Foraging responses of the great horned owl (Bubo virginianus) to changes in prey abundance

Includes bibliographical references (pages [59]-80)Predators may respond either numerically or functionally to fluctuations in prey availability. Generalist predators at mid-level latitudes are most likely to exhibit functional responses to changes in prey abundance. The functional response is characterized by a change in the predator’s prey consumption. This change in prey consumption may be caused either by the predator altering habitat use, thereby tracking its prey abundance across the landscape, or by the predator altering prey selection, such as switching to alternate prey when the most abundant prey declines. The prey and habitat selection of the Great Homed Owl (Bubo virginianus), a generalist avian predator, was investigated under conditions of fluctuating prey abundance from March 1994 through August 1995. The study site was located at Fermi National Accelerator Laboratory in northern Illinois. Prey abundance was estimated monthly in seven habitat locations characteristic of those utilized by Great Homed Owls. Small mammals were surveyed using standard mark-recapture techniques along three 200 m transects in each habitat. Larger mammals and avian prey were surveyed using visual flush counts along the same transects. Great Homed Owl habitat use was determined monthly using call playback surveys and visual observations. Great Homed Owl diet selection was estimated from pellets collected below frequently used perches throughout the site. There was a significant difference in the abundance of the three primary prey throughout the study. Great Homed Owls did not exhibit a switching behavior in response to changes in prey abundance. Rather, Great Homed Owls exhibited a strong and consistent preference for Microtus while they demonstrated a strong negative preference for Peromyscus. Sylvilagus were consumed approximately proportional to their availability. Great Homed Owls did not appear to track prey abundance across the landscape. Great Homed Owl habitat use was correlated with vegetative variables that were most likely related to nest site characteristics. Overall, Great Homed Owls behaved more similarly to small mammal specialists than to generalist predators as they are commonly characterized.M.S. (Master of Science