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 community composition in different riparian vegetation structures. We compared total abundance of diurnal breeding birds, individual bird species abundance, bird species richness, and vegetation composition and structure among streamside vegetation associations in Bear and Silvies valleys in eastern Oregon during 1993 and 1994. We collected bird and vegetation data along 4 replicate transects within each of 3 vegetation associations characterized by streamside vegetation structure: herbaceous xeric shrub, discontinuous mesic shrub, and continuous mesic shrub. Total bird abundance was greater (P < 0.001) in the continuous mesic shrub association than in the herbaceous xeric shrub and discontinuous mesic shrub associations during 1993 and 1994. Species richness was inconsistent between years among vegetation associations (P = 0.034); it was greatest (Ps less than or equal to 0.037) in the continuous and discontinuous mesic shrub associations in 1993, and greatest (Ps less than or equal to 0.003) in the continuous mesic shrub association in 1994. Yellow warbler (Dendroica petechia), song sparrow (Melospiza melodia), willow flycatcher (Empidonax traillii), American robin (Turdus migratorius), common snipe (Gallinago gallinago), bobolink (Dolichonyx oryzivorus), 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 niger), and American wigeon (Anas americana) were most abundant in the herbaceous xeric shrub association (Ps less than or equal to 0.032). Yellow warblers, willow flycatchers, and song sparrows, which almost exclusively depend on hydrophytic shrub thickets for nesting in the semiarid western United States, are especially threatened by the elimination or simplification of continuous associations of mesic shrub vegetation. We recommend that continuous associations of mesic shrub vegetation be maintained or restored where possible because this vegetation structure is associated with avian abundance, species richness, riparian-associate bird species abundance, and landscape-level biological diversity.