Loggerhead shrike COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Habitat conversion/degradation has been correlated with population declines of shrikes throughout North America (Yosef 1996). Telfer (1993) reported a 39% decline in unimproved pastureland between 1946 and 1986 in areas of the Prairie Provinces where shrikes have undergone the greatest declines. Telfer (loc. cit.) also studied shrike habitat use on the wintering grounds in Texas and found that only 17% of native grasslands remained. Samson and Knopf (1994) reported dramatic losses of native grasslands in Alberta (61% of mixed grass prairie), Saskatchewan (81% of mixed grass prairie and 85% of shortgrass prairie), and Manitoba (99% of both tallgrass and mixed grass prairie), as well as further south along the western and central Great Plains. Thus, the extensive loss of native grasslands (the preferred foraging habitat; Bent 1950) throughout the breeding, migration and wintering areas has also likely had a significant negative impact on Prairie Loggerhead Shrike populations. 

Although the exact sources of mortality have not been identified, it is clear that mortality of recently fledged young is high. Collister (1994) found that during the first 10 days following fledging, mortality of juvenile shrikes in Alberta was 33% in one year and ≥ 53% in another year.  In Indiana, Burton (1990) found a 46% mortality rate of juvenile shrikes within the first week of fledging.

There is extensive evidence of negative effects of pesticides on Loggerhead Shrikes (see summary in Yosef 1996). Declines in Loggerhead Shrike populations coincided with the introduction of organochlorines in the USA and Canada (Yosef 1996). Two studies have quantified relatively high levels of pesticides in shrikes breeding in Illinois (DDE; Anderson and Duzan 1978) and California (DDT; Rudd et al. 1981), while dieldrin has been implicated in the decline of Prairie Loggerhead Shrikes in Canada (G. Gox in Cadman 1985).

Pesticides may have direct negative effects via egg shell thinning (Morrison 1979) and consequent poor hatching success, impairment of behavioral development in juveniles, or death (Busbee 1977). In addition, pesticides may affect reproductive success and survival by significantly decreasing prey abundance. Cadman (1985) suggested that the sharp declines of Prairie Loggerhead Shrikes in Canada were correlated with the extensive use of dieldrin against grasshoppers, a primary source of food for breeding shrikes in the Prairie Provinces. Cadman cited evidence of a link between declines in clutch and brood sizes, hatching success, and the use of dieldrin in shrike breeding areas. On the wintering grounds, the recent spread of red fire ants has prompted the use of mirex (a pesticide), which can accumulate to relatively high levels in shrikes (Collins et al. 1974, Lymn and Temple 1991).

Collisions with vehicles are thought to be a major source of mortality for adult and juvenile shrikes. Shrikes often forage from hedgerows and barbed wire fences, often near roads, and several authors have documented extensive mortality in such situations (e.g., Robertson 1930, Miller 1931, Luukkonen 1987, Blumton 1989).

Predation on adults and eggs and nestlings appears to be higher near roads and hedgerows, which attract predators (DeGeus 1990). Feral cats have been noted as predators (Gawlik and Bildstein 1990, Scott and Morrison 1990), while raptors and carnivores in general have also been cited (Bent 1950).

In shortgrass prairie habitats, cattle may damage or kill the few trees available to nesting shrikes (D. Wiggins, pers. obs.). Populations of L. l. excubitorides in southeastern Colorado appear to actively avoid nesting in grasslands subject to summer cattle grazing, and instead nest in roadside shrubs and within fenced cattle exclosures (D. Wiggins, pers. obs.). 

Weather conditions may have significant negative effects on reproductive success. Porter et al. (1975) found that 9 of 12 nests in eastern Colorado were destroyed during severe thunderstorms, and such storms are a frequent source of nest losses on the Great Plains of eastern Colorado (Susan Craig, pers. comm.). Poor weather (cold, wet) is likely even more of a problem on the periphery of the species’ range (e.g., in Canada).

Finally, within the Manitoba population, reproductive success appears to be declining in step with the observed population declines. The most detailed data set (range-wide) on reproductive success has been collected in Manitoba, and several measures of reproductive success have declined since 1993, when the shrike population began a precipitous drop in numbers (Table 4).

Table 4. Measures of reproductive success in Manitobafrom 1987-1993, relative to 1994-2002
(data from Ken De Smet, pers. comm.).
Factor 1987-1993 1994-2002
a) Clutch size
6.18
6.17
b) Mean brood size at 8-10 days
5.28
4.79
c) Nest success
62.5%
57%
d)  b x c (fledged young per initiated nest)
3.30
2.73

West Nile Virus killed many captive Loggerhead Shrikes in Ontario in 2002 and there is a documented death from Alberta in 2003 (R. Bjorge, in litt. 2004).

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