Graduate Thesis Or Dissertation
 

Habitat associations of bird communities in shrub-steppe and western juniper woodlands

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  • The encroachment of western juniper into mountain big sagebrush plant communities and the subsequent development of juniper stands results in significant changes in vegetation structure and complexity. This study described and compared vegetative structure and bird community measures among four cover types representing a broad range of structural stages among semi-arid, upland plant communities in central Oregon. Avian communities and vegetation were sampled in 4 vegetative structural stages (cover types), including: 1) post-burn grassland (<5years post-burn); 2) mountain big sagebrush-Idaho fescue shrub-steppe; 3) mid-successional western juniper; and 4) old-growth western juniper. The point count, or variable circular plot (VCP), method was used to survey birds in May and June of 1998 and 1999. Bird density estimates for each transect in each year, mean relative abundance, and species richness, evenness, and diversity were compared among cover types. Sixteen of 22 vegetation measurements were significantly different (P<_0.05) among the four cover types. Mean bird species richness was greatest in the old-growth and mid-successional juniper types, with means of 22.9 and 23.6 species transecf 1, respectively. Bird species diversity (H) was similar between the old-growth (H'=2.499) and mid-successional (H'=2.416) cover types. Mean species richness was lower in the grassland cover type (17.6 species transect ) and lowest in the shrub-steppe cover type (11.5 species transect 1). Bird species diversity was similar in grassland and shrubsteppe, H=1.820 and H=1.644. Habitat complexity was evaluated by comparing the diversity of coverage of plant structural types. Indices of habitat complexity were calculated for each transect by using physiognomic cover diversity (PCD). Bird species diversity was positively correlated with PCD (r2=0.535; P=0.0013). Total bird density was significantly greater in 1999 than in 1998 (P<_0.05). Total bird density was greatest in old-growth juniper (6.551 birds ha ) and lowest in post-burn grasslands (3.569 birds ha 1). Total bird density was similar in shrub-steppe and mid-successional juniper (5.968 and 5.542 birds ha-1, respectively). Old-growth juniper had the highest total densities of both tree and cavity nesters. Total density of ground nesting birds was greatest in the grassland type. Total density of shrub nesting species was highest in the shrub-steppe type. Species consistently detected within all four cover types were Empidonax spp. flycatchers, brown-headed cowbirds, and northern flickers. Species detected in juniper stands but not other types included chipping sparrows, mountain bluebirds, mountain chickadees, Cassin's finches, American robins, ash-throated flycatchers, and dark-eyed juncos. The most common species in the shrub-steppe were also present in mid-successional juniper, but at lower densities. Species composition in mid-successional juniper was a mixture of shrub-associated and tree-associated bird species. Many bird species that regularly occurred in old-growth juniper also occurred in mid-successional juniper, however, density and richness of shrub nesting species was lower in old-growth juniper. Mountain chickadees, Cassin's finches, chipping sparrows, brown-headed cowbirds, mountain bluebirds, and dark-eyed juncos, Empidonax spp. flycatchers, ashthroated flycatchers, and northern flickers were correlated with juniper canopy cover, canopy volume, total tree density, and average tree height. Vesper sparrows, western meadowlarks, green-tailed towhees, and horned larks were correlated with vertical density of herbaceous vegetation, total vertical density of all vegetative functional groups, and perennial grass cover. Brewer's sparrows, sage sparrows, sage thrashers, and horned larks were associated with total cover of all shrub species, cover of green rabbitbrush and sagebrush, vertical density of shrubs, and bare ground. In the absence of fire or direct management actions, the transition of mountain big sagebrush plant communities to juniper woodlands will likely result in decreased densities or loss of shrub nesting and sagebrush obligate birds. However, sites in the early stages of juniper transition, with an intact shrub component, may provide both shrub and tree-foliage nesting bird species with suitable breeding habitat. Old growth juniper stands provide both tree and cavity nesting species with breeding habitat, but shrub-nesting species will likely be absent or at very low densities because of low shrub cover. On a landscape level, a mixture of various ages of juniper stands, as well as juniper-free mountain big sagebrush and grassland will likely result in the greatest diversity of bird species.
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