RESEARCH

NORTHERN SAW-WHET OWL (Aegolius acadicus)

SPECIES PROFILE


Description:
The Northern Saw-whet Owl (Aegolius acadicus) is the smallest owl in eastern North America, and among the smallest on the continent, only ~20 cm in length, and typically weighing just 75 - 105 grams.

Aside from small size, the Northern Saw-whet Owl can be recognized by its white breast with broad reddish-brown vertical streaks, and its relatively pale face and whitish streaks on the crown.

Sexes are identical in colouration, but females are roughly 20% larger.  Banders have traditionally determined sex through wing length, but more recently a method combining weight and wing chord has been employed.  Juveniles (plumage from fledging through to their first moult, prior to fall migration) have a striking appearance, with a dark brown back and face contrasting with a white forehead and solid tawny underparts.

Similar species:
The slightly larger Boreal Owl is sometimes confused with the Northern Saw-whet Owl.  Key differences are the colour of the beak (black for the Saw-whet, pale yellowish for the Boreal), pattern on the crown (brown with pale streaks for the Saw-whet, brown with white spots for the Boreal), and size (~20 cm for the Saw-whet, ~25 cm for the Boreal).  


Northern Saw-whet Owl roosting in a dense White Cedar.  Photo 2002 Leslie M. Hunt


Habitat:
Northern Saw-whet Owls prefer to roost in dense conifers, often relatively young cedars, spruces, and pines.  Commonly the trees chosen for roosting are along the edge of sizeable woodlots, and both the forest and adjacent more open areas are used for hunting.  For breeding, the Northern Saw-whet Owl tends to favour relatively mature forests, more often coniferous than deciduous, but is fairly flexible in that regard.  Areas near water seem to be favoured for nesting. 

Range:
The breeding distribution of the Northern Saw-whet Owl extends across most of southern Canada, from Nova Scotia through to British Columbia.  The population also extends north along the Pacific Coast to southern Alaska, and down through the Rocky Mountains as far as California and even into Mexico.  Some Northern Saw-whet Owls are non-migratory, but at least some move south for the winter, with some individuals wintering as far south as Nevada in the west and North Carolina in the east.

Behaviour:
The Northern Saw-whet Owl is generally nocturnal.  During the day these owls usually roost in dense vegetation, generally preferring to stay motionless even if discovered, counting on concealment and the network of branches to protect them from potential predators.

Vocalizations:
Generally Northern Saw-whet Owls are heard only for a relatively short period during the breeding season, typically March to May.  For the rest of the year they tend to remain silent, aside from snapping their beaks in a defensive manner.  The breeding call is a monotonous series of "toots", repeated at a frequency of roughly three notes per two seconds.  This call carries well, audible to a distance of roughly 300 metres in wooded areas, and as far as a kilometre over water.  Vocalizations are almost always strictly nocturnal.  Several other sounds have been described, including a nasal whine, and a high-pitched twitter, but these are heard relatively rarely.

Diet:
The Northern Saw-whet Owl hunts mostly mammals.  Mice in particular are heavily favoured, followed closely by voles, and to a lesser extent shrews.  Small songbirds are taken occasionally.  Rarely they will hunt animals as large as chipmunks and Rock Doves, but more typically prey is under 40 grams in weight.

Though there is little data on the hunting strategies of Northern Saw-whet Owls, observations suggest that they often pounce on their prey from above, either from a perched position, or following a short flight.  This species has very highly developed ears, suggesting that it should be able to hunt successfully in near complete darkness.

Life history:
Generally the Northern Saw-whet Owl is monogamous, but when prey is abundant some males may take a second mate.  Where individuals are non-migratory, it is thought that pair bonds are likely maintained from year to year.  It is uncertain to what extent the species is migratory, and whether movements which occur in fact represent a typical return migration, or may instead be indicative of some degree of nomadism in this species, as in some other owls.

Courtship begins in late winter or early spring.  In addition to the vocalizations described above, the male engages in active courtship displays, including a flight in which he circles a perched female 15-20 times before landing, following which he gradually shuffles closer to her.  The male often carries prey throughout this display, and eventually drops it beside the female.  After the female has eaten it, the male vocalizes, and the female typically flies off, pursued by the male, often ending in copulation.

Nesting usually occurs in cavities excavated by woodpeckers, often those created by Flickers or Hairy Woodpeckers.  In eastern North America, nesting ranges from March through to early July, with the laying of eggs most commonly occurring in mid-April.  Usually 5 - 6 eggs are laid, 48 - 72 hours apart.  Eggs are approximately 25 x 30 mm, weighing just under 10 grams.  The female is solely responsible for incubation.  Estimates on the duration of incubation vary, but several authors agree it averages just under four weeks.  The chicks are sometimes capable of very short flights by around four weeks of age, but the average fledging age is thought to be around 33 days.

Conservation issues:
Mortality rates are thought to be approximately 60% for first-year Northern Saw-whet Owls, and roughly 50% for older individuals.  The oldest known individual in the wild lived to just over 10 years, but an average lifespan is probably closer to just 3 - 4 years.

Data on the status of the Northern Saw-whet Owl's population is limited.  Though it is a fairly numerous bird, with an estimated Canadian population of 50,000 to 150,000 pairs (Kirk and Hyslop 1998), some have suggested that the population may be slowly declining.

Recommended references:
Cannings, R.J.  1993.  Northern Saw-whet Owl.  No. 42 in The Birds of North America, ed. A. Poole and F. Gill, Philadelphia: Academy of Natural Sciences; Washington, DC: American Ornithologists' Union.

Johnsgard, P.A.  2002.  North American Owls.  Second edition.  Smithsonian Institution Press, Washington DC, USA, pp 234-241.

Kirk, D.A., and C. Hyslop.  1998.  Population status and recent trends in Canadian raptors:  a review.  Biological Conservation 83: 92-118.

2002- The Migration Research Foundation Inc.