Largest of the two Lanius species that breed in North America, the Northern Shrike is a species of boreal affinity, nesting widely but sparsely in the taiga and taiga–tundra ecotone in both the Palearctic and Nearctic. The species formerly was considered conspecific with L. excubitor, the Great Gray Shrike of the Western Palearctic, but now is treated as separate by the American Ornithological Society (1) on the basis of differences in plumage and mtDNA (see Systematics: Related Species). In North America, 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 and early winter. Winter irruptions are common, with peaks generally occurring every 3–6 years, but the reasons for such movements (e.g., cyclical small mammal populations) are little understood. In some years, individuals are found as far south as Arizona, New Mexico, Oklahoma, Missouri, Virginia, and Maryland.
The Northern Shrike feeds 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). This species 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 than themselves, including robins, jays, and doves. Although not noted for their song, they can sing rhythmic and complex songs, and are capable mimics. Both sexes are commonly heard singing in late winter (February–March), especially on sunny days.
Nearctic populations exhibit some geographic variation, with the western birds tending to be heavier, paler overall, and with more white in the tail than populations east of Hudson Bay. However, it remains unclear whether North America hosts 1 or 2 breeding subspecies (see Systematics: Subspecies).
Because the Northern Shrike is uncommon in the Nearctic and breeds in remote, sparsely populated regions of North America, the species is less well-studied than the more southerly Loggerhead Shrike (2). Studies on L. borealis in the Nearctic are summarized below. Miller (3) investigated the systematics and natural history of North American shrikes and laid the groundwork for future investigations. Davis (4) began a series of studies on the cyclic and irruptive winter movements of shrikes, based on data from the Christmas Bird Count (5, 6, 7, 8, 9, 10, JDP). Bent (11) summarized both published and unpublished information on the natural history of the Northern Shrike. Cade (12, 5) 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 (13). Atkinson (14, 15, 16) and Atkinson and Cade (17) studied overwintering ecology and food habits in Idaho. Recently, Brady and Paruk color-banded over 90 individuals overwintering in northern Wisconsin and studied territory size, plumage differences (18), and determined resting metabolic rates (19). Otherwise, information on the ecology and behavior of the Northern Shrike in North America is widely scattered in often obscure literature.