Errington proposed that predators mainly kill substandard prey, because dominant individuals force subordinate ones into poor habitats, where the predation risk is higher. We studied the prey choice of breeding male Tengmalm's owls (Aegolius funereus funereus) in 1992, when vole densities crashed. We trapped small mammals in the main habitat types in 21 owl territories, and simultaneously identified prey items cached by the same owls in their nest boxes. The main prey of owls in western Finland are the common vole (Microtus epiroticus), field vole (M. agrestis), bank vole (Clethrionomys glareolus), and common shrew (Sorex araneus). Common voles were the preferred prey of owls, followed by field voles, bank voles, and common shrews. Prey captured by owls tended to be lighter and smaller than those available in the field. This tendency was significant for field voles and common shrews. Field voles, common voles, and female common shrews captured by owls tended to have more internal fat than those available in the field. This tendency was significant for male field voles and female common shrews. Owls appeared to choose small individuals of some, but not all, prey species, and that these prey items were not in poor physiological condition.
Dietary evenness, prey choice, and human–environment interactions
