E-Fauna BC Home

by Shawn Hilton, Keystone Wildlife Research Ltd.

Introduction

Northern Spotted Owls are a medium-sized owl, approximately 45-48 centimetres long, weighing 490-850 grams with females being slightly larger than males (COSEWIC 2008). Adult weights may fluctuate during the year, especially during the breeding season when food is prioritized for newly hatched young.

Adults are dark brown with a barred tail, white spots on the head and breast and facial disks. The eyes are dark brown. Juvenile and sub-adult owls look very similar to adults once downy feathers are dropped, but can still be distinguished by tail feathers: on young owls the tail feathers are pointed with white tips, while on adults they are rounded and mottled (COSEWIC 2008).

Spotted Owls are often confused with Barred Owls (Strix varia), and the two occasionally hybridize (Gutierrez 1996).

Biology

NESTING

Nesting habitat for the Spotted Owl is typically old-growth forests with coarse woody debris, snags and a high canopy cover. Spotted Owls do not build their own nests but rely on naturally formed platforms (e.g., mistletoe infestations, snag tops, large stick nests constructed by other species), or cavities within large trees (Spotted Owl Best Management Practices Working Group 2009). Some sites are used by different owls over multiple generations (S. Hilton, pers. obs.).

Spotted Owls may not breed every year (Chutter et al. 2004); although there is observed high nest-site fidelity where territorial owls revisit annually, regardless if breeding occurs. When breeding does occur only one brood is produced, which typically comes from one or two eggs laid. The number of eggs laid and young fledged is likely linked to prey availability (Gutierrez 1996).

FORAGING ECOLOGY

The Spotted Owl is considered a prey specialist, although preferred prey species may vary geographically. Analysis of pellets from Spotted Owls in the Chilliwack and Squamish Forest Districts showed that northern flying squirrels (Glaucomys sabrinus) comprised 43% of the diet, bushy-tailed woodrats (Neotoma cinerea) made up 13.4% of diet, and deer mice (Peromyscus maniculatus) made up 12.9% of the diet (composition by number; Horoupian et al., unpublished report, 1998). Analysis of pellets from two sites in the Lillooet Forest District (one nest site, one roost site) showed that bushy-tailed woodrats were the most common prey item (67.4% of diet by number), with deer mice the second-most common prey item (11.6% of diet by number); remains of flying squirrels were not found in any pellets (n = 35 pellets; Hilton & Hilton 2002). In the United States, an analysis of Spotted Owl diets showed that flying squirrels were a big component of diet in owls occurring from Washington to central Oregon, and shifted towards dusky-footed woodrats (Neotoma fucipes) and voles in southwestern Oregon and northwestern California (Thomas et al. 1990).

Habitat

The Northern Spotted Owl is strongly associated with old growth forest structure throughout its entire range in North America (Forsman et al. 1984; Carey 1989; Ganey & Balda 1989; Carey et al. 1990, 1992; Solis & Gutierrez 1990; Bart & Forsman 1992; Mills et al. 1993). Spotted Owls appear to show preference for forests with Douglas-fir as the leading tree species, high canopy closure; complex vertical canopy layering; presence of many large wildlife trees; and volumes of coarse woody debris that can be in excess of 268 m3 per hectare (Solis & Gutierrez 1990; Mills et al. 1993; Spotted Owl Management Plan 1999). These habitat characteristics provide thermal cover, protection from predators, forage for prey species, nesting trees, and roosting areas for Spotted Owls (Spotted Owl Management Plan 1999).

In B.C., Spotted Owls are located in coastal, interior, and transitional (coastal-interior) ecosystems, and it is possible that habitat preferences vary slightly between the three areas. Spotted Owl habitat in B.C. has been divided into different categories and differ by biogeoclimatic subzone. As stated in the Spotted Owl Management Plan (1999):

"Superior habitat type for wetter, maritime ecotypes (CWHdm, CWHvm1, CWHvm2, and MHmm1) entails: 1) canopy closure in the range of 60-85% with 3 or more canopy layers; 2) a multi-species canopy (with preference for Douglas-fir and western hemlock) dominated by trees >50 cm dbh (with half of the stand >75 cm dbh); 3) a stand density of 150-250 stems per hectare; 4) 10 or more large trees (>75cm dbh) per ha with deformities; 5). >250 m3 per ha Coarse Woody Debris (CWD), most of which should be longer than 5 m and have a diameter of greater than 50 cm, and 6) at least 25% of the patchy understory should be shrubs."

Spotted Owl pairs have high site fidelity (Forsman et al. 1984). Home range sizes vary throughout the Pacific Northwest, from 606 +/- 302 ha (Carey et al. 1990) to 11,052 ha (Lehmkuhl & Raphael 1993) for a single Spotted Owl pair. In BC, radio-telemetry on 3 Spotted Owl pairs in the Squamish Forest District (1999 – 2001) revealed pair home ranges of 1700 to 4600 ha (mean = 3100 ha; Blackburn and Godwin 2003). Management of Spotted Owls in B.C. is based on an annual pair home range size of 3200 hectares, which is incorporated into land management objectives (Spotted Owl Management Plan 1999).

Global Distribution

The Spotted Owl is found from southwestern British Columbia through to Mexico.

British Columbia Distribution

The Northern Spotted Owl population in B.C. is at the periphery of the species’ range, and is sparsely distributed throughout old growth forests in southwestern British Columbia. The species occurs throughout the Chilliwack, Squamish and Cascades Forest Districts. There have been historical sightings in the Sunshine Coast Forest District (Campbell & Campbell 1984) but few surveys have been conducted in the District to confirm continued presence. Spotted Owls in British Columbia have been found to reside primarily from sea level up to an elevation of 1400 m.

Conservation

From 1992 to 2002 there was a decline of up to 67% in the BC Spotted Owl population (Chutter et al. 2004). There are estimated to be fewer than 25 Spotted Owls left in BC (Spotted Owl Population Enhancement Team (SOPET) 2007).

Taxonomy

There are three recognized subspecies of Spotted Owl in North America.

1) Mexican Spotted Owl (Strix occidentalis lucida) is east of the Cascade ranges in the Midwest
2) California Spotted Owl (Strix. occidentalis occidentalis) is only in California, from south of San Francisco Bay through to almost the Mexican border
3) Northern Spotted Owl (Strix occidentalis caurina) is found from southwestern British Columbia through to San Francisco Bay (COSEWIC 2008).