Spotted Owl Biology

A Reference for North and Central American Owls

Name: Spotted Owl – Strix occidentalis
Other Common Names: Arizona Spotted Owl (lucida); Mexican Spotted Owl (lucida); California Spotted Owl (occidentalis); Northern Spotted Owl (caurina); Western Barred Owl; Wood Owl.

Subspecies: All three races of the Spotted Owl are found in the United States, one of which has a range that stretches into Canada and one into Mexico.
S.o. caurina is the northern most race and faces some of the greatest danger of deforestation. It ranges along the West Coast from SW British Columbia to San Francisco in the coastal ranges.
S.o. occidentalis is found both along the west slope of the Sierras in California and in the coastal ranges from San Francisco to San Diego.
S.o. lucida ranges from S. Utah south through Arizona, New Mexico, and W. Texas down into Central Mexico.

Measurements and Weights:
Wingspan: 40 – 50 in.
Length: 16.5 – 19 in.
Tail: 8 1/4 – 9 in.
Average Weight: Male: 21 oz.
Average Weight: Female: 23 oz.

Description: A medium size owl lacking ear tufts. Male and female are identical in plumage. Head and upper parts brown irregularly spotted with white. Under parts buff with brown and white ovals or barring. Lacks the vertical chest and flank streaking of its close relative, the similar Barred Owl. The Spotted Owl is also slightly smaller and darker than the Barred Owl. It has a large rounded facial disc that is pale brown with indistinct concentric rings of darker brown encircling the eyes. The facial rim or border is dark brown; lacks ear tufts; the bill is yellowish-green to straw colored; iris dark brown; claws blackish-brown; feet feathered. The Spotted Owl call is a very bold sound in the forest that often betrays its location. Although mostly nocturnal, it will often call at dusk or on a dark day.

Young: Initially pure white. The juveniles are a buffy-brown with brown barring and some white spotting on the back. The head is mostly buffy-brown. The white crown spotting and bolder over all colors develop as the owlet matures.

Habitat:

The Spotted Owl inhabits old growth forests. Most birds (90%) will be found in 200+ year old multi-layer forests and few in less than 100-year-old forests. Their dependence of dense old growth forests is probably related to a need for large trees for nesting and roosting sites, and variables associated with abundance and availability of appropriate prey. The preference for multi-layer forests is attributed to a need for temperature gradients for roosting. The density and distribution of Spotted Owls is subsequently also often determined by the distribution of federal land that has usually been protected from destruction and/or logging. Studies also show that the vast majority of Spotted Owls reside in forests where the dominant trees in the area have greater than a 32-in. breast height diameter. Examples of preferred habitats include mixed forests, coniferous and wooded canyons dominated by Douglas Fir, redwood, pine-oak, western red cedar, and ponderosa pine.

Food and Feeding:

Depending on the region and prey availability, the chief prey items include flying squirrels, wood rats, hares, and/or rabbits. Mice and moles also contribute to a diet that is 90% small mammals. There also seems to be a correlation between prey size and breeding success. The mean weight of prey items @ 4.1 oz. were found for successfully breeding owls. A mean weight of 2.8 oz. for prey items were found for owls that were unsuccessful breeding. Other prey include birds and smaller owls, invertebrates, and some reptiles.

Breeding:

Most nest sites are in natural tree cavities. Additional nest sites include platform nests (mostly constructed by other raptors, wood rats, or squirrels), clusters of mistletoe, and cavities or potholes in cliff or cliff ledges. Most nest sites are used for more than 1 year and are typically 60 – 100 feet high. Usually 2 eggs are laid but as many as 4 are possible. Eggs are laid from March through mid-May and incubation lasts 28 – 32 days. The female will brood the young for an additional 8 – 10 days. The young will leave the nest at 32 – 36 days old onto nearby branches. Since flight feathers, at this point, are not fully developed, the young often fall to the ground but will soon climb up nearby trees to perch. At 40 – 45 days old most owlets can fly short distances. Survival rate for the young Spotted Owls is low.

Movements and Life Span:

The Spotted Owl is generally sedentary except in areas where harsh winters and snow force downslope movements. In areas of milder climates, winter ranges may only expand, depending on prey availability. Therefore once a Spotted Owl is located, it can usually be found in the same area for many years… you have found its home.
The lifespan for banded Spotted Owls has been found to be quite long. Wild owls often live for 16-17 years.

Conservation: The Spotted Owl faces the danger of the loss of its unique habitat type (in the Pacific Northwest only about 10% of the old growth forests remain intact). It also faces encroachment from the Barred Owl, low rate of reproductive success, and low survival rates for the juveniles. All this certainly makes the status of this bird of great concern.