Largest of the two Lanius species that breed in North America, the Northern Shrike is a species of boreal affinity across the Holarctic, nesting widely but sparsely in the taiga and taiga–tundra ecotone. In the Nearctic the Northern Shrike breeds from Labrador and Quebec to western Alaska, and many, but not all, individuals migrate south to southern Canada and northern United States during late fall/early winter. Winter irruptions are common, with peaks generally occurring every 3–6 years, but reasons for such movements (e.g., cyclical small mammal densities) are little understood. In some years, individuals are found as far south as Arizona and New Mexico in the west, Oklahoma and Missouri in the Midwest, and northern Virginia and Maryland in the East.
Northern Shrikes feed on larger prey than the similar, and more southerly distributed Loggerhead Shrike (Lanius ludovicianus), especially during winter months, when their diet consists of a higher percentage of vertebrate prey (birds and mammals). The Latin binomial, L. excubitor, means “butcher watchman,” an apt name for this capable and alert predator that impales its prey on thorns, barbed-wire fences or wedged in forks of branchlets. Lanius have a strong, sharp, hooked bill with a small falcon-like "tooth" (tomium) on either side of the upper mandible and a corresponding notch in the lower mandible. They are commonly observed perched atop a tall tree or shrub, surveying the landscape, where they appear innocuous and non-predatory. However, should a flock of finches or starlings fly overhead, they will actively pursue them. Although they are not strong direct flyers, they are persistent, and will often follow prey into thick bushes. They have been known to take down birds larger then themselves, including robins, jays, and doves. Although not noted for their song, they can sing quite rhythmic and complex songs, and are capable mimics. Both sexes are commonly heard singing in late winter (Feb-Mar), especially on sunny days.
Neartic populations exhibit some geographic variation, with the western subspecies (L. excubitor invictus) being slightly heavier, paler overall, and with more white in the tail than the central and eastern subspecies (L. e. borealis). Palearctic populations (L. e. excubitor) are quite variable and may include more than one species ( Harris and Franklin 2000 ). Because this species is uncommon and breeds in remote, sparsely populated regions of North America, they are not as well-studied as the more southerly distributed Loggerhead Shrike ( Yosef 1996 ) or Palearctic populations, both of which nest closer to population centers ( Kuczynski et al. 2010 ). Studies on excubitor in the Nearctic are summarized chronologically below.
Miller 1931c investigated the systematics and natural history of North American shrikes and laid the groundwork for future investigations. Davis 1937a began a series of studies on the cyclic and irruptive winter movements of shrikes, based on data from Christmas Bird Counts, that has continued to the present ( Cade 1967 , Davis 1974e , Davis and Morrison 1988 , Atkinson 1995 , Petersen and W. E. Davis 1997 , Hess 2000 , JDP). Bent 1950 summarized both published and unpublished information on the natural history of the Northern Shrike. Cade 1962 and Cade 1967 studied the hunting behavior and food habits 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 1991 , Atkinson 1993 , Atkinson 1997 , Atkinson and Cade 1993 studied overwintering ecology and food habits in Idaho. Recently, Brady and Paruk have uniquely color-banded > 90 individuals overwintering in northern Wisconsin and studied plumage differences ( Brady et al. 2009 ), territory size, and determination of resting metabolic rates ( Paruk et al. 2015 ). Otherwise, information about the Northern Shrike is widely scattered in often obscure literature as short notes or segments in faunistic and ecological accounts.
Breeding and nonbreeding biology is better documented and understood for Palearctic excubitor and this rich literature will be incorporated ( Cramp and Perrins 1993b , Antczak et al. 2004 , Antczak et al. 2012 , Tryjanowski and Martin 2005). Studies of special relevance are a series of papers from 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 ( Huhtala et al. 1977 ). Also there are a number of important German works on behavior and ecology (e.g., Mester 1965 , 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 1992, 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 last decade, numerous papers have been produced on the breeding and nonbreeding behavior and ecology of excubitor in Poland ( Antczak et al. 2004 , Antczak et al. 2005, Antczak et al. 2010 , Antczak et al. 2012 ; Hromada et al. 2002 , Hromada et al. 2003 , Hromada et al. 2008 ; Tryjanowki et al. 2005). These latter series of papers lay the foundation for future comparative behavioral and ecological studies between Nearctic and Palearctic populations.