Northern spotted owl (Strix occidentalis caurina) recovery strategy: chapter 6
3. Population Abundance
Owl abundance is reported differently by different authors. Some authors report the total number of individuals, but, in addition to breeders, this includes single territorial adult owls, floaters (non-territorial owls), and those younger than breeding age (typically less than 3 years old) and thus not contributing to recruitment. COSEWIC uses the number of mature birds that are capable of reproduction, but this can include non-breeders and/or floaters. The number of potential breeding pairs is most frequently used because it represents the portion of the population that contributes to recruitment. The term potential is used because paired owls do not necessarily reproduce every year.
In the following sections, cited owl abundance estimates are given in terms of the number of potential breeding pairs. Simply extrapolating between COSEWIC’s counts of mature birds and the number of potential breeding pairs is not recommended because the COSEWIC’s count also includes non-breeders and/or floaters.
3.1 Global Range
The global population of Spotted Owls was estimated at 3778 potential breeding pairs in the early 1990s (Gutiérrez et al. 1995). A more recent estimate is about 6000 pairs (2300, northern California; 2900, Oregon; 860, Washington; 33, British Columbia; Forsman 2003). Most reports now use a range of 3000 to 6000 potential breeding pairs. The higher numbers reported in recent years reflect more comprehensive inventory and are, therefore, more accurate than earlier estimates.
3.2 Canadian Range
Within the Chilliwack and Squamish forest districts before European settlement, the Spotted Owl population in Canada was estimated at approximately 500 potential breeding pairs (Blackburn et al. 2002). In 1991, the Canadian population was estimated at fewer than 100 potential breeding pairs, based on low response rates during field inventories conducted from 1985 to 1988 (Dunbar et al.1991; Dunbar and Blackburn 1994). The most recent estimate of the size of the Canadian population, based on analysis of trend data, is less than 33 potential breeding pairs in 2002 (Blackburn and Godwin 2003).
3.3 Proportion of Global Abundance inCanada
The Canadian population of Spotted Owls represents approximately 0.5 to 1% of the global population of 3000 to 6000 potential breeding pairs. Because the Canadian portion of the owl’s range is approximately 8%, these population estimates appear far below a level of abundance that would be expected if the population were equally represented across the species’ range. However, this can partly be explained by the owl’s use of larger home range sizes in northern portions of its range. This would typically result in lower owl densities relative to geographic extent here than in areas where home ranges are smaller.
3.4 Population Trend
Between 1992 and 2001, at least 64 occupied sites were detected in British Columbia within the Squamish, Chilliwack, and Cascade forest districts. Analysis of the occupancy of owls at 40 of these sites in the Chilliwack and Squamish forest districts between 1992 and 2001 suggests a population decline of about 49% (90% C.I., 40 to 57%) at an average annual rate of 7.2% (90% C.I., 5.5% to 8.9%, Blackburn et al. 2002). Results from 2002 suggest that the population declined an additional 35% between 2001 and 2002. Overall, based on a linear multiplicative model, the Spotted Owl population in British Columbia declined by 67% between 1992 and 2002 at an average rate of 10.4% per year (Figure 3; Blackburn and Godwin 2003). Based on the 100 pairs of Spotted Owls in British Columbia estimated by Dunbar et al. (1991), the observed 67% decline between 1992 and 2002 suggests that the current Spotted Owl population in British Columbia may be fewer than 33 breeding pairs of owls (Blackburn and Godwin 2003). Based on a historic population estimate of about 500 potential breeding pairs of owls (Blackburn et al. 2002), the current population estimate suggests that the population may have declined by as much as 90% since European settlement.
The observed large decline in Spotted Owl numbers is not exclusive to British Columbia. In the United States, monitoring of Spotted Owls at 15 different demographic study areas between 1985 and 1998 suggests a range-wide annual population decline of 3.9% (95% C.I., ±3.6%) for female Spotted Owls (Franklin et al. 1999). The Cle Elum demographic study area in Washington exhibited a decline of about 60% between 1992 and 2002 (Forsman et al. 2002c), although populations elsewhere in the United States showed less dramatic declines.
Figure 3. Estimated number of occupied survey areas among the 40 survey areas, 1992 to 2002 (90% C.I.). (Blackburnand Godwin 2003)
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