Brandt's Vole density affects nutritional condition of Upland Buzzard Buteo hemilasius on the Mongolian Grassland Steppe

Abstract

We evaluated the nutritional condition of nestling Upland Buzzards by using the non-invasive ptilochronology technique. Recognizing that each growth bar on a feather represents 24h of growth, this technique uses the width of growth bars as an index of a bird's nutritional condition at the time the feather was being grown. Despite the fact that ptilochronology has been used in raptors, there is no experimental evidence that growthbar width reflects dietary adequacy in Upland Buzzards. Field work was conducted during the 2007 breeding season in Central Mongolia, in two separate areas that had different densities of Brandt's Vole, the main prey of the focal species. The average Brandt's vole density of 18 different plots in the Eej Khad study area was 441.6 ind/ha. In the desert steppe, the density was zero during the same period of time. Growth-bar width of nestlings was significantly different and wider in vole-rich areas than in areas lacking voles (3.9 vs. 2.24 mm/24-h period). Similarly, there was a significant difference in the number of fault bars; the average was 2.2 in vole-rich areas and 0.5 in areas with no voles. Prey abundance also influenced the average clutch size that was larger in vole-rich areas (4.1 vs. 2.8), resulting in the fledging of a greater number of young (3.8 vs. 2.4).