Short-eared owl (Asio flammeus) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort

Population trend data for Short-eared Owls comes from two primary sources: the Breeding Bird Survey (BBS) and the Christmas Bird Count (CBC). The Breeding Bird Survey monitors populations of breeding birds, but is restricted to areas with roads and volunteer participants. Because of these restrictions, the BBS only has sufficient sample sizes in the southern part of the Short-eared Owl’s range and its results must therefore be viewed with caution. The Short-eared Owl’s wintering range is well covered by the CBC, however; about 2000 of these counts are done annually across North America. While most owls are poorly monitored by both the CBC and BBS because of their nocturnal habits, the Short-eared Owl is largely diurnal and relatively conspicuous. The Ontario Breeding Bird Atlas also provides a way to measure changes in range between the first atlas in 1981-1985 and the second atlas in 2001-2005.

Abundance

The nomadic nature of Short-eared Owls has made quantitative assessment of population trends problematical (Cadman and Page 1994, Clayton 2000). Partners In Flight estimate the global population at 2,000,000, the North American population at 700,000 and the Canadian population at about 350,000 (see Partners in Flight databases). In the Maritime Provinces, Erskine (1992) estimated a total of 100 breeding pairs, with some fluctuation in numbers between years. The estimated provincial totals were 60 pairs in New Brunswick, 10 pairs on Prince Edward Island, and 30 pairs in Nova Scotia (Erskine 1992).

Trends

In the northeastern United States, Short-eared Owls are now considered endangered (at the state level) in several states and are no longer found as a breeding species in many historical portions of the species’ range (Holt 1986, Holt and Melvin 1986). In Canada, Short-eared Owls have declined significantly in abundance in the southern part of the breeding grounds since long-term survey work began in the late 1960s (Figure 3).

Data collected on Canadian Breeding Bird survey routes (Table 1, Downes and Collins 2007) suggest a continuing decline across southern Canada. A better perspective on the trend in Short-eared Owl numbers is to view the overall trend data in Canada from the late 1970s until 2006 (Figure 3). Although the number of Short-eared Owls appears more or less “stable” during recent years, the overall abundance is very low and has declined significantly since the 1970s.

Table 1.  Results of Breeding Bird Survey data for Short-eared Owls in Canada.
In column P (statistical significance): * = p<0.05.
Region 1968-2006
Trend
1968-2006
P
1968-2006
N
1968-1985
Trend
1968-1985
P
1968-1985
N
1986-2006
Trend
1986-2006
P
1986-2006
N
1996-2006
Trend
1996-2006
P
1996-2006
N
Canada
−6.3
*
134
−14.0
*
69
3.1
 
86
−1.7
 
70
Alberta
−8.8
*
50
−11.9
 
26
−2.0
 
37
−4.7
 
33
Saskatchewan
−8.6
 
34
−15.2
 
22
−1.8
 
15
 
 
Manitoba
−14.0
 
18
-
 
 
-
 
 
-
 
 

Figure 3. Annual population indices for Short-eared Owl from Breeding Bird Survey data from Canada, 1968 to 2006 (Downes and Collins 2007: www.cws-scf.ec.gc.ca).

Figure 3.   Annual population indices for Short-eared Owl from Breeding Bird Survey data from Canada, 1968 to 2006 (Downes and Collins 2007: www.cws-scf.ec.gc.ca).

Data from the U.S. Breeding Bird Survey (Sauer et al. 2007) suggest that Short-eared Owls have undergone a significant, long-term population decline in Montana (−7.4%/year since 1980), but have remained stable in North Dakota and Washington. For the United States overall, the trend from 1980-2006 was a decline of -3.7%/year (P = 0.07).

A recent analysis of Christmas Bird Count data found a significant annual decline of 3.07% over the past 40 years. Since a high proportion of these birds are likely from Canadian breeding populations, this is likely a good estimate of the Canadian population trend. This translates into a total decline of 27% over the past 10 years. Christmas Bird Count data from the United States are graphed in Figure 4. CBC results also show a steep decline in the numbers of Short-eared Owls wintering in British Columbia (Figure 5).

Figure 4. Long-term trend in the number of Short-eared Owls seen on Christmas Bird Counts in the United States. The negative correlation is significant (Spearman rank correlation: rs = - 0.72, P < 0.001).

Figure 4.   Long-term trend in the number of Short-eared Owls seen on Christmas Bird Counts in the United States. The negative correlation is significant (Spearman rank correlation: rs = - 0.72, P < 0.001).

Figure 5. Temporal pattern of abundance of Short-eared Owls on Christmas Bird Counts in British Columbiafrom 1960 to 2004. As can be seen from the relatively high numbers of owls on counts during the 1960s and early 1970s, the Fraser River delta (where most of these sightings come from) was once a major wintering area for Short-eared Owls. The decline is statistically significant (linear regressions, Y = 4.256 – 0.002X, R² = 0.538, P < 0.001).

 Figure 5.   Temporal pattern of abundance of Short-eared Owls on Christmas Bird Counts in British Columbiafrom 1960 to 2004. As can be seen from the relatively high numbers of owls on counts during the 1960s and early 1970s, the Fraser River delta (where most of these sightings come from) was once a major wintering area for Short-eared Owls. The decline is statistically significant (linear regressions, Y = 4.256 – 0.002X, R2 = 0.538, P < 0.001).

Yukon Territory

The Short-eared Owl is an uncommon summer resident and migrant in the Yukon, with strong fluctuations in the number of breeding pairs from year to year (Sinclair et al. 2003). Breeding has been documented along the coastal plain and inland tundra areas, while observations during summer suggest that they may also breed at scattered sites in the southern Yukon. There are currently no long-term data available on population trends.

Northwest Territories

No population status or trend information available.

Nunavut

No population status or trend information available.

British Columbia

Short-eared Owls are currently listed as Vulnerable in British Columbia due to a declining population trend. The (historically) relatively high density of wintering and breeding Short-eared Owls on the Fraser River delta make the area the most amenable in Canada for analysis of long-term population trends. Campbell et al. (1990) suggested that habitat loss and degradation was responsible for a long-term and dramatic decline in the number of wintering (Figure 5) and breeding owls.

Alberta

Numbers have been very low since 1990, with the exception of 1997, which was a high vole year on the southern Prairies (Clayton 2000, Poulin et al. 2001). Clayton (2000) concluded that, despite the difficulty in accurately tracking Short-eared Owl population trends, the available data were suggestive of a significant decline in abundance over the past 30 years.

Saskatchewan

Smith (1996) suggested that Short-eared Owls have declined in abundance and are now generally rare breeders in the province, with the exception of the Last Mountain – Quill lake area where they are still “fairly common”.

Manitoba

Short-eared Owls been observed only rarely on surveys since 1980 and the species is now considered an “occasional breeder” in the province (Taylor 2003).

Ontario

Although quantitative information is lacking, several authors have suggested that abundance is much lower now in southern Ontario than in the mid-1900s (Hunt 2004). At Point Pelee National Park, the status of Short-eared Owls appears to have changed from circa 1900, when it was considered common in the Park (Taverner and Swales 1907, 1908), to uncommon but regular (Wormington 2006, A. Wormington, pers. comm.). Cadman and Page (1994) concluded that the status of Short-eared Owls had changed from a common breeding species and migrant in the early 1900s, to a rare breeder and uncommon migrant ca. 1990. In the Lake St. Clair area, the mix of marshland and tallgrass prairie supported a few (apparent) breeding pairs, but in some years up to 100 wintering owls (Wood 1949). This area is now under heavy agricultural use with fewer observations of Short-eared Owls in recent years. Thus, in addition to the apparent declines in the abundance of breeding birds in southern Ontario, the strong decline in the number of fall migrants observed in southern Ontario also suggests that the number of breeding birds may have declined in northern Ontario as well. The Ontario Breeding Bird Atlas (Cadman et al. 2008) shows a strong increase in the number of atlas squares occupied between 1985 (84 squares) and 2005 (158 squares); this increase is due to population or distribution changes in northern Ontario.

Quebec

There are insufficient data for estimating trends of Short-eared Owl abundance in Quebec, especially northern areas. Surveys in the 1990s suggest the largest concentration of summering birds occurs along the St. Lawrence River estuary in the southern portion of the province (Gauthier and Aubry 1996).

New Brunswick

Nocturnal owl surveys in 2001 detected no Short-eared Owls, but this was not surprising given that survey routes were largely in forested habitat (Whittam 2001). The most recent atlassing work suggested a total of approximately 60 breeding pairs in New Brunswick (Erskine 1992).

Nova Scotia

There are no long-term BBS data available for Nova Scotia. Erskine (1992) estimated an annual total of 30 breeding pairs in the province.

Prince Edward Island

There are no long-term population data available for Prince Edward Island, but a decrease in the amount of open habitat on the island (from approximately 70% in 1900 to ca. 45% in 2007; R. Curley, pers. comm.. 2007) has probably led to a long-term decrease in the number of breeding pairs. The current (2007) estimate for the maximum number of breeding pairs on the island is five (R. Curley, pers. comm. 2007).

Newfoundland/Labrador

A recent summary of the known historical status in Newfoundland and Labrador concluded that there has likely been little change in the distribution and abundance of Short-eared Owls over the past century (Schmelzer 2005). That is, Short-eared Owls have always been uncommon to rare in coastal grasslands and marshes. Dedicated Short-eared Owl surveys are now being carried out in Newfoundland and Labrador, but the data are not sufficient to estimate recent population status or trends (Schmelzer 2005; Schmelzer pers. comm.).

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