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.
