Diet of Northern Saw-Whet Owls in Southern Wisconsin

Abstract

We described 623 roosts of Northern Saw-whet Owls (Aegolius acadicus) in southern Wisconsin from 1986 to 1990. Roosts were in seven species of trees, with 97.9% in white (Picea glauca) and Norway (P. abies) spruces, red (Pinus resinosa) and jack (P. banksiana) pines, and eastern red cedar (Juniperus virginiana). Mean roost height was 4.05 * 2.2 1 m (range 0.15-l 1.20 m) in a 9.15 + 3.40 m tall tree. Roosts averaged 46.6 ? 43.4 cm from the trunk on a 150.4 + 69.6 cm long limb. Roost characteristics varied according to tree species, size, and shape. Mean roost height ranged from 1.5 + 0.4 m in eastern red cedars to 6.9 + 1.3 m in red pines. Roost height correlated positively with tree height and negatively with distance of roost from trunk. Distance of roost from trunk correlated positively with limb length. Directions of roosts relative to the trunk were random. Mean roost height and height of roost tree increased with time. Most roosts afforded good cover from above and most sides. Saw-whet Owls chose roosts that provide concealment, not those of a particular height. Roosts conferred thermal benefits on owls. Behavior of roosting Sawwhet Owls suggests that the owls’ motionlessness when approached by humans is a camouflaging strategy.