Two species of shrikes occur in North America, but only the Loggerhead Shrike
(Lanius ludovicianus) is unique to this continent. Shrikes differ from other songbirds in that their diet regularly includes small
vertebrate prey. In the absence of a raptorial foot used by larger predatory birds to handle live prey, shrikes have evolved the unique tactic of impaling prey on sharp objects such as thorns and barbed wire. This behavior has earned the species the nickname
Between seven and eleven subspecies of the Loggerhead Shrike have been identified based on range, morphology and plumage coloration. In Alberta, the Loggerhead Shrike is included on the
Yellow List of species that are not at immediate risk of
extirpation, but may require specific management attention because of concerns about long-term population declines. Loggerhead Shrikes are considered
threatened in western Canada by the
Committee on the Status of Endangered Wildlife in Canada (COSEWIC 1999).
Loggerhead Shrikes are birds that love open places. Throughout their range, their habitat typically includes grasslands interspersed with scattered trees and shrubs that provide nesting and perching sites. A variety of habitats often occur within breeding territories, including cultivated cropland, transportation rights-of-ways, and shelterbelts. Shrikes typically hunt from dead trees, tall shrubs, utility wires and fences, and may impale their prey on sharp twigs, thorns, or barbed wire. These features may also be important components of habitat selection by the Loggerhead Shrike.
The species composition of grassland in shrike habitat also varies geographically, but the preferred ground cover in all areas is typically used for livestock grazing. The preference for short grass appears logical, as shrikes rely heavily on insects that live in grassy areas, and the insects would be more easily detected and captured in short cover. Shrikes have also been known to use cropland and bare ground for foraging, although these habitats would typically be found adjacent to grasslands and are unlikely to be critical components of shrike habitat.
Shrikes from the northern portion of their breeding range winter in the central and southern United States and Mexico. Here, as on the breeding ground, shrikes prefer expanses of grassland, sometimes interspersed with trees and shrubs. However, the dependence of wintering shrikes on woody growth is apparently less than during the breeding season.
The Loggerhead Shrike is a medium-sized songbird. In the southern parts of the species' range, the Loggerhead Shrike is a permanent resident, but in more northerly areas, including Alberta, the species is migratory. Males arrive on the breeding grounds before females. Nests are built in a variety of trees and shrubs. The female, alone, builds the nest over a period of six to eleven days. The nest, which is generally a bulky cup of rootlets, forbs and bark strips lined with finer material, is generally concealed below the crown in a crotch or on a large branch.
Loggerhead Shrikes consume a wide variety of vertebrate and
invertebrate prey, and appear to adjust diet to local prey availability. Numerically, invertebrates make up the largest part of the diet, especially during the breeding season. Vertebrates make up a larger part of the winter diet and probably make up the
greatest percentage of
biomass of the diet at all times of the year. Generally, small mammals and birds make up the majority of vertebrate prey, although small reptiles and amphibians are also consumed. In Alberta, vertebrate prey are primarily Thirteen-lined Ground Squirrels, Meadow Voles, and Sagebrush Voles.
By late August, most birds have departed from Alberta for wintering grounds in the southern United States and
Mexico, although sightings as late as 5 November have been made. Birds apparently migrate individually, travel during the day, and move short distances at a time while feeding on route. Band recoveries from the Canadian prairies indicate that breeding birds from this area may winter primarily in Texas and Oklahoma. Two birds banded in Alberta in 1933 were recovered during the winter in central Texas, and one bird banded near Atlee in June 1992 was recovered in March 1995 at Fort Hood, Texas.
It is possible that birds from the Canadian prairies also migrate into Mexico, but the
lack of observers in that country reduces the probability of obtaining band recoveries south of the continental United States.
Reprinted from Alberta Wildlife Status Report No. 24 (1999), with permission
from Alberta Sustainable