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Island Loggerhead Shrike

NPS
 

Scientific Name
Lanius ludovicianus anthonyi

Introduction
The island loggerhead shrike is an endemic, genetically distinct sub-species of loggerhead shrike found on Santa Rosa and Santa Cruz Islands in the northern Channel Islands and on Santa Catalina Island in the south. The island loggerhead shrike is a robin-sized bird that hunts like a small hawk, preying on insects and small animals, including small birds. Populations on all three islands are greatly diminished from historical populations. A close relative, the endemic San Clemente Island loggerhead shrike (Lanius ludovicianus), is found only on San Clemente Island and is federally listed as endangered.

Quick and Cool Facts

  • The name "loggerhead" means of possessing a head that is disproportionately larger than the rest of the body.
  • Another name for the shrike is "butcher bird", owing to its tendency to impale their prey on thorns. The reason appears to be due to their lack of talons that would accommodate holding their prey while consuming it.
  • Lanius, the scientific name for the shrike, is derived from the Latin word meaning "butcher". Historically, the Santa Cruz and Santa Rosa Islands population of island loggerhead shrikes was variously reported as "extremely common" to "comparatively common" and "fairly common."
  • Recent surveys indicate that the island loggerhead shrike occurs in very small population sizes.
  • The U.S. Fish and Wildlife Service has been petitioned to list the subspecies under the Endangered Species Act, the objective of a 2009-2010 project was to obtain a rigorous and defensible estimate of northern island loggerhead shrike abundance on both islands.

Appearance
The island loggerhead shrike is a robin-sized bird. It is the darkest of all forms of this species. It is similar in coloration to the mainland loggerhead shrike, but the gray of upper parts is still darker (nearly slate color) especially on the top of the head, and more uniform, the shoulder feathers, almost wholly deep gray; white spot at the base of primary wing feathers much smaller; under parts of body much more strongly tinged with gray, becoming distinctly gray on sides and flanks; lateral tail feathers with much less of white, this extending slightly less than an inch for the tip on inner web of exterior tail feathers.

Range
Although loggerhead shrikes are widely distributed throughout the continental United States, the island loggerhead shrike is an endemic, genetically distinct subspecies of loggerhead shrike found on Santa Rosa, Santa Cruz, and Santa Catalina Islands.

Habitat
Like other shrikes, island loggerhead shrikes require rather open areas for hunting, with sufficient vertebrate and invertebrate prey, but also adequate perches, and enough shrub cover nearby for nesting. They prefer grasslands, open coastal sage scrub habitats on terraces, and brushy canyon slopes (Collins 2008).

Feeding
Island loggerhead shrikes are efficient search-type predators; their diet apparently is related more to prey abundance, detectability, and size than to specific prey type. Although the diet of the island loggerhead shrike has not been studied, it is likely similar to that of the San Clemente loggerhead shrike an endemic subspecies of shrike that occurs only on San Clemente Island in the southernChannel Islands (Collins 2008). That shrike feeds on a wide variety of prey, including various aerial and ground-dwelling arthropods (crickets,grasshoppers, beetles, earwigs, lepidopteran larvae, flies, truebugs, ants, bees, and wasps), small ver­tebrates (lizards, birds, and mice), molluscs (land snails), isopods (pill-bugs), arachnids (spiders and scorpions), chilopods (centipedes), thysanurans (silverfish), and diplopods (millipedes). For foraging, shrikes require open and semi-open habitats with scattered taller vegetation. Shrikes generally search for prey from a variety of perches from 3 to 45 feet above the ground.

Reproduction
Protection of nests against predation is important. On the islands, shrike nests are susceptible to predation by island foxes, and, on Santa Cruz Island, by the islandscrub-jay. Therefore, nests typically are placed in a variety of arborescent shrubs, includ­ing lemonade berry, California sagebrush, Catalina cherry, toyon, blue elderberry, and various oaks. Pairs generally raise a single brood and will renest following nest failure. Island loggerhead shrikes have an extended breeding season, which enables pairs to raise two broods in some years.

Conservation Status
Although loggerhead shrikes are widely distributed and abundant, the northern island subspecies is classified as a species of special concern by the California Department of Fish and Game and has been petitioned for federal listing under the Endangered Species Act.

Recent exhaustive surveys on Santa Cruz and Santa Rosa Islands provided the first reliable estimates of island loggerhead shrikes (Stanley et al. 2012). Estimated population size in 2010 was 240 on Santa Rosa and only 42 on Santa Cruz, and historical shrike populations may have been much higher (Collins 2008). Higher shrike numbers on Santa Rosa may reflect higher habitat quality on that island. There may be more habitat with short vegetation, which shrikes prefer, on Santa Rosa Island. Shrikes also require adequate perches from which to hunt, and suitably thick shrubs in which tonest. Changes in vegetation due to removal of non-native ungulates will have as-yet undetermined effects on shrike habitat.

Very little is known about the ecology of the island loggerhead shrike. Basic information is lacking concerning breeding biology, habitat relations and the importance of predation. Because of this, the reasons for its apparent decline and the difference in population size between Santa Rosa and Santa Cruz Islands remain unclear.

The island shrike's cousin, the San Clemente Island shrike, was listed as Endangered in 1977 and at one point was considered the most endangered animal in North America. In 1998 there were only 12 adults remaining in the wild. The population has since increased due to captive breeding and reintroduction, coupled with innovative predator control: predatory island foxes were fitted with shock-collars to prevent them from depredating shrike nests (Cooper et al. 2005).

Additional Information

  • National Audubon Society
  • BirdLife International
  • Northern Channel Islands Loggerhead Shrike Study
  • Collins, P.W. 2008. California Bird Species of Special Concern, Shuford, D. and T Gardali, eds., Studies of Western Birds No. 1, Western Field Ornithologists and California Fish and Game. 278-283.
  • Cooper, D.M., e.L.Kershner and D.K.Garcelon. 2005. The use of shock collars to prevent island fox (Urocyon littoralis) predation on the endangered San Clemente loggerhead shrike (Laniusludovicianus mearnsi). Pages 287-297 in D.K. Garcelon and C.A. Schwemm, eds., Proceedings of the Sixth California Islands Symposium. National Park Service Technical Publication CHIS-05-01, Institute for Wildlife Studies, Arcata, California.
  • Smith, Suxan M.The Ontogeny ofImpaling Behaviour in the Loggerhead Shrike, Lanius ludovicianus L. Behaviour, vol 42, N. 3/4(1972), pp.232-247.
  • Stanley, T.R., S. Teel, L.S. Hall, L.C. Dye, and L.L. Laughrin. 2012. Population size of island loggerhead shrikes on Santa Rosa and Santa Cruz Islands. Wildlife Society Bulletin 36(1): 61-69.

Did You Know?

1994 pygmy mammoth excavation, Santa Rosa Island

The world's most complete pygmy mammoth specimen was discovered on Santa Rosa Island in 1994. These miniature mammoths, only four to six feet tall, once roamed island grasslands and forests during the Pleistocene.