Spotted owl (Caurina subspecies) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

Prior to 1985, the Northern Spotted Owl in Canada was largely known from museum specimens. By 1985, only 28 records of the subspecies were known, suggesting that it was a very rare and local resident within its range in Canada (Campbell and Campbell 1984, Dunbar et al. 1991, Dunbar and Blackburn 1994; Fraser et al. 1999). Due to the initial COSEWIC report by Campbell and Campbell (1984), surveys for Spotted Owls were begun in 1985. Between 1992 and 2002, more intensive and standardized surveys were conducted in 147 areas in the Chilliwack and Squamish forest districts. Forty of these 147 survey areas were then chosen and used to assess population trends. Only survey areas that were occupied at least once by a territorial Spotted Owl during the study were chosen. Search effort varied among years so some areas were not surveyed annually. Surveys were conducted at night using Spotted Owl call playbacks at stations along transects within each survey area (Blackburn et al. 2002; Harestad et al. 2004).

A cumulative frequency distribution was derived for the search effort required to detect the first Spotted Owl each year in each of 40 survey areas between 1992 and 2002. This distribution was used to determine a criterion for estimating the likelihood that Spotted Owls were absent from other individual survey areas. A minimum 13 hours of unsuccessful nocturnal survey effort was used to indicate a 90% chance that a surveyed area was vacant (Blackburn et al. 2002; Harestad et al. 2004). The methods used to calculate an adequate level of survey effort should give all readers confidence that the survey data reported for the period 1992-2002, and then thereafter, is reasonably accurate.

Beginning in 2002, intensive surveys of other areas thought to have suitable habitat for Northern Spotted Owls were initiated (Table 2), resulting in seven new sites being found by 2004 (Hobbs 2005, Keystone 2004). Four of these sites extended the range of the Northern Spotted Owl further north than previously known (Hobbs 2004). Survey effort by the BC Government in 2005 was about 50% greater than in 2004 (table 2) and resulted in another five new sites. Survey effort by the Ministry of Environment in 2006 declined by 80% (860 hrs) compared to 2005, and five sites known to be active in 2005 were not surveyed to standard. In 2006, only 14 active sites were found (Hausleitner 2006).

Table 2.  Survey effort for Northern Spotted Owls in British Columbia 2002-2006. Data from Beauchesne (2003), Hobbs (2004, 2005), Keystone (2004), Hausleitner 2006)
Year 2002 2003 2004 2005 2006
Survey areas >17 411 1493 149 33
Transects >17 ? 2854 249 ?
Call playback stations ? >852 31395 3342 577
Hours spent surveying ? ? 7546 835 158

1 32 by BC Government, 9 by MCA.
2 85 by MCA.
3 91 by BC Government, 58 by Keystone.
4 155 by BC Government, 130 by Keystone.
5 2112 by BC Government, 1027 by Keystone.
6 452 by BC Government, 302 by Keystone.

Even so, one of the key recommendations of the CSORT (Chutteret al. 2004) is to ensure that all Northern Spotted Owls in British Columbia are found and great effort has been expended over the years to ensure this. Surveys in recent years have focused on areas with the highest probability of finding “new” owls. Although it is possible that some owls remain undiscovered, there is no doubt that virtually all of the most likely areas have been surveyed.

Abundance

Prior to European contact, the size of the original Spotted Owl population in Canada was estimated at 500 breeding pairs (Blackburn et al. 2002). This estimate was based on the amount of suitable habitat estimated to have occurred at the time and using densities of Northern Spotted Owls found in relatively unfragmented landscapes. The Canadian population was estimated at fewer than 100 pairs in 1991, based on low response rates (0.03 pairs/km) during field inventories conducted from 1985 to 1988 (Dunbar et al. 1991; Dunbar and Blackburn 1994). By 2002, the Canadian population was estimated to be less than 50 pairs (Blackburn et al. 2002), but was more likely less than 30 pairs (I. Blackburn pers. comm. 2003; Cooper 2006) or 33 pairs (Harestad et al. 2004). Currently, the Ministry of Environment estimates the provincial population to be 16-60 individuals (CDC 2007), but the actual population is likely much closer to the lower number (19 owls are known in 2007).

From 1985 to 2002, 71 sites had been documented in British Columbia (Hobbs 2005); there were 79 by 2006 (J. Hobbs pers. comm.). In 2002 and 2003, 15 and 10 active Northern Spotted Owl sites, respectively, were confirmed (Hobbs 2004); but not all sites were surveyed. In 2004, 17 active sites and 25 adult owls (9 single owls and 8 pairs) were confirmed (Table 3). Only 4 of the 8 pairs were confirmed to breed (Hobbs 2005). Of these 17 sites, only 10 were from the 71 known sites prior to 2002, and 7 sites were new since 2002 (Hobbs 2005). In 2005, the total known number of Northern Spotted Owls in British Columbia was 23, 6 pairs and 11 single adults (Hobbs 2005; Table 3) but numbers had dwindled to 21 by year’s end as one adult female was killed by a predator at the nest and a lone adult was found dead in December (Hobbs 2005).

In 2006, numbers declined to 17 owls > 1 year old at 14 active sites (three pairs and 11 single birds) (Hausleitner 2006). Five sites with 6 birds documented in 2005 were not surveyed in 2006; presumably, some of these birds would be alive in 2006. An adult Northern Spotted Owl was found injured (and later died; Vancouver Sun report) on Highway 3 east of Hope in October, 2006. This bird was about 13 km from a known Northern Spotted Owl site with a pair of birds in 2005, and occupied by a male (unknown if he attracted a mate) in 2006: it is unknown if the dead owl was from that site J. Hobbs (pers. comm.). In 2007, there were 14 active sites with five pairs and nine single owls (J. Hobbs pers. comm.).

Table 3. Numbers of known active sites, pairs, single adults and breeding attempts by Spotted Owls in British Columbia 2002-2007. Data from Hobbs (2004, 2005) and J. Hobbs pers. comm. Uneven survey efforts between 2002 and 2003, and less intensive survey efforts in 2002-2003 compared to 2004-2005 explain some variations in the numbers of owls, pairs and nests found.
Year 2002 2003 2004 2005 2006 2007
Active sites 15 10 17 17 144 14
Pairs 2 4 8 71 3 5
Single adults 13 6 9 102 11 9
Pairs that attempted to breed 2 4 4 2 2  
Total known adult owls 17 14 25 233 17 19

1 reduced to 6 pairs after death of an adult female at the nest.
2 reduced to 9 after a lone adult was found dead in December 2005.
3 Reduced to 22 by December 2005.
4 Five active sites in 2005 were not surveyed in 2006.

The global population of Northern Spotted Owls was estimated at 3778 pairs in the early 1990s (Gutiérrez et al. 1995). A more recent estimate, based on more reliable inventory data, is about 6,000 pairs (2300-northern California, 2900-Oregon, 860-Washington, 30-British Columbia) (Forsman 2003). The Canadian population may have once been about 10% of the global population but now accounts for <0.02% of the global population.

Fluctuations and trends

Pre-European settlement, the Spotted Owl likely inhabited most mature and old forests found throughout the Pacific Northwest (USDI 1992). Populations of the Northern Spotted Owl are now declining throughout its range in Canada and the USA. In the United States, Northern Spotted Owls were the subject of the largest population dynamics analysis on any raptor. Between 1985 and 1998, demographic studies indicated a 3.9% annual decline in the population of territorial females within 15 study areas in Washington, Oregon and California (Franklin et al. 1999). A more recent meta-analysis of 14 study areas in the same three states found a -3.7% annual decline from 1985-2003 (Anthony et al. 2006). There is considerable variation between study areas, however. One study area in central Washington reported a 7% annual decline from 1992-2002 (Forsman et al. 2002b), whereas populations in one Oregon study area remained relatively stable from 1985-2002 (Forsman et al. 2002b).

In British Columbia, inventory in 40 of 147 survey areas between 1992 and 2002 indicate the Canadian population declined by approximately 67% during that time period, suggesting an average annual rate of -10.4% (Figure 6; Blackburn et al. 2002; Harestad et al. 2004). A similar total decline (-70%) over a similar time period (1990-2003) was reported for one population in adjacent Washington state in the central Cascade Range (Herter 2004). At the low population levels of 2002, extirpation of the Northern Spotted Owl from Canada was estimated to likely occur within 5-10 years (i.e., 2007-2012; Blackburn et al. 2002), or 2009 (Cooper 2006) if the rate of population decline continued (Figure 5). Without habitat protection and direct population augmentation, extirpation is now seemingly inevitable and will likely occur by 2012.

Rescue effect

There is potential for natural rescue from Washington State as Spotted Owl habitat and populations are contiguous between British Columbia and Washington. However, rescue potential is reduced because the lower Fraser River valley is now heavily urbanized or farmed and is likely a significant barrier to emigration as Spotted Owls do not normally cross such large open areas. At present there seems to be only a narrow corridor through the Cascade Range from which natural emigration to Canada could occur. The potential for rescue from Washington is further compromised as Spotted Owl populations there are also declining; slower than in Canada but faster than in Oregon and California (Anthony et al. 2006).

Figure 5. Estimated number of occupied survey areas among the 40 survey areas from 1992 to 2002 (with 90% C.I.). Figure from Cooper (2006), adapted from Blackburn and Godwin (2003).

Figure 5.   Estimated number of occupied survey areas among the 40 survey areas from 1992 to 2002 (with 90% C.I.)

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