Loggerhead shrike COSEWIC assessment and status report: chapter 6

Biology

Reproduction

Prairie Loggerhead Shrikes are summer residents in the northern half of their breeding range, including the Canadian Prairie Provinces. Shrikes arrive on the Canadian breeding grounds from April to June, typically raise a single brood of 4-6 young, and then begin fall migration sometime in August or September.

Both sexes are apparently involved in choosing the nest site and both gather nesting material, although only the female builds the nest (Yosef 1996). One egg is laid per day, and clutch size averages 5-6 eggs. Females perform all of the incubation and are fed extensively by their mates during the laying and incubation periods. The incubation period is approximately 16 days. Females brood the nestlings for the first 4 to 5 days, with males providing most of the food during this period. The nestling period lasts an average of 17-18 days, and the parents continue to feed the young during the post-fledging period.

If early season nesting attempts fail, the pair will typically renest within a few hundred meters of the failed nest. Second broods (following the successful fledging of a first brood) are rare in the northern part of the range (including Canada), but common further south (Yosef 1996).

Survival

Haas and Sloane (1989) found return rates of 28% for adult males and 5% for adult females in North Dakota. In Alberta, 1.2% of juveniles and 32% of adults were resighted in the following year, while 0.85% of juveniles and 16% of adults were resighted in Manitoba (Collister and De Smet 1997). Clearly, these return rates are not reflective of survival rates, given that adults and juveniles typically disperse from breeding and natal areas (see Movements/dispersal below). The lack of an accurate measure of adult and juvenile survival rates hampers attempts to accurately predict local population viability.

Juvenile mortality following fledging is apparently high, with 33-46% fledgling mortality the first 7-10 days after fledging (Yosef 1996). However, other studies (e.g., Blumton 1989) have shown relatively high fledgling survival. As a consequence, it is unclear whether fledgling survival is typically high, or whether the long dispersal distances of some fledglings (Blumton 1989, Haas 1995) has biased such estimates downwards.

Movements/dispersal

Collister and De Smet (1997) summarized breeding and natal dispersal in shrikes from Alberta and Manitoba. In Alberta, the mean distance moved by adults between successive years was 1.9 km. In the same area, young shrikes banded as nestlings were recaptured on average 12.4 km away from the natal site. In Manitoba, adults moved an average of 3.1 km between successive nesting sites, while natal dispersal averaged 15.4 km. However, there was considerable site fidelity, with 50% (10 of 20) of adult males and 27% (3 of 11) of adult females breeding on the same territory between captures.  Breeding dispersal was greater among females in both study areas, although the between-sex differences were significant only in Manitoba (23% of males, 9% of females). Breeding site fidelity in Manitoba appeared to be affected by fledging success, as 7% of adults that successfully raised young returned to the same territory, whereas only 2.3% of failed breeders returned.

Recovery of shrikes banded in Canada suggests that most of the prairie population winters in the southcentral USA (Burnside 1987). Two shrikes banded as nestlings in Alberta were recovered during winter in central Texas, while four shrikes banded in Saskatchewan were recovered in central Missouri (1 bird), southern Oklahoma (1), and in Texas (2). 

Interspecific interactions

Shrikes have been observed chasing a wide variety of birds, presumably in defense of foraging areas (Cadman 1985; Collister 1994; Woods 1994).  Northern Mockingbirds (Mimus polyglottos) and Burrowing Owls (Athene cunicularia) occasionally steal food from shrike caches (Yosef 1996). Fledgling shrikes are attacked/harassed by a number of species of passerine birds (Smith 1991). There are no indications that interspecific competition has contributed to local or range-wide population declines of Loggerhead Shrikes.

Adult shrikes attack and chase a wide variety of potential nest predators (see Yosef 1996). In Alberta, Collister (1994) documented feral cats, Black-billed Magpies (Pica pica), Long-tailed Weasels (Mustela frenata) and bull snakes (Pituophis melanoleucus) depredating shrike nests. While such predation may be more common in fragmented, agricultural habitats, there are currently no data to support the idea of increased predation rates on shrike nests in such habitats.

Behaviour/adaptability

Disturbance – Although it is typically assumed that Loggerhead Shrikes are tolerant of human disturbance around the nest site, there is conflicting evidence (Porter et al. 1975; Siegel 1980; Kridelbaugh 1983) as to how frequently shrikes desert the nest in response to disturbance. However, desertion rates are generally low. Only a single study (Campbell 1975) has assessed disturbance to foraging birds. The primary finding of Campbell’s study was that foraging birds are often victims of collisions with vehicles. Such mortality is thought to be especially extensive in winter, when shrikes often focus their foraging efforts along roadsides (Miller 1931, Cadman 1986).

Food/foraging – Loggerhead Shrikes are opportunistic foragers, typically adjusting their foraging to exploit the available prey base (Miller 1931, Craig 1978). Although they feed primarily on insects during the breeding season, vertebrates make up an increasing proportion of the diet during the winter months. Several studies (see Disturbance section above) have noted that during winter, shrikes feed primarily along roadways.

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