Largest of the 2 North American species of Lanius, the Northern Shrike is a breeder of boreal affinity in the Nearctic, nesting widely but sparsely in the taiga and taiga-tundra ecotone from Labrador and Quebec to western Alaska, unlike palearctic populations which breed across a much broader latitudinal range. In winter, some individuals move south into southern Canada and the northern part of the coterminous United States (farther south in some years), while depending on conditions others overwinter in subarctic regions at least as far north as the Arctic Circle.
Similar to the Loggerhead Shrike (Lanius ludovicianus) in hunting habits and use of a wide range of prey from small insects to mammals and birds its own size and larger, the Northern Shrike consumes a higher percentage of vertebrate prey than the former. Especially in winter, it is a determined pursuer of small birds and mammals, which it impales in typical shrike manner on thorns and barbed-wire or wedges in forks of branchlets.
This species is most often observed when it moves out of its northern haunts during winter to sojourn in more southerly latitudes, where prey may be more available. These movements are notably variable in extent from year to year. Typically perched atop a tall tree or shrub, exposed in a lollipop-like silhouette surveying its world, the Northern Shrike appears innocuous and non-predatory. Often tame and unsuspicious, it sometimes sings a feeble though pleasing and rhythmical song, even in winter, utterly belying its true nature, which it reveals the instant a mouse or small bird moves within its range of attack. The Latin binomial, Lanius excubitor, means "butcher watchman," an apt name for this capable and alert predator.
Because this species breeds in remote, northern regions and is generally rare in North America, much less is known about it in the Nearctic than in the Palearctic ( Cramp and Perrins 1993b ); it has also been much less studied than the more common, temperate zone Loggerhead Shrike ( Yosef 1996 ). Although North American populations of Northern Shrike exhibit relatively slight geographic variation, Palearctic populations are quite variable and may include more than 1 species.
Studies specifically about excubitor in the Nearctic are few. In Miller 1931c A. H. Miller published his classic doctoral dissertation on the systematics and natural history of North American shrikes, laying the groundwork for further investigations, but his treatment of excubitor was subsidiary to his main interest in ludovicianus . Also in the 1930s, David E. Davis began a series of studies on the cyclic and irruptive winter movements of shrikes, based on Christmas Bird Count data, a theme that has continued to the present ( Davis 1937a , Cade 1967 , Davis and Morrison 1988 , Atkinson 1995 , Petersen and W. E. Davis 1997 , Hess 2000 ). Arthur Cleveland Bent produced his natural history of the Northern Shrike in 1950, summarizing both published and unpublished information. Cade ( Cade 1962 , Cade 1967 ) studied the hunting behavior and food habits of excubitor both on its breeding grounds in Alaska and its winter haunts in upstate New York and produced the only paper on breeding biology in the Nearctic ( Cade and Swem 1995 ). Atkinson ( Atkinson 1991 , Atkinson 1993 , Atkinson 1997 , Atkinson and Cade 1993 ) studied winter ecology and food habits in Idaho. Otherwise, information about the Northern Shrike is widely scattered in often obscure literature as short notes or segments in faunistic and ecological accounts.
Consequently, we have relied for general and comparative information on the much richer literature on excubitor in the Palearctic ( Cramp and Perrins 1993b ). Studies of special relevance are a series of papers 1981-1995 by Viking Olsson on the ecology of excubitor in Sweden, as well as the Finnish work (in German) on breeding biology and food habits by K. Huhtala et al. ( Huhtala et al. 1977 ). Also there are a number of important German works on behavior and ecology, especially Mester ( Mester 1965 ), Ullrich ( Ullrich 1971 ), and an impressive series of papers by Martin Schön (summary in English 1995) based on his doctoral dissertation. Reuven Yosef's research on shrikes in Israel ( Yosef 1992b , Yosef and Pinshow Yosef and Pinshow 1988 , Yosef and Pinshow 1989 , Yosef and Pinshow 1995 ) has also been helpful in understanding general aspects of shrike biology in the Nearctic.