OWL SELECTION ON SIZE AND SEX CLASSES OF RODENTS: ACTIVITY AND MICROHABITAT USE OF PREY

Our aim was to study selective owl predation on size and sex classes of rodents in northwestern Argentine Patagonia and to determine which behavioral traits may be involved in differential prey vulnerability. Diet of Magellanic horned owls (Bubo magellanicus) was studied for 2 years. Jaw and pelvic measurements were used to estimate size and sex of rodents eaten by owls. Livetrapping of rodents was conducted simultaneously with diet analysis to estimate proportion of size and sex classes of rodents available and to examine movement and microhabitat use. Owls generally selected individuals of smaller size and avoided heavier individuals. Sex classes of prey were consumed according to availability, except that the female silky desert mouse (Eligmodontia morgani) was selected. Smaller individuals of E. morgani, the long-haired mouse (Akodon longipilis), and the yellow-nosed mouse (A. xanthorhinus) tended to use open microhabitat where risk of predation was higher. Vulnerability to predation seemed to be related to differential activity and use of microhabitat.