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COSEWIC assessment and update status report on the Burrowing Owl in Canada

Population Sizes and Trends

Estimates of Burrowing Owl population trends come from a variety of sources including: 1) large-scale breeding bird surveys carried out by volunteers on designated routes in Canada and the United States: 2) dedicated Burrowing Owl counts carried out by the Provinces (AB, SK, MB) in support of provincial conservation programs; 3) surveys of landowners participating in Operation Grassland Community (AB) and Operation Burrowing Owl (SK); and 4) surveys carried out by Burrowing Owl researchers on the Regina Plain, Grasslands National Park, and other research sites in the Prairie Provinces. Given the differences in methodology, scale, and observer effort among these surveys, the four survey types have different strengths and weaknesses. While the BBS and landowner surveys give results on the broadest scales, they are also more prone to observer/sampling error. Dedicated Burrowing Owl surveys carried out by provincial and federal biologists may give more accurate results, but are necessarily restricted to much smaller areas. Taken together, however, the four survey methods likely provide a relatively robust representation of the long-term population size and trends of Burrowing Owls in Canada.


Canada

In Canada, previous COSEWIC status reports on Burrowing Owls reported population sizes of 2000 pairs in 1977 (Wedgwood 1978), 2540 pairs in 1991 (Haug and Didiuk 1991), and 1010-1685 pairs in 1995 (Wellicome and Haug 1995). Data from 2004 suggest a total minimum population size of 795 individuals in Canada: 498 individuals in Saskatchewan, 288 individuals in Alberta and 9 individuals in British Columbia (National Burrowing Owl Recovery Team 2004). This number almost certainly underestimates the total Canadian population as relatively large areas of potentially suitable habitat in Alberta and Saskatchewan remain unsurveyed; the actual population may be as high as 1600 individuals (National Burrowing Owl Recovery Team 2006). However, this number comes from a year (2004) in which some local populations appeared to have increased from the previous year by as much as 73% (D. Todd, pers. comm., December 2004). It should be stressed that each of the COSEWIC status reports used different methods to estimate population sizes, thereby complicating long-term trend analysis.

Burrowing Owls are seen on too few Canadian Breeding Bird Survey routes to generate meaningful population trends. Recent trend data from dedicated Burrowing Owl surveys in Canada, however, show a clear decline in the number of owls since the late 1980s.


Manitoba

Data from Manitoba show a decline from 76 nests in 1982 to 0 nests in 1997 (Figure 3), and only 1 nest in 1999. Since 1999, only one pair has been confirmed breeding in Manitoba (2001), and no nests were documented in Manitoba in 2004 (De Smet 1997; updates from K. De Smet, September 2004). While it may be too early to consider the Burrowing Owl extirpated in Manitoba, it is almost certainly only an irregular breeder now in the province (probably <10).


Figure 3: Trend in the Number of Pairs of Burrowing Owls Found Nesting in Manitoba Since 1982

Figure 3: Trend in the number of pairs of Burrowing Owls found nesting in Manitobasince 1982 (updated from De Smet 1997; K. De Smet, pers. comm., September 2004).

Updated from De Smet 1997; K. De Smet, pers. comm., September 2004.


Alberta

In Alberta, standardized surveys have been carried out near Hanna (104 quarter sections) and Brooks (128 quarter sections), sites that contain large blocks of suitable habitat and have been surveyed since the late 1980s and early 1990s, respectively (Wellicome 1997). In the Hanna blocks area, the number of Burrowing Owl nests has decreased from a high of over 30 in 1991 to 2 nests or less since 2001 (Figure 4). Similar surveys near Brooks (Figure 5) show a less dramatic decline, but only five nests/100 km2 in 2002 and 2004. On a larger scale, data from Operation Grassland Community in Alberta also show a long-term negative trend (Figure 6). Taken together, the standarized survey and OGC data from Alberta suggest significant declines in the density of breeding Burrowing Owls in the province. The OGC data in particular indicate a decline from about 240 nests in 1991 to about 25 in 2001.


Figure 4: Trend in the Number of Nests/100 km2 on Survey Blocks Near Hanna, Alberta

Figure 4: Trend in the number of nests/100 km square on survey blocks near Hanna, Alberta. The negative trend is statistically significant (Rs = - 0.89, P = 0.01, n = 9).

The negative trend is statistically significant (Rs = - 0.89, P = 0.01, n = 9).


Figure 5: Trend in the Number of Nests/100 km2 on Survey Blocks Near Brooks, Alberta

Figure 5: Trend in the number of nests/100 km2on survey blocks near Brooks, Alberta.

Note that surveys were not performed in 1996 and 2003, and that an incomplete survey was carried out in 1993. Data are from Russell (2002) and the Burrowing Owl Recovery Team meeting (2004).


Figure 6: Trend in the Number of Burrowing Owls Reported by Operation Grassland Community Landholders in Alberta Between 1989 and 2004

Figure 6: Trend in the number of Burrowing Owls reported by Operation Grassland Community landholders in Alberta between 1989 and 2004.

Unpublished data provided by L. Tomyn, Operation Grassland Community.


Saskatchewan

In Saskatchewan, surveys carried out by Operation Burrowing Owl cover a large proportion of the historical range of the species in the province. However, the survey includes only data from participating landowners and should consequently be viewed as a rough approximation of actual population trend in the province.

Figure 7 displays the trend in the number of Burrowing Owls reported by private landowners enrolled in Operation Burrowing Owl in Saskatchewan. The data show a significant decline in the estimated number of breeding pairs from around 1000 in the late 1980s, to less than 100 pairs since 2000.

Studies carried out on the Regina Plain have monitored the population status and breeding success of Burrowing Owls since 1987. Figure 8 shows the long-term population trend on the Regina Plain, with a significant decline in numbers from 1987 to 1999, and low numbers of breeding pairs since that time.


Figure 7: Burrowing Owl population trend at Operation Burrowing Owl sites in Saskatchewan between 1987 and 2004

Figure 7: Burrowing Owl population trend at Operation Burrowing Owl sites in Saskatchewan between 1987 and 2004.

Unpublished data provided by K. Dohms, Operation Burrowing Owl.


Figure 8: Trends in the Number of Burrowing Owls Found During Surveys on the Regina Plain Area in Southcentral Saskatchewan

Figure 8: Trends in the number of Burrowing Owls found during surveys on the Regina Plain area in southcentral Saskatchewan.

The study areas originally surveyed since 1987 (P.C. James) and since 1994 (T.I. Wellicome) are geographic subsets, wholly contained within the larger study area surveyed since 1997 (R.G. Poulin and L.D. Todd).

Finally, at Grasslands National Park and the adjoining Dixon ranch, the number of nesting pairs has increased since 1998 (Figure 9). However, the extent to which this increase may be due to improved surveying techniques and increased survey coverage is not clear (G. Holroyd, pers. comm., October 2004). Breeding success in the Grasslands Park area has varied strongly among years, with between 1 and 4 young produced per nesting attempt (Figure 9). Such annual variation is typical, and the overall reproductive success of Burrowing Owls appears to be comparable or even slightly higher than that recorded in Great Plains states in the U.S (McDonald et al. 2004).


Figure 9: Breeding Success (left panel) and Number of Breeding Pairs at Grasslands National Park and Dixon’s Ranch, Saskatchewan

Figure 9: Breeding success (left panel) and number of breeding pairs at Grasslands National Park and Dixon’s ranch, Saskatchewan.

Unpublished data provided by G. Holroyd, Canadian Wildlife Service.

In summary, the various data sets from Saskatchewan suggest a significant long-term decline in the number of Burrowing Owls at the provincial level, with evidence of a small, stable population at Grasslands National Park in the extreme south of the province. Modest population increases within the last few years are interpreted to be the result of good productivity; however, 2005 data from the Regina Plain show another downward trend following poor productivity in 2004 (Figure 8).


British Columbia

Burrowing Owls were extirpated in British Columbia sometime in the 1980s (J. Surgenor, pers. comm., 2004). Efforts aimed at re-establishing a viable population in the province started with the release of captive-raised birds in the Thompson-Nicola region in 1983 and the release of families transplanted from Washington to the south Okanagan Valley from 1983 to 1988 (Dyer 1990). Although the program has succeeded in establishing small numbers of captive-raised birds that breed and migrate, as of 2004, only 9 individual owls were found in surveys in southern British Columbia. Of these, 4 were raised in the wild, 2 were returning captive-reared birds, and 3 were of unknown origin (J. Surgenor, pers. comm., October 2004). Apparently, larger numbers (i.e., > 50 birds/year) of releases will be necessary to re-establish a self-sustaining breeding population in British Columbia.


Summary of Breeding Season Surveys

Survey data from the Prairie Provinces discussed above suggest a population decline from about 1315 pairs (AB, 240; SK, 1000; MB, 75) to 125 (25, 100, 0) through the 1990s. This represents a decadal decline of about 91%. This trend has decreased in the last 10 years (1994-2004) to approximately 57% based on the same data sources (ca. 325 pairs to ca. 140 pairs; Figures figure3, figure6 and figure7).


Wintering Range

Burrowing Owls from the Canadian prairies are known to migrate through the southern Great Plains states (James 1992) and are thought to winter largely in Mexico (G. Holroyd, unpubl. data). In the northern portion of their wintering range, Burrowing Owls occur within some Christmas Bird Counts (hereafter, CBCs). CBCs are carried out throughout North America and are one-day, fixed-radius surveys conducted each year in late December or early January. Analysis of the long-term trend in the number of Burrowing Owls seen on CBCs in Texas (where a few Canadian owls may winter) shows a statistically significant decline since 1960 (see Figure 10; Spearman Rank Correlation Rs = - 0.42, n = 43, P < 0.01). However, these data must be interpreted with caution as wintering Burrowing Owls in Texas are largely of unknown origin (i.e., it is unclear to what extent owls from the Canadian prairies winter in Texas). Similar analysis of CBC data from California, where B.C. owls are thought to winter, shows a strong, significant decline from 1960 to 2003 (Figure 10; Rs = - 0.66, n = 43, P < 0.001).

In summary, since 1995, Burrowing Owl populations have continued to decline in all areas of Canada and the species is no longer a confirmed breeder in Manitoba and portions of southeastern Saskatchewan.