Larger predators consume prey of greater mean size and include a wider diversity of prey in their diets than their smaller counterparts occurring in the same communities. There is some controversy as to whether these patterns result from opportunistic feeding behavior or from prey size selectivity leading to food-niche segregation among predators. This study examined the effects of body size on the diet of avian predators in the sagebrush habitat of north-eastern Utah. The assessment was based on data collected from the analysis of pellet contents and was deliberately confined to mammalian components, almost exclusively from rodents. A significant positive relationship was found between predator size and both average and maximum prey size, but no such correlation was found for the minimum body size of prey. In general, there was considerable overlap in the rodent prey taken by different raptors, suggesting opportunistic feeding behavior in these predators. However, the size (and species) of rodent prey that contributed most to the consumed biomass was different for each bird species and correlated well with its body size. The revealed pattern of larger raptors acquiring most biomass (energy) from larger prey, implies food selectivity based on its energetic profitability, and niche segregation that could facilitate the coexistence of a high diversity of avian predators in the sagebrush habitat. The possible role of food limitation and competition in the evolution of body size in raptors and the consequences of size-dependent predation are discussed.