Scientific Name: Aegolius acadicus
Common Name: Saw-Whet Owl

(Information for this species page was gathered in part by Ms. Kristin Baer for an assignment in Biology 220W, Spring 2007)

The northern saw-whet owl (Aegolius acadicus) is a year-round resident of Pennsylvania and an overwintering migrant species of states to our east, west, and south. Its unusual name is derived from the sound of its calls. These calls have been variously described as resembling the sound of water dripping into a half-filled pail or the pinging that is generated when a saw blade is being sharpened (or “whetted”). The northern saw-whet vocalizes only during its breeding season (March to May) and uses its song both to attract mates and to mark its breeding territory.

Range
The northern saw-whet is the smallest owl found in the eastern half of the United States. Its range extends from the forests of the northeast, around the Great Lakes, across most of Canada, into Alaska, and southward down the line of the Rocky Mountains and the western coastal states. A year-round population of northern saw-whets is also found in the mountains of central Mexico.

Appearance
The northern saw-whet owl is 7 inches tall and weighs between 80 and 90 grams (2.8 to 3.2 ounces), Female owls tend to be 25% larger than males. The saw-whet is brown with a white, red-brown streaked belly. Their large yellow eyes are topped by a V-shaped white patch and surrounded by a white facial disc that blurs laterally into brown. They have black beaks and brown and white speckled crowns.

Activity cycle
Like most owls, the saw-whet is nocturnal and tends to be most active just after dusk and near dawn. It spends the daylight hours roosting on low branches in dense forests or shrub lands. It relies on camouflage for its safe concealment and if approached by a human typically reacts by maintaining its position and immobility. This behavior has led many people (and bird field guide authors) to describe these ferocious little owls as “tame.”

Prey
The saw-whet is an active hunter that takes deer mice (Permyscus leucopus) for 66% of all of its prey. Meadow voles and shrews also make up significant percentages of prey taken. They also consume song birds and locally abundant prey species like the intertidal and shoreline crustaceans on islands of British Columbia.

Mating and Reproduction
The northern saw-whet breeds primarily in densely forested sites. Northern coniferous forests, thickets of secondary successional forest growth, and shrub lands are common habitats in which these owls can nest and reproduce. Nests are constructed in both naturally occurring tree holes and abandoned nest holes made by large woodpeckers (like the pileated woodpecker). They also will nest in artificial nest boxes. Nest sites near water are highly favored.

Females typically lay 5 or 6 white, oval eggs and then will, with only occasional short trips away from the nest for defecation or pellet regurgitation, continuously incubate the clutch for 21 to 28 days. During this incubation period the male will bring the female food but will not actually participate in the nest brooding. There have been observations of males caring for more than one female on nests at one time, but this must have occurred in habitats with very abundant food supplies.

The females remain in the nest and keep the tree hole very clean during the incubation period and through the first 2 ½ weeks of the nestlings’ lives. The owlets grow very rapidly, though, and 18 days after egg hatching, the female begins to roost outside of the tree hole. In the next 2 weeks before the owlets fledge the nest cavity will steadily fill with feces, rotting pieces of uneaten prey, and regurgitated pellets. Survival of the entire cohort of nestlings depends on the local abundance of prey. If prey densities are limiting, the younger, latter hatching owlets will not survive.

The fledged owlets roost together near the brood nest for the next 4 weeks. During this period of time they are fed, primarily by the male. During this period of time the female, liberated from the maintenance duties of her brood, may mate with another male and begin the rearing of a second clutch of eggs.

Hunting
The saw-whet utilizes its excellent senses of vision and hearing to locate prey. Typically, the owl will perch on a low branch and opportunistically swoop down and take whatever prey it happens to detect below it. Its sharp talons efficiently grab and quickly kill most of the prey species it encounters. As mentioned before, deer mice are its overwhelmingly most abundant food. During cold weather, the saw-whet may cache extra kills in tree holes where they can remain frozen and preserved for many months. Before consuming these cached food items, the owl will warm them with its body (almost like it was incubating eggs!). Adult deer mice and comparably sized prey are torn into two pieces and usually eaten at two different meals. The small saw-whet cannot easily swallow its prey whole.

Like all owls, the saw-whet relies on intestinal enzymes to break down the digestible body parts of its food. The un-digestible hair, bones, claws, feathers, or arthropod exoskeletons are then regurgitated in the form of a pellet. Examination of these pellets is an excellent way to access the specifics of an owl’s diet.

The northern saw-whet owl is a widely distributed and relatively common species throughout large sections of North America. Its small size, excellent camouflage, and preference for relatively unmanaged habitats, though, make its sighting an uncommon experience. As long as complex woodland habitats are maintained, however, the saw-whet owl will continue to liven spring nights with its calls and help to keep wild deer mice populations under control.