ESTIMATION OF BODY-MASS OF VOLES FROM CRANIA IN SHORT-EARED OWL PELLETS

Voles comprised more than 95% of the diet of short-eared owls (Asio fl ammeus) breeding in western Montana. Regression analyses were used to estimate body mass of meadow voles (Microtus pennsylvanicus) and monta ne voles (M. montanus) from measurements of crania found in pellets of these owls. Most voles eaten by the owls were in a size class typical of subadults, and only a few adult voles were eaten. Mean body mass o f the two species of voles appearing as prey items in pellets did not differ significantly. Body mass estimated from cranial dimensions by r egression is superior to the common practice of assuming that all prey items are adults, or to calculation of mass from single skeletal meas urements.