Grazing Influence on Selected Parameters of the Avian Community on a Texas Hill Country Ranch
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Many regionally declining populations of prairie and shrubland birds breed in the Edwards Plateau ecoregion of Central Texas. Additionally, Central Texas supports a wintering bird community rich in ground foraging sparrow species. Livestock grazing can have species specific and mixed results for local bird communities and other wildlife. I examined the degree to which grazing influences bird foraging frequency and bird abundance, richness, diversity, and evenness relative to herbaceous vegetative ground cover at Freeman Center, a 1,701 ha working cattle ranch in the Balcones Canyonlands subregion of the Edwards Plateau. For one year, I conducted avian surveys and herbaceous ground cover surveys on two grazed and two ungrazed pastures using twenty, 100 meter fixed radius point count sites and twenty, 100 meter transects extending from each site. I included a total of 383 Daubenmire frame surveys, 135 point count surveys, and 184 avian walking transect surveys in various analyses. I used General Linear Models (GLMs) to analyze herbaceous ground cover surveys in grazed and ungrazed sites. I incorporated significant herbaceous vegetation predictors into General Linear Mixed Models (GLMMs) to analyze breeding and wintering bird abundance, richness, diversity, and evenness. I also included site as an intercepts-only random factor. I built an additional GLMM to analyze avian winter ground foraging counts. I identified a total of 138 avian species from Freeman Center between January 2014 and May 2015. All breeding bird indices were significantly different among breeding seasons. Breeding bird richness positively correlated with forb cover (β = 0.187, Z = 3.000, P = 0.003). Breeding bird diversity was positively correlated with tallest green grass (β = 0.631, Z = 2.400, P = 0.016) and forb cover (β = 0.171, Z = 2.710, P = 0.007). Except foraging counts, no wintering bird indices were significantly correlated with herbaceous ground cover predictors. Wintering bird foraging counts were positively correlated with forb cover (β = 0.809, Z = 3.070, P = 0.002). Breeding and wintering bird abundance, richness, diversity, and ground foraging counts were all higher in grazed sites than ungrazed sites. Results suggest that moderate grazing pressure promotes forb production and native forbs are important for breeding and wintering birds in the Texas hill country. Future studies should analyze herbaceous plant diversity and the dominant herbaceous plants in study sites throughout the Edwards Plateau. Judgment deferred rotational grazing should be appropriate when ranch managers have the knowledge, experience, resources, and prudence to make best-management decisions based on climate, rainfall, and sustainability. A multi-year study is necessary to assess long-term cattle use and the affects of climate and rainfall on the health and future of ranch operations and wildlife diversity at Freeman Center.