Breeding bird community composition in relation to riparian vegetation structure in grazed habitats Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/h702qb27c

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  • Riparian zones provide habitat for breeding birds in the semiarid western United States; however, there are few data available that address the effects of livestock grazing strategies on riparian habitats and avian communities. Documenting avian community composition in different riparian vegetation communities and relating vegetation communities to livestock grazing strategies may identify management alternatives that are sustainable from a wildlife habitat perspective, and may permit constructive coalitions between agricultural industry and environmental groups. I compared diurnal breeding bird abundance, individual species abundance, and species richness, and vegetation composition and structure among 12 streamside riparian areas of Bear and Silvies valleys in eastern Oregon during 1993 and 1994. Bird and vegetation data were collected along four replicate transects within each of three riparian vegetation communities characterized by vegetation structure: herbaceous, discontinuous willow (Salix spp.), and continuous willow. These riparian vegetation communities were grazed under summer season-long, summer short-duration, and fall short-duration livestock grazing strategies, respectively, >5 years before the study. Differences in riparian vegetation among communities were primarily related to shrub structure by experimental design. The continuous willow community had more shrub cover overall (P<0.001) and within each 1-m height interval from 0-4 m (P<0.013) than the herbaceous and discontinuous willow communities. The herbaceous community had no shrub cover >1 m in height. Willows extended farther (P=0.031) from the steam edge in the continuous willow community than in the discontinuous willow community. No willows were detected in the herbaceous community. I detected 4,016 birds representing 56 species along the transects. Total bird abundance was greater (P<0.001) in the continuous willow community than in the herbaceous and discontinuous willow communities. Species richness was inconsistent (P=0.034) between years within communities; it was greatest (P<0.037) in the continuous and discontinuous willow communities in 1993, and greatest (P<0.003) in the continuous willow community in 1994. Total bird abundance and species richness in 1994 increased with willow volume (r²>0.707, P<0.001). Of 23 bird species with >20 individual detections over both years, 13 species were most abundant in one or two vegetation communities (P<0.088). Seven species (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 willow community. Three species (savannah sparrow [Passerculus sandwichensis], black tern [Chlidonias niger], and American wigeon [Anas americana]) were most abundant in the herbaceous community. Willets (Catoptrophorus semipalmatus) were most abundant in the herbaceous and discontinuous willow communities whereas cinnamon teal (A. cyanoptera) were most abundant in the herbaceous and continuous willow communities. Red-winged blackbirds (Ageliaus phoeniceus) were inconsistent (P=0.032) between years within communities: they were most abundant in the continuous willow community in 1993, and most abundant in the continuous willow and herbaceous communities in 1994. Hydrophytic woody vegetation within semiarid rangeland environments increases structural complexity and is associated with avian abundance and diversity. Yellow warblers, willow flycatchers, and song sparrows, which depend on hydrophytic shrubs for nesting almost exclusively in the semiarid West, are especially threatened by the elimination or simplification of woody riparian vegetation. I suggest that riparian vegetation structure and composition, which is associated with avian abundance, species richness, riparian associate bird species, and landscape-level biological diversity, be maintained where possible. Seasonal light (<30% use) fall short-duration grazing seemed to be compatible with the maintenance of woody riparian vegetation whereas summer season-long and summer short-duration grazing is likely incompatible.
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