By Michael J. Caduto
Barred Owl. Photo credit: Michael J. Caduto
A barred owl has been stalking our bird feeder this winter, ever since a thick blanket of snow covered the ground. It is not hunting songbirds, but instead angles for the small mammals that glean dropped seeds. One morning, during a brief ten minutes, the owl caught two meadow voles and a short-tailed shrew, swallowing them whole.
Barred owls mostly hunt at night, but they sometimes feed during the day, especially when food is scarce. But what of songbirds? Short days afford little time to forage, even as greater energy is needed to stay warm and survive the long frigid nights. Plentiful food and shelter are essential for sustaining vital energy stores.
Thick evergreens and dense shrubs are common nocturnal winter roosts. One of our prolific bird feeders hung near a row of bushy white pines. Come sunset, blue jays, juncos, mourning doves and cardinals flew into the pines and settled for the night, somewhat protected from cold winds, snow and rain.
Purple finches will roost en masse in pines and hemlocks, but redpolls—which are abundant this winter—prefer the snow hollows under dense bushes. Turkeys and crows are often flock in treetops to form winter roosts. Crows will congregate in groups of several hundred and will return to the same roost each night from up to twenty miles away. The large pine groves in the Norwich-Hanover area shelter thousands of crows.
Birds also spend the night in hollow trees, in old nest cavities, under peeling bark, in vines that cover buildings and under eaves. Winter roosts often relate to how each species lives during the breeding season. Birds that nest in tree cavities often go there for winter shelter, including chickadees, titmice, nuthatches, screech owls and both downy and hairy woodpeckers.
Food is critical. Most songbirds have just enough fat reserves to survive one extremely cold night. Retaining body heat to save this scarce energy is a challenge. Depending upon the species and its environment, avian body temperatures range from 104-109°F. So coats of insulating feathers grow heavy as winter approaches. A goldfinch’s and redpoll’s winter cloak is 50 percent thicker than summer plumage. Puffball birds increase their insulation by 50 percent, simply by fluffing up feathers.
In extreme cold, birds tuck in their necks, legs and wings to avoid losing heat from extremities. Reducing exposed surface area by one fourth decreases heat loss by an equal amount. Another heat-saving strategy takes advantage of the fact that larger bodies have less skin exposed to the cold relative to their size, so they’re better at retaining heat. Some birds band together and pool heat by creating a communal body mass. Wintering white-breasted nuthatches will huddle in groups of more than two-dozen. A “3-dog night,” in avian terms, is a “24-nuthatch night.”
Instead of a hollow tree, ruffed grouse dig under snow or dive-bomb into a snow bank where they hollow out a tunnel. Within this warm snow shelter, grouse expend half as much energy as they would out in the open, so they can rest there for up to three days. Plus their heat-stingy plumage sports downy plumes that branch from the main feathers.
If these heat-conservation techniques prove inadequate and a bird’s body temperature drops, their cold-weather biology kicks in. By day, birds stay active to keep warm. But on very cold days, and after sunset, birds shiver to generate body heat. In his fascinating book, Life in the Cold, Peter Marchand says a shivering goldfinch creates more than five times the body heat it would normally generate, and can survive for up to 8 hours at –76°F. Chickadees are able to consume 1/5th less energy by actually dropping their nocturnal body temperature by 20°F.
An ingenious heat exchanger safeguards a bird’s body heat when naked feet start to chill. As warm arterial blood is pumped into the feet it comes into close contact with the cold blood returning in the veins. This contact warms up the venal blood before it returns to the body’s core, and cools the arterial blood as it enters the feet.
As well equipped as birds are for surviving winter nights, there are ways we can help: Offer high-energy suet or a peanut butter bell and scatter seed on the ground for juncos, mourning doves and other ground feeders. Place birdhouses under eaves on the south side of the house. Line nest boxes with dried grass for cozy winter roosts. Pile sheltering brush piles in the woods. Come spring, plant thick evergreens for winter cover and berry-bearing shrubs for cold-weather food. Leave dead trees standing for birds that roost in cavities. Cut a small access hole in a shed for winter birds to enter. And don’t forget to leave that barn door open by just a…feather.
Portions of this article adapted with permission from Through a Naturalist’s Eyes: Exploring the Nature of New England by Michael J. Caduto.