Annual weight cycle in wild screech owls

The annual weight cycle of wild birds of prey has received little attention in the past, primarily because of the difficulty in capturing and recapturing them. Screech Owls (Otus asio) are resident in our northern Ohio study area and readily occupy nest boxes established for Wood Ducks (Aix sponsa) (see VanCamp and Henny 1975). In addition to nesting in the boxes, the owls use them as roosting and feeding stations during the winter and are easily captured in the boxes during the day. Therefore, the owls presented an unusual opportunity to obtain information on their annual weight cycle.

Between January 1975 and December 1978, Screech Owls were captured, weighed, and banded. Several were subsequently recaptured and weighed again to form the basis of this report. It is difficult to sex live Screech Owls in the field because they are not sexually dimorphic; however, Sherman (1911) noted that females did all of the incubating. We sexed adult owls with young at nest sites by the presence (female) or absence (male) of a brood patch. Owls captured away from nests during the nesting season without brood patches, or at other times of the year, were classified as sex unknown, unless they were previously captured at a nest with young. Screech Owls in northern Ohio have young in the nests from mid-April through early June.