Effects of cattle grazing on bird abundance and diversity in Conservation Reserve Program grasslands

Abstract

Grassland bird populations have declined across North America, likely due to conversion of contiguous grassland to cropland. The Conservation Reserve Program (CRP) has restored grassland habitat and benefits grassland bird populations. However, cattle grazing as a mid-contract management practice in CRP is currently restricted, despite the important role that large herbivores historically played in grassland ecosystems. Conservative grazing may increase spatial heterogeneity and plant diversity, which might support higher densities of some grassland bird species. My objective was to determine how experimental cattle grazing affected species-specific abundance (density and occupancy), species diversity, and community similarity of grassland birds on CRP grasslands across the longitudinal extent of Kansas during the 2017-2019 avian breeding seasons. Half of the 108 fields were grazed by cattle during the growing seasons of 2017 and 2018 and were rested from grazing in 2019. For all analyses, I ran separate model sets for mesic eastern and arid western Kansas. Using distance sampling methods along line transects, I modeled densities of four songbird species and relative abundance of brown-headed cowbirds (Molothrus ater), due to its violation of distance sampling

assumptions. Although densities varied for some species between CRP plantings (CP2 versus CP25), grazing had no substantial effects on densities of songbird species analyzed. I modeled multi-season occupancy of three gamebird species and Henslow’s sparrow (Centronyx henslowii), but grazing did not affect occupancy of any species. Species diversity in eastern Kansas was higher in grazed fields and lower in spring burned fields. Otherwise, non-metric multidimensional scaling revealed no patterns of community dissimilarity between grazed and ungrazed fields. Conservative stocking of cattle during the nesting season might not detrimentally affect bird abundances in CRP grasslands and might temporarily increase bird species diversity, though these responses would likely vary regionally and across species.