Western screech-owl (Otus kennicottii) COSEWIC assessment and status report: chapter 6
Biology
General
Apart from some aspects of its diet and its environmental needs, the Western Screech-owl is not a well-studied species. It is a nocturnal species, foraging (Hayward and Garton 1988) and generally calling only at night. It is essentially non-migratory in British Columbia (Campbell et al. 1990). Pairs are territorial throughout the year and can be heard calling at all times of the year to varying degrees. See Cannings and Angell (2001) for more information on topics such as social behaviour and vocalizations.
Reproduction
The Western Screech-owl is probably monogamous, with pair formation beginning in January or February (Cannings and Angell 2001). Most birds are thought to begin breeding at a year of age, but average age of breeding is not known (Cannings and Angell 2001). Eggs in BC are recorded as being laid between 17 March and 31 May, with about half recorded in the middle half of April, while young were found between 19 April and 21 August, with half recorded in May (Campbell et al. 1990). Out of 50 fledgling birds in Idaho, the average date for leaving the nest was May 18 (Ellsworth and Belthoff 1997). The species is single-brooded, and lays on average 3.4 eggs in the coastal and Great Basin regions of North America (Murray 1976), but can lay between two and seven eggs (Cannings and Angell 2001).
Nests are in natural tree cavities or nest boxes where provided, with no nesting material used. Of the 62 nests recorded by Campbell et al. (1990) in BC, over three-quarters were in nest boxes, but of the remaining nests, 26% were in natural cavities within Black Cottonwood, Red Alder, Douglas-fir, Western Redcedar and Western Hemlock trees, and 13% were in cavities excavated by Pileated Woodpeckers (Dryocopus pileatus) or Northern Flickers (Colaptes auratus). All nests in natural cavities were in trees with diameter at breast height (dbh) of greater than 25 cm. Of two nests found near Victoria, one was in a nest-box on a 71 cm dbh Douglas-fir and another in an 35 cm dbh unidentifiable deciduous tree (probably Red Alder, Hobbs pers. comm. 2001). In the second tree, the entrance cavity was about 15 m up from ground, about 11 cm in diameter and the cavity was 58 cm deep from top to bottom. Two nests in the Okanagan region were in cottonwood trees (Cannings et al. 1987) and Bent (1938) also mentions use of cottonwoods by both O. k. macfarlanei and O. k. kennicottii. In northwestern Washington, Bowles (1906) usually found nests in a natural cavity of an oak or fir stub, but also found at least one nest in a hole excavated by flickers.
Survival
The longest-lived bird on record in the wild was a California bird that was almost 13 years of age (Clapp et al. 1983), while another in Idaho lived to 11 years (Doremus in Cannings and Angell 2001). Average life span is likely on average much shorter. Breeding females and males in Idaho had an average life span of 1.73 and 1.83 years respectively (Doremus in Cannings and Angell 2001).
Many Western Screech-owls die in collisions with motorized vehicles (Hawbecker 1938, Dorst, pers. comm., 2001, Hobbs, pers. com. 2001, Holmes, pers. comm., 2001). Campbell et al. (1990) cite several road-kills in British Columbia, as well as mortalities along train tracks. Dyer (pers. comm. 2002) reports that four screech-owls were killed along roads in the south Okanagan in 2001--a significant number considering the low total population there.
Movements/dispersal
Ellsworth and Belthoff (1997) studied the dispersal of young screech-owls in Idaho. They found that on average birds dispersed 10.6 km (females 14.7 and males 5.1) to presumed over-wintering sites. The furthest a bird was recorded as dispersing was 36 km.
Nutrition
Western Screech-owls have an extremely varied diet. They have been recorded as eating small mammals, birds, fish, reptiles, and a wide variety of insects and other invertebrates (Ross 1969, Earhart and Johnson 1970, Brown et al. 1986, Johnsgard 1988, Marti et al. 1993). In some locations the diet appears to be composed of a mix of taxa while other individuals, depending on the season, appear to specialize on the food that is most readily available.
In coastal regions of BC, the species has been recorded as eating fish, birds, beetles, lepidopterans, and other insects (Munro 1929, Guiguet 1949, Ryder 1973 (in Cannings and Angell 2001), Hazelwood 1994), while in the interior one bird had eaten crickets and a caterpillar (Cannings 1987), and another had eaten a Northern Flicker (Colaptes auratus, Munro 1929). Western Screech-owls in BC undoubtedly eat small mammals, as their diet in one study in Idaho was almost completely composed of mammals (Marks and Marks 1981).
Interspecific interactions (Predation)
Great-horned Owls (Bubo virginianus), Spotted Owls, Barred Owls (Strix varia), and Raccoons (Procyon lotor) all prey on Western Screech-owls (Johnsgard 1988, Cannings and Angell 2001). As recently as in Johnsgard (1988) Barred Owls were not reported as having preyed upon Western Screech-owls, but there is an increasing body of mostly anecdotal evidence that convincingly shows that Barred Owl predation may be frequent. Ryder (pers. comm. via Clulow 2001) saw a Barred Owl eating a Western Screech-owl in the Langley area. Birders have also seen Barred Owls either fly in silently when a Western Screech-owl tape is played (Levesque 2000, Hobbs, pers. comm., 2001, Clulow, pers. comm. 2001, Darling pers. comm. 2002) or to fly directly at a Western Screech-owl or tape machine playing a screech-owl call (Levesque 2000, Acker, pers. comm., 2001, Darling pers. comm. 2002). Barred owls have only recently become established in BC, and are a new predator for the Western Screech-owl in many parts of its range. The Barred Owl arrived in the northeastern part of BC about 50 years ago through natural range expansion, and reached the coast in the mid-1960s (Campbell et al. 1990). Only in the 1980s did it become common in southwestern BC (Dunbar et al. 1991). For more information on Barred Owl-Western Screech Owl interactions see Limiting Factors and Threats.
Adaptability
Western Screech-owls are adaptable in that they will readily take to using nest-boxes (Campbell et al. 1990, Doremus pers. comm., 2001). Also, it seems likely that the species is very adaptable to different food sources as they appear to eat almost any live prey that is of appropriate size. On the other hand, they are presumably somewhat restricted in their habitat use in that (where nest boxes are not provided) they need trees that are large enough to accommodate their nest cavities.
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