The effect of broadcasting great horned owl vocalizations on spotted owl vocal responsiveness

One hypothesis advanced for the association of Spotted Owls (Strix occidentalis) with mature forest has been avoidance of competitors and predators such as Great Horned Owls (Bubo virginianus). Great Horned Owls also have been identified as an issue of concern for the conservation of Spotted Owls. Thus, knowledge of Great Horned Owl presence in Spotted Owl territories could be valuable when evaluating trends in Spotted Owl survival. If Spotted Owls avoid Great Horned Owls because of risk of predation, we hypothesized that Great Horned Owl vocalizations should affect Spotted Owl calling behavior. Therefore, we experimentally examined vocal responsiveness of male Spotted Owls after Great Horned Owl vocalizations were played in their territories. We found little evidence that broadcasting Great Horned Owl vocalizations in Spotted Owl territories affected relatively short-term (24 hr) responsiveness of male Spotted Owls. Heterospecific response rates were also low for both species. Thus, our prediction that the presence of Great Horned Owls (i.e., simulated calling by Great Horned Owls) would affect Spotted Owl responsiveness was not supported, at least on the temporal scale at which we conducted the experiment. Our results suggested that surveys to estimate Great Horned Owl presence on Spotted Owl study areas would not confound surveys for Spotted Owls in those areas if at least 24 hr passed between surveys for each species.