Western screech-owl (Otus kennicottii) COSEWIC assessment and status report: chapter 8
Limiting Factors and Threats
Factors that regulate populations have been little studied (Cannings and Angell 2001), but some factors that are important can be presumed. For instance, the availability of suitable nest cavities must affect the ability of the Western Screech-owl to successfully breed, although if nest boxes are provided they will readily use them (Deal, pers. comm., 2001, Doremus, pers. comm., 2001). It has been suggested that forestry operations negatively affect screech-owl habitat both by the removal of habitat through timber harvesting and through the removal of dead trees and snags which serve as potential nest cavity trees (Darling, pers. comm., 2001, Fraser, pers. comm., 2001). Yet, the relationship between Western Screech-owls and forestry operations has not been studied. Forest fires, perhaps like logging, may also temporarily remove habitat for interior populations (Hayward, pers. comm., 2001).
Habitat loss, both on the south coast and in the interior of BC is a threat, as mentioned in the Habitat Trends section. This is of particular concern for O. k. macfarlanei which occurs over a much smaller portion of the country than the coastal population, and primarily in declining riparian habitats that are favoured for development, agriculture and forestry. MWLAP (2001) estimates that there is about 15,600 ha of appropriate habitat for the species in the southern Okanagan. R. Cannings (pers. comm., 2001) feels that an estimate about twice this size would include all of the appropriate habitat for O. k. macfarlanei in Canada.
Almost all anecdotal reports from the Lower Mainland and from southeastern Vancouver Island, as well as two from Washington state, cite Barred Owl predation as a probable cause of Western Screech-owl declines, or at least they state that when Barred Owl numbers increase Western Screech-owl numbers decline (Acker, pers. comm., 2001, Aldcroft, pers. comm., 2001, Clulow, pers. comm., 2001, Fraser, pers. comm., 2001, Hobbs, pers. comm., 2001, Ryder via Mackenzie, pers. comm., 2001, Toochin, pers. comm., 2001). As noted, the Barred Owl has only in the last few decades become common in BC, as the species spread southwestward into areas that it had not previously occupied. Fraser (pers. comm., 2001) hypothesizes that Barred Owls and Great-horned Owls are doing particularly well in south-eastern Vancouver Island because of the introduction and establishment of eastern mammal species that they might be using as a food source. Both the Grey Squirrel (Sciurus carolinensis) and the Eastern Cottontail (Sylvilagus floridanus), have in recent decades become well established in large parts of southeastern Vancouver Island and the Lower Mainland, but do not occur anywhere else in the province in significant numbers (Nagorsen in prep).
It is completely unknown if Barred Owls are negatively affecting Western Screech-owl populations outside the south coast of BC--certainly Barred Owls are present, and are reasonably common in these parts of the screech-owls’ range. For instance, in the Nimpkish Valley survey, between 19 and 27 Barred Owls were recorded in a season’s surveys (Setterington 1998) and in the Campbell River Watershed, 16% of all owl detections were Barred Owl detections (in contrast to 28% that were Western Screech-owl) (Mico and Van Enter 2000).
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