Aegolius acadicusnorthern saw-whet owl(Also: saw-whet owl)

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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. (Bisbee, 1998; Cannings, 1993; Long, 1998)

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. (Cannings, 1993; Long, 1998; The Raptor Center, 2000; Tufts, 1986)

Physical Description

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. (Cannings, 1993; Government of Alberta, 2000; Long, 1998; The Raptor Center, 2000)

  • Sexual Dimorphism
  • female larger
  • Range mass
    65 to 110 g
    2.29 to 3.88 oz
  • Range length
    18 to 21.5 cm
    7.09 to 8.46 in
  • Range wingspan
    45 to 60 cm
    17.72 to 23.62 in
  • Average basal metabolic rate
    0.654 W
    AnAge

Reproduction

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. (Cannings, 1993)

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. (Bisbee, 1998; Cannings, 1993; Tufts, 1986)

  • Breeding interval
    Northern saw-whet owls breed once yearly.
  • Breeding season
    Northern saw-whet owls breed between March and July.
  • Range eggs per season
    4 to 7
  • Average eggs per season
    5
    AnAge
  • Range time to hatching
    26 to 28 days
  • Range fledging age
    4 to 5 weeks
  • Range time to independence
    6 to 8 weeks
  • Average age at sexual or reproductive maturity (female)
    1 years
  • Range age at sexual or reproductive maturity (male)
    1 (low) years

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. (Cannings, 1993)

  • 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 years. (Cannings, 1993)

  • Range lifespan
    Status: wild
    7 (high) years
  • Range lifespan
    Status: captivity
    16 (high) years
  • Average lifespan
    Status: wild
    124 months
    Bird Banding Laboratory

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. (Cannings, 1993; The Raptor Center, 2000; Tufts, 1986)

  • Range territory size
    1 (low) km^2

Home Range

The home ranges of two males that were tracked with radio transmitters were 1.42 and 1.59 square kilometers. (Cannings, 1993)

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. (Cannings, 1993; Government of Alberta, 2000; The Raptor Center, 2000; Tufts, 1986)

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. (Cannings, 1993; The Raptor Center, 2000)

  • Primary Diet
  • carnivore
    • eats terrestrial vertebrates
  • Animal Foods
  • birds
  • mammals
  • insects

Predation

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. (Cannings, 1993)

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. (Cannings, 1993)

Economic Importance for Humans: Positive

Northern saw-whet owls help humans by killing rodents that many people consider to be pests.

  • Positive Impacts
  • controls pest population

Economic Importance for Humans: Negative

There are no known adverse affects of northern saw-whet owls on humans.

Conservation 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. (BirdLife International, 2004; Cannings, 1993)

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

Kari Kirschbaum (editor), Animal Diversity Web.

Jia Yan (author), West Windsor-Plainsboro High School, Joan Rasmussen (editor), West Windsor-Plainsboro High School.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

polymorphic

"many forms." A species is polymorphic if its individuals can be divided into two or more easily recognized groups, based on structure, color, or other similar characteristics. The term only applies when the distinct groups can be found in the same area; graded or clinal variation throughout the range of a species (e.g. a north-to-south decrease in size) is not polymorphism. Polymorphic characteristics may be inherited because the differences have a genetic basis, or they may be the result of environmental influences. We do not consider sexual differences (i.e. sexual dimorphism), seasonal changes (e.g. change in fur color), or age-related changes to be polymorphic. Polymorphism in a local population can be an adaptation to prevent density-dependent predation, where predators preferentially prey on the most common morph.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

stores or caches food

places a food item in a special place to be eaten later. Also called "hoarding"

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

visual

uses sight to communicate

References

"The Owl Pages - Northern Saw-Whet Owls" (On-line). Accessed July 12, 2000 at http://www.owlpages.com/species/nsawwhet/index.html.

BirdLife International, 2004. "IUCN Red List of Threatened Species" (On-line). Accessed December 03, 2004 at http://www.redlist.org/search/details.php?species=48588.

Bisbee, M. 1998. Meet Them at the Wildlife Park: The Saw-Whet Owl. The Gray News (online edition), Vol. 30, No. 1.

Cannings, R. 1993. Northern saw-whet owl (Aegolius acadicus). Pp. 1-20 in A Poole, F Gill, eds. The Birds of North America, Vol. 42. Philadelphia, PA: The Academy of Natural Sciences and Washington DC: The American Ornithologists Union.

Government of Alberta, 2000. "Alberta's Watchable Wildlife" (On-line). Accessed July 10, 2000 at http://www3.gov.ab.ca/srd/fw/watch/owl_saw.html.

Grondahl, C., J. Schumacher. "Northern Prairie Wildlife Research Center Home Page" (On-line). Accessed July 10, 2000 at http://www.npwrc.usgs.gov/resource/birds/owls/aegoacad.htm.

Long, K. 1998. Owls - A Wildlife Handbook. Boulder, Colorado: Johnson Books.

The Raptor Center, 2000. "Raptor Facts - Saw-Whet Owl" (On-line). Accessed July 10, 2000 at http://www.ahc.umn.edu/ahc_content/colleges/vetmed/depts_and_centers/raptor_center/index2.cfm?nav=53535&CFID=687163&CFTOKEN=88288799.

Tufts, R. 1986. "Nova Scotia Museum of Natural History: Birds of Nova Scotia" (On-line). Accessed July 10, 2000 at http://museum.gov.ns.ca/mnh/nature/nsbirds/bns0219.htm.