DIFFERENTIAL PREDATION OF SIZE AND SEX CLASSES OF MICE BY THE BARN OWL, TYTO-ALBA

Simultaneous studies on an insular population of house mice Mus domesticus and an avian predator, the barn owl Tyto alba, showed that owls differentially take small female mice from the population. Owls hunted at night over open vegetation, when mice were also active. Fluorescent pigment tracking and trapping showed that juvenile mice, especially females, used open vegetation more than adults, and hence were potentially most at risk of predation. On an hourly basis, observations of hunting owls were correlated strongly with the numbers of juvenile females using open vegetation, and less strongly with the numbers of juvenile males. A removal experiment showed that adults partially confine juveniles to open vegetation by interference. These findings suggest that fitness of adults is probably increased by foraging in dense vegetation where resources are abundant and risk of predation from owls is low. In contrast, the foraging decisions of juveniles are balanced both against the risk of predation in open vegetation and interference from adults where cover is dense.