Effects of grazing on vegetation structure, prey availability, and reproductive success of Grasshopper Sparrows

Abstract

Populations of Grasshopper Sparrows (Ammodramus savannarum) have been declining, and ag- ricultural practices, such as grazing by domestic cattle (Bos taurus), are likely contributing factors. Grazing can alter the composition and structure of vegetation and influence prey availability, and such changes can impact the nesting success of grassland birds. Our objective was to examine the nesting success of Grasshopper Sparrows in grazed and ungrazed habitats on the Blue Grass Army Depot in Madison County, Kentucky. Clutch sizes of female Grasshopper Sparrows nesting in grazed and ungrazed areas differed significantly, with mean clutch sizes of 4.48 in ungrazed areas and 3.91 in grazed areas. In addition, nest success was higher in ungrazed areas (70%) than grazed areas (25%). Insect sweeps revealed that invertebrate biomass in ungrazed areas was greater than in grazed areas, and analysis of vegetation indicated that grazed areas had less litter, more shrubs, and shorter, less dense vegetation than ungrazed areas. Most unsuccessful nests were depredated, and the higher predation rates on nests in grazed areas may have been due to differences in vegetation structure. Shorter, less dense vegetation in grazed areas may make it easer for predators to observe adults and locate nests, while taller, denser vegetation in ungrazed areas may provide greater concealment. While the results of previous studies suggest that light to moderate grazing can produce habitat suitable for Grasshopper Sparrows, more intense grazing, as on our study area (one animal unit/ha), creates habitat less suitable for these sparrows.