Loggerhead shrike COSEWIC assessment and status report: chapter 2

Executive Summary

Loggerhead Shrike
Excubitorides Subspecies
Lanius Ludovicianus Excubitorides

Species information

English name: Loggerhead Shrike excubitorides subspecies, French name: Pie-grièche migratrice de la sous-espèce excubitorides

Scientific name: Lanius ludovicianus excubitorides (Swainson).

The Loggerhead Shrike excubitorides subspecies (hereafter Prairie Loggerhead Shrike) is a medium-sized songbird, approximately 21 cm in length. Adults are boldly coloured with a blend of black, white and gray, and are particularly conspicuous in flight. The best field mark is the black facial mask, which completely covers the eye (in the similar Northern Shrike, Lanius excubitor, the black facial mask does not extend above the eye). The bill is black with a hook at the tip of the upper mandible.

Distribution

The Prairie Loggerhead Shrike breeds from Alberta, Saskatchewan and Manitoba south through the Great Plains to northern Mexico. The winter distribution is poorly known, but is thought to be primarily the south-central United States (e.g., Oklahoma, Texas, Missouri) and Mexico.

Habitat

The Prairie Loggerhead Shrike inhabits a wide variety of open habitats including grasslands, sand-sage, pastures, agricultural areas, and open woodlands, with small shrubs and brushy trees providing nesting and foraging areas. Its habitat choice is still poorly understood, as many apparently suitable areas are not used.

Biology

Prairie Loggerhead Shrikes typically return to Canadian breeding areas from late April through early June. Nests are usually placed in thorny and/or dense shrubs and small trees within open habitats. Clutches of 4-7 eggs are initiated from late May through early July (June peak), and a single brood is raised. Family groups remain together for at least 2 weeks following fledging, but family groups apparently dissolve thereafter as most southward migrants are solitary birds. Shrikes reach their wintering grounds in the fall, and remain until the following spring.

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). Populations are declining strongly in Manitoba, are apparently stable in Alberta, while the trend in Saskatchewan remains unclear. Difficulties in accurately assessing population trends stem from the different results of dedicated shrike surveys, and continent-wide Breeding Bird Surveys. In the U.S. portion of the range, population trends are variable, with declining populations in the eastern (e.g., Minnesota and Iowa) and the southern portions of the range (e.g., Kansas, Oklahoma, Texas, New Mexico), but apparently stable populations in the northwest (e.g., Montana, Wyoming).

Limiting factors and threats

The primary limiting factor and threat to viable populations is thought to be conversion of grassland to cropland, and degradation of remaining grasslands. Other factors thought to have contributed to population declines include 1) accumulation of toxins (from pesticide applications) via the prey base; 2) collision with vehicles resulting from foraging (and nesting) activities around roadways; 3) increased predation at nest sites due to increasing habitat fragmentation; and 4) declines in prey abundance as a result of pesticide applications and habitat fragmentation. All of these factors may act to decrease reproductive success and adult survival.

Special significance of the species

Prairie Loggerhead Shrikes are closely related to the eastern subspecies L. l. migrans, which has disappeared from much of its former range in Canada and the northeastern United States. Formerly common throughout their range, Prairie Loggerhead Shrikes are now rare and Endangered in the northern and eastern portions of their range, with little sign of abatement in population declines in recent years.

Existing protection or other status designations

The Loggerhead Shrike is protected internationally (Canada, Mexico, USA) by the Migratory Birds Convention Act (1916).  It is classified as G4 (apparently secure globally, although it may be quite rare in parts of its range, especially at the periphery) by NatureServe; provincial NatureServe rankings are S2S3B in Manitoba, S4B in Saskatchewan and S3B in Alberta. This taxon is listed as Endangered in Manitoba, as a Species at Risk in Saskatchewan, and a Sensitive Species in Alberta; it is listed as Endangered, Threatened or a Species of Concern in several U.S. states.

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed  under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species and include the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal organizations (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biosystematic Partnership, chaired by the Canadian Museum of Nature), three nonjurisdictional members and the co-chairs of the species specialist and the Aboriginal Traditional Knowledge subcommittees. The committee meets to consider status reports on candidate species.

Definitions (after May 2004)

Species
Any indigenous species, subspecies, variety, or geographically or genetically distinct population of wild fauna and flora.

Extinct (X)
A species that no longer exists.

Extirpated (XT)
A species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A species facing imminent extirpation or extinction.

Threatened (T)
A species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)Footnotea
A species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)Footnoteb
A species that has been evaluated and found to be not at risk.

Data Deficient (DD)Footnotec
A species for which there is insufficient scientific information to support status designation.

 

Canadian Wildlife Service

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

 

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