Loggerhead Shrike
Last Update/Review: June 17, 2002
Status | Description | Habits | Reproduction | Food | Limiting Factors | Management and Outlook
Status
Like many of the 74 species of shrike in the world, the loggerhead shrike has declined over much of its range. Between 1970 and 1991, this species has shown the most significant downward trend of any songbird in the southern Prairie Provinces.
The loggerhead shrike has been designated by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) as threatened in western (prairie) Canada and endangered in Ontario and Quebec.
Today, the nesting range of the loggerhead shrike extends across southern Alberta, Saskatchewan, and Manitoba south to Mexico. It winters from Kansas to Mexico. Historically, these birds were widespread in Alberta and occurred throughout the prairies, north and west into the aspen parkland and west into the foothills. As early settlers moved west and north clearing land, they created additional suitable habitat for shrikes. By the 1960s, this species was recorded as far northwest as the Peace River area in Alberta. However, since the mid-1980s, records as far west as Calgary or north to Edmonton are rare.
The Alberta population estimated for 1993 is in excess of 2000 pairs. Although the danger of extinction is not immediate, fewer shrikes occur today than 20-30 years ago. Fortunately, there is still time to develop and implement sound management strategies that could halt the population decline of loggerhead shrikes and ensure viable populations are maintained.
Internationally, the loggerhead shrike is protected by a treaty between Canada, the United States and Mexico (Migratory Birds Convention Act 1916). This treaty makes it illegal to harm the adults, young, eggs, or nests of loggerhead shrike anywhere in these countries. Therefore, while working to conserve this species on its breeding grounds in Alberta, we can be optimistic that the birds receive protection during the nonbreeding season as well.
Description
The loggerhead shrike is a medium-sized songbird—slightly smaller than a robin—grey above and white below, with a black "mask" through the eyes. The mask runs from behind the shrike's eyes to the beak and across the forehead, and acts to reduce glare that might impair the bird's ability to see prey in bright sunlight. Males and females have similar markings and cannot be distinguished easily in the field. Although they are songbirds, shrikes possess beaks similar to those of birds of prey, with a hook at the end and tooth-like structures on the upper mandible. By using this raptorial bill to kill their prey, shrikes are the only truly predatory songbirds.
The
loggerhead shrike can be confused easily only with the other North
American shrike species, the northern shrike. Although their breeding
ranges do not overlap, both species are found at mid-latitudes during
winter. The northern shrike breeds across northern Canada, its range
barely reaching into the provinces, and it winters as far south as
Colorado. Both species favor brushy open habitat and exhibit similar
behavior. The northern shrike is slightly larger (30 percent heavier),
has a lighter gray color, and a narrower mask that fails to reach above
the eyes. However, the two species can be difficult to tell apart if
a good view is not obtained or the bird in question is not studied
carefully. A strong clue to which species is most likely to be encountered
is the season. A shrike seen in southern Alberta from early October
to mid-April is most likely a northern. On the other hand, a shrike
seen between May I and September I in southern Alberta is almost certainly
a loggerhead. Special care must be taken in that part of the province
during late April and September when either species may be encountered.
Habits
Most loggerhead shrikes arrive in southern Alberta during the first week of May and begin their return journey south by the end of August. The specific wintering grounds of loggerhead shrikes that breed in Alberta are unknown. A few birds banded in the Prairie Provinces have been recovered in Texas, Missouri and Oklahoma; however, the recoveries occurred in fall and early winter and may not represent wintering birds. It is possible that many shrikes from Canada might be going as far south as Mexico. Shrikes that remain year-round residents in the southern United States may occupy all suitable habitat and prevent Canadian birds from wintering there.
Loggerhead shrikes are visual hunters and require tall perches from which to survey the surrounding countryside for potential prey. Isolated trees, power poles, fenceposts and tall shrubs provide suitable lookout sites. When a prey item is located, the shrike descends quickly to attempt to capture it and, if successful, often flies directly to the nest. loggerhead shrikes are active throughout the day although strong prairie winds or intense midday heat will force them to retreat to the shelter of a shrub or tree. On calm days they are usually visible, perched on a favorite lookout.
Most
shrike species world-wide are birds of edge habitat and the loggerhead
shrike is no exception. Open country, thinly wooded or scrubby land
with clearings, meadows, pastures, shelterbelts, and thickets along
roads and other rights-of-way are preferred habitats. Areas of dense,
ungrazed or lightly grazed grassland are important habitats for potential
prey. When prey move into adjacent open, grazed areas, the shrikes
are able to see and capture them more readily. The transition between
aspen parkland and grassland areas provides ideal habitat.
Shrikes sometimes impale various types of prey on thorny trees, shrubs and barbed wire. This behavior is the basis for the term "butcher bird," another common name given to the loggerhead shrike. Impaling dead prey has several functions. The shrike is a songbird, without the strong talons of a hawk.
Using thorns or barbs helps the shrike to hold on to larger prey while it is torn into bite-size pieces with the strong hooked bill. Impaling prey also appears to be part of a courtship display that lets a female know that this male's territory does indeed contain prey and, perhaps more importantly, that he can provide it. Impaled prey are also an emergency food source used by shrikes when fresh prey are unavailable (such as during bad weather or on a cold morning). Finally, caching prey by impaling it nearby allow the female to remain close to the nest and protect her young.
Adult loggerhead shrikes show strong attachment to the breeding area used the previous year, often nesting within a kilometre or two of the previously used nest. Young birds seem to wander widely and seldom return to the area where they were raised. Although a few areas in southeastern Alberta contain high densities of loggerhead shrikes, many pairs of birds breed in isolated farm shelterbelts and other sites with no close neighbors. Shrikes defend territories of 3-20 hectares; the size depends on habitat quality. Loggerhead shrikes are bold and show little fear of people. It is not uncommon for adult birds to attack researchers who come too close to nest sites.
Reproduction
A dense shrub, either isolated or on the edge of a shrubby patch, usually is selected for a nest site. Thorny buffalo berry and willow are preferred. Other favored nesting sites are caragana, Manitoba maple and poplar. Loggerhead shrikes construct a rather bulky nest of twigs lined with grass and cattle hair tightly woven into a deep cup. Nests are seldom built in sites more than 2-3 m above ground and average 1.3 m in height. Loggerhead shrikes are strongly territorial, but in good habitat, nests can be as close as 100 m apart.
Usually 6-7 yellowish white speckled eggs are laid in the last few weeks of May. Incubation takes approximately 16 days and most hatching occurs during the first 2 weeks of June. If the initial nesting attempt fails during incubation or in the first few days after hatching, a second attempt may be made at a new nest site less than 100 m away. Only the female incubates the eggs. The male finds food and feeds her until the eggs hatch, after which both parents provide prey to the young. After 16 days, the chicks leave the nest, and by 21 days, they are making short flights. At approximately 35 days, the young are independent and forage for themselves. Both adults and young begin to migrate south in early August and most birds have departed by the end of that month. There are no records of shrikes migrating in groups hence loggerhead shrikes appear to be solitary migrants.
Food
In Alberta, loggerhead shrikes prey on grasshoppers, crickets, beetles and other invertebrates as well as mice and voles, juvenile ground squirrels, horned larks, vesper sparrows and lark buntings. Most prey are transported in the bill although heavier prey are sometimes carried in the shrike's feet.
During the breeding season, most of the items in the diet are insects, whereas in winter, half the prey items may be small rodents. However, vertebrate prey such as small rodents and songbirds are relatively large and those eaten during the breeding season may be critical for successful reproduction. During periods of bad weather, a cached vole can provide the key nutrition that allows nestlings to survive until fresh prey is available. Similarly, early in the season, adults whose fat reserves are low after a migration of 3000 km or more may require vertebrate prey in order to regain the energy needed for breeding.
Loggerhead shrikes exhibit several hunting strategies. The most common, as described previously, is watching from an elevated perch. In habitats where perches are scarce, or in tall grass where visibility is limited, shrikes may hover over potential prey sites. This technique expends a large amount of energy and appears to be reserved for areas where food abounds or areas where prey have been previously sighted. Occasionally shrikes will attempt to chase down songbirds, usually young or injured individuals. Loggerhead shrikes are not strong fliers and have difficulty catching healthy adult birds on the wing.
Limiting Factors
Many factors have been suggested as contributing to declines in loggerhead shrike populations. Pesticide contamination, increased competition with other species such as the American kestrel, increased human disturbance, and climatic change may have contributed to this species' threatened status. Reduction of suitable habitat is also felt to be important.
Suitable breeding and wintering habitat has been disappearing in most areas of North America, primarily because of intensive agricultural practices that have resulted in the removal of shrubs and shelterbelts. In Alberta, 79 percent of native grassland has been lost, the majority converted to cultivation. Abandoned farmsteads, shelterbelts, roadsides and railway rights-of-way currently provide the loggerhead shrike with suitable habitat—open grazed areas adjacent to areas with perches and dense ungrazed vegetation. Research conducted in 1991 suggested that, at least in certain areas of southern Alberta, all remaining good quality shrike habitat supported at least one breeding pair.
The
most important factor contributing to the mortality of young is adverse
weather. Wet, rainy days associated with cold temperatures can cause
the death of all young in a nest. Loggerhead shrikes are at the northern
limit of their range in Alberta and are therefore living close to their
limit of tolerance with regard to climate.
However, because the species has survived thus far in the presence of this adverse environmental factor, climate is unlikely to be significant in the overall decline of this species in Alberta. Mortality on the breeding grounds can also occur because eggs and young are susceptible to a variety of predators including weasels, snakes, crows and magpies. Both fledglings and adults are preyed upon by owls, falcons and hawks.
The mortality of adults during migration or on wintering grounds may be a limiting factor. Recent research in southeastern Alberta has indicated that fewer shrikes return each year from wintering grounds than the number that nested the previous year. Unfortunately, because wintering areas and migration routes are unknown, specific causes of nonbreeding mortality have not been identified.
Loggerhead shrikes are usually welcomed in prairie communities as a natural pest control. Unfortunately, there have been those who felt that the butcher bird, because it occasionally preys on other birds, should be killed. Nests, eggs and adults have been destroyed. Thankfully, this attitude is no longer common and these handsome birds are becoming better known and appreciated. It is illegal to kill loggerhead shrikes or disturb them or their nests at any time of the year in Alberta.
Management and Outlook
The outlook for the loggerhead shrike in Alberta is uncertain. The population, although relatively high, has declined in recent decades. Stabilization of the population is a key management priority. Alberta, along with other provinces with populations of loggerhead shrikes, is a member of the National loggerhead shrike Recovery Team. Team members work to coordinate research and develop management strategies to conserve this species.
Loggerhead
shrikes require a diverse landscape. Unfortunately, such habitat generally
is limited in occurrence and some of these areas are in danger of being
lost. Management for this species must include preservation of existing
habitat wherever possible. For example, the Canadian Pacific Railway
right-of-way that stretches from Princess to Empress harbours the highest
density of loggerhead shrikes in Alberta and should be managed to ensure
the species' long-term survival.
Another important management strategy is to encourage farmers and ranchers to maintain existing shelterbelts and hedgerows, as well as abandoned farm sites with trees and shrubby areas. One way to make these areas favorable for nesting loggerhead shrike is to fence off a modest area around the nest bush. This fencing will serve two purposes. First, it will allow vegetation to remain ungrazed within the fenced area, thereby providing good habitat for prey species. Second, by excluding cattle, the nest bush or tree will not be damaged nor the birds disturbed due to rubbing or trampling.
In Alberta, the Operation Burrowing Owl program is being expanded to include a variety of threatened prairie wildlife. The loggerhead shrike was the first species added to the program, renamed Operation Grasslands Community in 1994. This program encourages landowners to become stewards (caretakers) of their grasslands and the species that occupy them. (If you are interested in becoming a grassland steward call your local Natural Resources Service office.)
Enhancing loggerhead shrike populations can involve an uncomplicated and inexpensive management program. Increasing habitat for shrikes can be as simple as planting a few appropriate shrubs in an unused corner of your pasture or field, fencing them off, and telling other landowners, who may implement similar habitat management. To preserve existing breeding sites, land within 400 m of an active nest should be restored to the natural grassland/shrubland habitat upon which these birds rely. Furthermore, although shrikes are tolerant of some human activity, human disturbance within 100 m of a nest should be minimized during the mid-May to mid-August breeding season. Landowners with shrikes on their property have a unique opportunity to actively particiipate in the conservation of these exciting birds and witness the natural history of the loggerhead shrike first-hand.
