Estimation of food consumption from pellets cast by captive Ural Owls (Strix uralensis)

There is considerable data in the literature on the diet of the Ural Owl (Strix uralensis) based on pellet analysis. Though it is possible to identify prey items by this method, the volume of food consumption is still unknown. The population of Ural Owls in Japan is declining due to the reduction of old-growth forest and the concurrent loss of natural nest cavities in older trees. More information, including dietary requirements, is needed to conserve suitable habitat and manage for this species. In this study, ingested food and cast pellet mass were quantified to determine if food consumption could be predicted from pellet mass for captive Ural Owls. Three caged adult owls were acclimatized for 2 months. Three common and natural Ural Owl prey species were fed to the captive owls: Microtus montebelli Apodemus speciosus, and A. argenteus. There was no significant difference in the caloric value for each prey species used. Therefore, the body weight (mass) of prey species was considered a suitable index of food energy. The energy intake by owls was positively correlated with dry pellet weight: estimated calorie intake (kcal) = 33.82 x dry pellet mass (g) + 7.38. The influence of owl activity and seasonal changes in energy budgets on this relationship need to be determined.