By Jia Yan
Geographic Range
Northern saw-whet owls are found only in North America. Their breeding range includes southern Alaska, southern Canada, most of the United States and some high elevation sites in central Mexico. ()
Biogeographic Regions:
nearctic 
(native ); neotropical (native ).
Habitat
Northern saw-whet owls inhabit woodlands of all types throughout their range. Though they seem to be most abundant in coniferous forests, they are also common in deciduous and mixed conifer-deciduous forests. During migration and winter, saw-whet owls inhabit a wide variety of habitats over a range of altitudes and latitudes. They may be found in rural or even suburban environments. The primary habitat requirements seem to be perches for hunting and dense vegetation for roosting. ()
These animals are found in the following types of habitat:
temperate .
Other:
suburban ; agricultural ; riparian .
Physical Description
(2.29 to 3.87 oz; avg. 3.08 oz)
(7.09 to 8.46 in)
(17.72 to 23.62 in)
Northern saw-whet owls are the smallest owls in eastern North America. At approximately 75 g, males weigh about as much as an American robin. Females weigh slightly more, at about 100g. The body lengths of males and females are 18 to 20 cm and 20 to 21.5 cm respectively. The wingspan of an adult ranges from 45 to 60 cm Northern saw-whets have dark-colored bills, eyes with yellow-pigmented irises, heavily feathered legs and feet, a tail with three bars, and a wide, reddish-brown body with white streaks on the abdomen. Their large, round heads are reddish brown to brown, have a large, grayish facial disk in the center and are streaked with white on the top. The neck is speckled with white. Northern saw-whet owls depend on this plumage for camouflage while roosting and hunting. .
Male and female saw-whet owls are similar in appearance, though females are slightly larger than males. Juveniles are chocolate-brown with a pattern of large white spots above their bills that extend over their eyes.
There are two recognized subspecies of northern saw-whet owl (Aegolius acadicus). Aegolius acadicus brooksi is found only on Queen Charlotte Island in British Columbia. It is similar to Aegolius acadicus acadicus, which is found throughout the rest of the range, except that the underparts are buff instead of white. Other than this subspecies, there is little geographic variation in appearance. ()
Some key physical features:
endothermic ; bilateral symmetry ; polymorphic .
Sexual dimorphism: female larger.
Reproduction
Northern saw-whet owls breed once yearly.
Northern saw-whet owls breed between March and July.
Northern saw-whet owls are typically monogamous, though polygyny can occur when prey are abundant. It is likely that females of this species are sequentially polyandrous, leaving the male and nestlings during to mate with another male and raise a second brood. Though this behavior most likely occurs, it has not been confirmed. There is no evidence that pairs remain together for more than one season.
Males establish a territory and begin advertising for a female in late winter and early spring. Males advertise by calling to a female who may call back if interested in the male. Pairs have been seen allopreening (tending to the feathers of one another), which may serve to build a pair bond. ()
Mating systems:
monogamous .
Northern saw-whet owls breed between March and July. Males begin advertising for a mate by calling in late winter and early spring. Once a pair has formed, the female selects a nest site. The nests are 2 to 12 m high, usually in natural cavities or abandoned woodpecker holes, often ones made by Northern Flickers and Hairy Woodpeckers. The female lays 4 to 7 (usually 5 or 6) eggs at two-day intervals. She also incubates the eggs, beginning soon after the first egg is laid. Meanwhile, the male brings her food and defends the territory. The eggs hatch after 26 to 28 days of incubation. The chicks are altricial at hatching; their eyes remain closed for the first 7 to 10 days and they must be brooded by the female. The male provides food to the female, who tears it into pieces and feeds it to the chicks until they are about 18 days old. After this, the female leaves the nest to roost elsewhere, and the male, and sometimes the female, continues to provide food to the chicks. The chicks leave the nest when they 4 to 5 weeks old. They are able to fly reasonably well at this time, but continue to be fed by the male for at least a month afterward. The young become independent from the parents 6 to 8 weeks after fledging. Juveniles complete their first molt and grow adult plumage when they are one year old. They also become sexually mature and may begin breeding at one year old. ()
Key reproductive features:
iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; oviparous .
There is a clear division of parental responsibilities by northern saw-whet owls. The female selects the nest site, lays and incubates the eggs for 26 to 28 days and broods the chicks for at least 18 days. She also tears food up into smaller pieces and feeds it to the chicks. During this time, the male provides all of the food to the female and the chicks, and protects the nest area.
After 18 days, the female may join the male in providing food to the chicks, or she may leave the nest area completely, presumably to find another mate and raise a second brood. The male continues to feed the chicks for at least a month. ()
Parental investment:
altricial ; pre-fertilization (provisioning, protecting: female); pre-hatching/birth (protecting: female); pre-weaning/fledging (provisioning: male, protecting: male, female); pre-independence (provisioning: male, female, protecting: male, female).
Lifespan/Longevity
Captive saw-whet owls have lived as long as 16 years. In the wild, the longest known lifespan of a northern saw-whet owl was 7 ears. ()
Behavior
Northern saw-whet owls are nocturnal. They are active at night, and roost silently in thick vegetation during the day. This species is also migratory. Though some individuals may stay in the same area year-round, the majority of northern saw-whets move south in autumn. Northern saw-whet owls are solitary. ()
Home Range
The home ranges of two males that were tracked with radio transmitters were 1.42 and 1.59 square kilometers. ()
Communication and Perception
Northern saw-whet owls communicate and perceive their environment using touch, sound and vision. They detect prey by sight and sound. In fact, their hearing is so well developed that they can locate prey by hearing alone. Northern saw-whets use visual cues and vocalizations to communicate. For example, males with neighboring territories may exchange calls to establish territorial boundaries. During courtship, males vocalize to attract a mate, and pairs sometimes allopreen (preen each others feathers), using touch to strengthen or establish a pair bond. ()
Food Habits
Northern saw-whet owls hunt at night, from about 30 minutes after sunset to about 30 minutes before sunrise. They hunt from a low perch, detecting prey by sight and sound. Northern saw-whet owls have excellent hearing; their asymmetrical skull allows them to locate prey using sound alone. When a prey item is located, the owl drops out of the perch onto the prey, capturing it with the talons. The prey is torn apart and eaten in pieces. Larger prey may be partially eaten and stored on a branch to eat over the course of several hours.
The northern saw-whet owl diet consists primarily of small mammals, particularly deer mice. Voles, red-backed voles, shrews (g. Sorex, Blarina and Cryptotis), shrew-moles, pocket-mice, harvest-mice, bog lemmings, heather voles, red tree voles, jumping mice and house mice are also common prey items. Juveniles of larger mammals, including pocket-gophers, chipmunks and squirrels (Tamiasciurus and Glaucomys) are occasionally taken, as are insects, such as beetles and grasshoppers. Small birds are also occasionally taken, primarily during migration when they are active at night. ()
Primary Diet:
carnivore (eats terrestrial vertebrates).
Animal Foods:
birds; mammals; insects.
Foraging Behaviors:
stores or caches food .
Predation
- great horned owls (Bubo virginianus)
Great horned owls are the only species that has been directly observed predating northern saw-whet owls. However, other large owls, such as long-eared owls and barred owls presumably also prey on northern saw-whet owls.
When approached by a predator or a human at night, northern saw-whet owls give a “ksew” call. During the day, they assume an erect posture and flatten their feathers against the body. If the predator continues to approach, they usually exhibit a “fright” reaction, bobbing the head, shifting from foot to foot, defecating, bill-snapping and finally flying away. ()
Ecosystem Roles
Northern saw-whet owls impact the populations of small mammals that they eat. They also host at least nine species of external parasites. ()
Economic Importance for Humans: Negative
There are no known adverse affects of northern saw-whet owls on humans.
Economic Importance for Humans: Positive
Northern saw-whet owls help humans by killing rodents that many people consider to be pests.
Ways that people benefit from these animals:
controls pest population.
Conservation Status
IUCN Red List: [link]:
Least Concern.
US Migratory Bird Act: [link]:
Protected.
US Federal List: [link]:
No special status.
CITES: [link]:
Appendix II.
State of Michigan List: [link]:
No special status.
Global population estimates for northern saw-whet owls range from 200,000 to 600,000 individuals. Though population trends have not been studied, populations of northern saw-whets are probably declining slowly due to habitat loss. Starvation and parasites are documented causes of nestling mortality. Adults are frequently killed by collision with vehicles.
Northern saw-whet owls are protected under the U.S. Migratory Bird Act and CITES Appendix II. They are ranked as a species of least concern by the IUCN, and are not protected under the U.S. Endangered Species Act. ()
Other Comments
The Northern Saw-Whet Owl's common name comes from the "skiew" call it makes when it is alarmed. This call is said to sound like a saw being whetted.
Contributors
Jia Yan (author), West Windsor Plainsboro High School.
Joan Rasmussen (editor), West Windsor-Plainsboro High School.
Kari Kirschbaum (editor), Animal Diversity Web Staff.
