THE NEST DEFENSE BEHAVIOR OF EASTERN SCREECH-OWLS - EFFECTS OF NEST STAGE, SEX, NEST TYPE AND PREDATOR LOCATION

The responses of male and female Eastern Screech-Owls (Otus asio) to a human approaching the nest were examined. Intensity of nest defense a s measured by distance of approach, number of flights and dives, and n umber of vocalizations was more pronounced during the nestling stage t han during the incubation stage. These results generally support both the age-investment and positive-reinforcement hypotheses. In addition, male owls defended nestlings more vigorously than did females. As in many other owls, male screech-owls are smaller than females (reversed sexual dimorphism), and the increased maneuverability of the smaller m ales may have contributed to this difference. Other factors (e.g., the use of a relatively large predator), however, could also be involved. Males also responded with greater intensity as the human intruder mov ed closer to the nest, supporting the hypothesis that ''well-armed'' p arents should respond vigorously as a predator gets close to a nest. S uch a response informs the potential predator of the direction it must move to reduce the likelihood of injury.