Breeding density and brood size of Common Buzzard Buteo buteo in relation to snow cover in spring

In a 500 km2 study area situated 100-400 m above sea level in southern Norway, breeding density and offspring production of Common Buzzard Buteo buteo was studied in relation to snow depth in spring during 1985-97. Because a thick snow layer will reduce the availability of the preferred prey, Field Vole Microtus agrestis, snow may reduce both Common Buzzard breeding density and offspring production. In the study area, where snow depth in spring increases with increasing elevation, there is a marked rise in mean altitude from 160 to 200 m approximately 15 km from the coast line. Below this border, there were 30 Common Buzzard pairs per 100 km2, whereas at higher altitudes, there were only 15 pairs per 100 km2. The overall mean brood size of successfully nesting Common Buzzards was highest in peak vole years, but it also tended to be negatively related to snow depth in April. Large broods (3 fledglings) were mainly found in years with high vole numbers, low or intermediate snow depth and a low amount of precipitation in May-June

Selas V., ARDEA 89 (3): 471-479.