Loggerhead shrike COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

In Canada, reported population sizes are 118 individuals in Manitoba (in 2002), 14000-15000 in Saskatchewan (as of 1999), and 6000 individuals in Alberta (1999).

The Canadian population of Prairie Loggerhead Shrikes has apparently been declining since the 1960s (Cadman 1985, Telfer 1993, Cade and Woods 1997). As has occurred in the eastern USA and Canada with the migrans subspecies, the breeding range of prairie shrike populations has contracted southward as former pastures have been converted to agriculture and aspen parkland habitat has reverted to forest.

Pooled BBS data from Alberta, Saskatchewan and Manitoba (Connie Downes, pers. comm) show a mean annual decline of 4.5 % since 1968, suggesting a total decline of 80 % over that period and 37 % decline over 10 years (Figure 4). Recent BBS data suggest that populations may be stabilizing at low levels in Alberta (see Table 1), continuing to decline Manitoba, while the trend remains unclear in Saskatchewan. BBS data are difficult to interpret (statistically) in Manitoba due to small sample sizes and resulting poor statistical power (see discussion under Manitoba, below). Shrike populations declined drastically in the Prairie Provinces from 1966-1979, and although those declines have lessened recently, the pattern is still towards declining populations (see Figure 4). Recent updates to the BBS dataset, incorporating data from 2001 and 2002, have typically resulted in stronger declines in shrike populations both in Canada and the USA (unpubl. data; http://www.mp2-pwrc.usgs.gov/bbs/). Thus, while there is some degree of statistical uncertainty with the BBS data, the overall pattern from those data suggest a continuing (albeit less rapid) decline in shrike populations in Canada.

Table 1. Loggerhead Shrike trend results from North American Breeding Bird Surveys. Data were taken from Sauer et al. (2002) and are given for the breeding range of L.l. excubitorides. Trend indicates the percentage of change/year.
Region 1966-2002
N
1966-2002
Trend
1966-2002
P
1966-1979
N
1966-1979
Trend
1966-1979
P
1980-2002
N
1980-2002
Trend
1980-2002
P
Alberta
22
- 4.0
0.21
7
- 12.6
0.02
19
0.5
0.91
Saskatchewan
33
- 9.4
0.01
15
- 12.3
0.00
25
- 3.0
0.10
Manitoba
10
- 13.1
0.22
-
-
-
8
-3.3
0.76
Canada
75
- 10.0
0.00
32
- 16.6
0.00
54
- 2.5
0.12
North Dakota
28
- 0.6
0.76
12
- 6.0
0.21
27
3.5
0.07
Montana
24
2.6
0.34
10
- 10.5
0.44
23
2.6
0.40
South Dakota
34
- 1.2
0.40
23
4.6
0.21
27
- 2.0
0.22
Nebraska
42
- 2.7
0.07
27
- 10.1
0.03
36
- 0.1
0.96
Wyoming
63
- 2.2
0.19
17
- 2.1
0.63
60
1.3
0.40
Colorado
49
2.5
0.25
10
1.2
0.79
49
0.9
0.67
Kansas
44
-2.8
0.00
34
- 5.0
0.03
43
- 4.3
0.00
Oklahoma
57
- 5.6
0.00
33
- 5.1
0.00
55
- 5.5
0.00
New Mexico
54
- 5.4
0.00
21
- 11.1
0.00
51
- 3.1
0.02

Alberta

Data from provincial monitoring schemes show a similar picture. In Alberta, where BBS data suggest that populations may have stabilized in the 1980s, recent survey work has found breeding shrikes along the southern periphery of the Central Aspen Parkland, an area previously thought to harbour few shrikes (Kiliaan and Prescott 2002). Surveys in the core of the species’ range in Alberta in 1993 and 1996 indicated a stable population there over that period, with 90 shrikes observed in 1993 and 96 in 1996 (Bjorge and Kiliaan 1997). However, roadside surveys in 1998 and 2003 suggest a 34% decline (from 1.96 to 1.29 pairs/km) over that 5-year period (D. Prescott, pers. comm. 2004). Similarly, recently updated data on the BBS web site suggest that the number of shrikes on BBS routes has declined in Alberta by 6.8% per year from 1993-2002, a number similar to that in Saskatchewan during the same period (see Table 2). However, there is often considerable uncertainty regarding the accuracy of BBS data for species with low population sizes and the Alberta trend is not statistically significant (see the discussion under Manitoba).

Figure 4. Breeding Bird survey indices of Loggerhead Shrikes fromAlberta, Saskatchewan, and Manitoba(data from Connie Downes, Canadian Wildlife Service, in. litt.).

Figure 4. Breeding Bird survey indices of Loggerhead Shrikes fromAlberta, Saskatchewan, and Manitoba (data from Connie Downes, Canadian Wildlife Service, in. litt.).
Table 2. Provincial results for Loggerhead Shrikes from the last decade of the Breeding Bird Survey, 1993-2002. (www.mp2-pwrc.usgs.gov/bbs/).
Province Routes Trend (% change/year) P
Alberta
16
- 6.81
0.20
Saskatchewan
19
- 6.31
0.02
Manitoba
5
- 2.60
0.85

Saskatchewan

Table 3 summarizes the numbers of breeding shrikes observed on transect surveys in Saskatchewan. While these data show little indication of a long-term decline in abundance, the decline from 1993 to 2003 is cause for concern (for a similar pattern, see the paragraph below on Manitoba results). However, analysis of the last ten years (1993-2002) of BBS data for Loggerhead Shrikes shows a statistically significant (P = 0.02) decline of 6% per year in Saskatchewan. These data suggest that in 2002 the population had declined to 54% of the 1993 total.

Table 3. Results of roadside transect surveys for breeding shrikes in Saskatchewan(from Collister 1999 and A. Didiuk in litt. 2004).
Region Routes km Pairs per 100 km
1987
Pairs per 100 km
1993
Pairs per 100 km
1998
Pairs per 100 km
2003
Northeast
6
1175
1.02
1.11
0.85
0.87
Northwest
8
1290
3.49
6.05
5.89
5.89
Southeast
10
1971
1.47
2.74
1.67
0.36
Southwest
8
1473
2.58
4.48
3.94
4.89
Totals
32
5909
2.10
3.57
3.00
2.90

Manitoba

In Manitoba, annual censuses have been carried out since the original COSEWIC status report in 1986. The number of shrikes increased from 1987 (265 pairs) until 1993 (327 pairs), but has since declined, with only 59 pairs found in 2002 (K. De Smet, pers. comm.; see Figure 5). In contrast, Manitoba BBS data for the same period suggest an annual decline of 2.6% per year, or a decline from 327 pairs in 1993 to 257 pairs in 2002. Thus, as mentioned earlier in this report, BBS data must be viewed cautiously, especially within small, declining populations such as in Manitoba. In this case, dedicated surveys likely provide a much more accurate picture of population trends than does BBS methodology.

Figure 5. Results of annual surveys for nesting Loggerhead Shrikes in Manitoba. Rs = Spearman rank correlation.

Figure 5. Results of annual surveys for nesting Loggerhead Shrikes in Manitoba. Rs = Spearman rank correlation.

Wintering range

On the wintering grounds, Loggerhead Shrikes have undergone declines within the Christmas Bird Count regions in the USA. Canadian (Alberta and Saskatchewan) populations of the Prairie Loggerhead Shrike are known to winter in the southern Great Plains states (Burnside 1987). Analysis of long-term trends in the numbers of shrikes seen on Christmas Bird Counts in Colorado and Kansas, as well as Oklahoma and Texas (the wintering areas of Canadian excubitorides) show statistically significant declines, with at least 50% declines in numbers from 1959-2002 (see Figure 6).

Figure 6. Pattern of abundance of Loggerhead Shrikes on Christmas Bird Counts in the southcentral USA.

Figure 6.  Pattern of abundance of Loggerhead Shrikes on Christmas Bird Counts in the southcentral USA.

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