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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
P.A. 2002. North American Owls. Second edition.
Smithsonian Institution Press, Washington DC, USA, pp 234-241.
and C. Hyslop. 1998. Population status and recent trends in
Canadian raptors: a review. Biological Conservation 83: