Estimating individual weight of prey items for calculation of the biomass in the diet of long-eared owl (Asio otus): is it worth of extra effort?

A method is presented for calculation of biomass in the diet of the long-ea red owl (Asio otus) using individual weights of prey items. Weights were es timated from skull measurements. The method is compared with those in which biomass is calculated using mean weights of the prey species obtained from literature or from small mammal trappings within the study area. Differenc es were significant when energy income was calculated, but less so when the relative importance of particular prey species for the owl was considered. Use of reduced adult weight for water voles (Arvicola terrestris) is also suggested for calculations of prey biomass in long-eared owl diet.