Space Use and Home-Range Size of Barn Owls on Santa Barbara Island

Spatial overlap between predators and prey is often a key component of predator-prey interactions. Barn Owls (Tyto alba) are important predators of some species of conservation concern on the Channel Islands in southern California; therefore, understanding patterns of owl space use on these islands could provide insights on variations in predation risk that may be useful for conservation efforts of Barn Owl prey. In this study, our objectives were to investigate home-range size and space use by individual owls on Santa Barbara Island, which at 2.6 km² is the smallest island within the Channel Islands National Park. Specifically, we were interested in owl space use in relation to the spatial distribution of owl prey, in particular the state-listed Threatened Scripps's Murrelet (Synthliboramphus scrippsi)—a small nocturnal seabird whose largest breeding colony in California is on this island and whose nesting habitat is strictly along the island's perimeter. In contrast, the distribution of the Barn Owl's primary prey, deer mice, includes both murrelet habitat and the island interior. We therefore conducted a radiotelemetry study of Barn Owls in combination with a novel technique of applying colored reflective tape to colored plastic leg bands to aid in the identification of individual owls at night. Home-range size estimates for 3 owls were 0.02–0.53 km² using the 100% minimum convex polygon method and were 0.06–1.12 km² using a fixed-kernel method. Owl resight locations for 8 marked individuals were no farther than 1.24 km apart, which suggests that owl home ranges do not generally encompass the entire island. Nocturnal observations of owls also tended to be not far from their diurnal roost sites, which were located close to the edges of the island and near murrelet nesting habitat. This spatial overlap suggests there may be patchiness in predation risk for the owls' seabird and rodent prey in relation to proximity to owl roosts.