The irruption watch is on! Will Common Redpolls, Bohemian Waxwings, White-winged Crossbills, Snowy Owls and Pine Grosbeaks grace us with their presence this winter? Initial observations are promising as redpolls, Bohemian Waxwings and Snowy Owls have been reported widely in the state already.
These irruptive migrants are driven from their northerly breeding grounds by lack of food. Birds are remarkably tolerant of cold weather as long as they can find sufficient sustenance to keep their greedy furnaces stoked. When food on the Arctic tundra or the taiga becomes scarce, be it birch seeds, conifer seeds, lemmings or fruit, birds dependent on a particular food must move south or perish.
The Northern Shrike is another irruptive species. They are absent in Maine in some winters and occasionally common. At least a few Northern Shrikes have been seen in Maine. We’ll explore the biology of Northern Shrikes in today’s column.
The breeding distribution of Northern Shrikes spans the northern part of North American from Quebec to Alaska. When they withdraw from their breeding areas due to lack of food, they come farther south in the western United States, reaching southern Utah and Nevada. In the east, Northern Shrikes rarely occur south of Massachusetts and New York. This species also occurs in Eurasia where it is called the Great Grey Shrike.
Northern Shrikes are boldly marked birds, reminiscent of a bulky Northern Mockingbird. Northern Shrike adults have a gray back and crown with a narrow black mask through the eye. The wings are black with a prominent white patch. The tail is black with white outer tail feathers. The gray dorsal surface, white wing patches on black wings and white in the tail make it easy to dismiss a shrike as a mockingbird. The undersides are whitish-gray.
Immature Northern Shrikes look like washed out versions of adults. The upper parts are generally buff-colored; the mask is thinner and less darkly colored. The most distinctive feature is the prominent scaling on the breast and belly.
The similarity with mockingbirds ends when you check out the bill. The bill of a Northern Shrike is strongly hooked, used to kill small mammals, rodents and sometimes insects. A shrike is a hawk wannabe!
Northern Shrikes habitually perch at the top of a tall shrub or tree, appearing peaceful and docile. But when a potential prey is spotted, the shrike springs into action. A shrike usually captures insects or small mammals with its bill. Birds are pursued through the air and usually captured with the feet. Although the toes of shrike are not shaped like the talons of a hawk or owl, the feet are very strong. Friends of mine who band shrikes wear leather gloves when taking shrikes out of mistnets to avoid wounds from shrike feet clamping down on the banders’ hands.
Once a prey item has been secured, the shrike quickly kills it with a bite to the neck, severing the spinal cord at the level of the cervical vertebrate. The prey is typically impaled on thorns or barbed-wire, often left as a cache for later use. This macabre impaling behavior is the basis for the folk name of butcherbirds for shrikes.
While studying the effect of winter bird feeding on Black-capped Chickadees in a remote section of the North Woods east of Flagstaff Lake, I quickly learned when a Northern Shrike was in the neighborhood. The chickadees, woodpeckers and other birds frequenting the feeder would freeze. The cacophony of birds at the feeder halted; dead silence prevailed. Woodpeckers pressed themselves tightly against a tree trunk. The birds were perched stock-still in mortal feat. I never saw a shrike take a bird in this situation but they often captured a vole that made the unfortunate decision to emerge from the snow pack to grab a fallen sunflower seed.
[Originally published on December 7, 2014]