiarid West; however, there are few data available that address avian communi ty composition in different riparian vegetation structures. We compare d total abundance of diurnal breeding birds, individual bird species a bundance, bird species richness, and vegetation composition and struct ure among streamside vegetation associations in Bear and Silvies valle ys in eastern Oregon during 1993 and 1994. We collected bird and veget ation data along 4 replicate transects within each of 3 vegetation ass ociations characterized by streamside vegetation structure: herbaceous xeric shrub, discontinuous mesic shrub, and continuous mesic shrub. T otal bird abundance was greater (P < 0.001) in the continuous mesic sh rub association than in the herbaceous xeric shrub and discontinuous m esic shrub associations during 1993 and 1994. Species richness was inc onsistent between years among vegetation associations (P = 0.034); it was greatest (Ps less than or equal to 0.037) in the continuous and di scontinuous mesic shrub associations in 1993, and greatest (Ps less th an or equal to 0.003) in the continuous mesic shrub association in 199 4. Yellow warbler (Dendroica petechia), song sparrow (Melospiza melodi a), willow flycatcher (Empidonax traillii), American robin (Turdus mig ratorius), common snipe (Gallinago gallinago), bobolink (Dolichonyx or yzivorus), and Vaux's swift (Chaetura vauxi) were most abundant in the continuous mesic shrub association (Ps less than or equal to 0.088). Savannah sparrow (Passerculus sandwichensis), black tern (Chlidonias n iger), and American wigeon (Anas americana) were most abundant in the herbaceous xeric shrub association (Ps less than or equal to 0.032). Y ellow warblers, willow flycatchers, and song sparrows, which almost ex clusively depend on hydrophytic shrub thickets for nesting in the semi arid western United States, are especially threatened by the eliminati on or simplification of continuous associations of mesic shrub vegetat ion. We recommend that continuous associations of mesic shrub vegetati on be maintained or restored where possible because this vegetation st ructure is associated with avian abundance, species richness, riparian -associate bird species abundance, and landscape-level biological dive rsity.