Most may think that birds migrate southward to escape the cold. Why is it then, that some birds migrate one year and not the next (for example - blue jays), or migrate such short distances you wonder why they even bother? Availability of food is the primary criterion determining where birds spend their winter. Flycatchers, swallows, nightjars, and other species that depend almost entirely on airborne insects for their diet are forced to migrate south when autumn weather begins to kill flying bugs. Wading birds are imperiled by frozen water. Raptors are dependent on warmth-induced air currents to keep them aloft while hunting.
The birds that remain in snow-covered areas in winter - and only twenty to thirty species ordinarily do so in North America - must be able to live off what food they can find. These birds such as sparrows, cardinals, pigeons, woodpeckers, and finches rely on fall berries and seeds, unharvested field crops, garbage, roadkill, hibernating insects and their larvae, seeds, and nuts. A few species make efforts to store food caches. A blue jay is a diligent collector of seeds and nuts during autumn, hiding them in tree crevices and under liter on the ground, but jays usually lose what they stockpile by eating an entire days hoarding in just a few minutes. The gray jay is more skilled and uses salivary excretions to form sticky lumps of food called boluses, which is stores on twigs.
Besides finding enough food, birds rely on a number of tricks to beat the challenge of winter's cold weather. One way they do that is by increasing body fat. Body fat is another important factor in winter survival. Larger birds are able to conserve energy longer and more efficiently. At night most birds seek shelter in dense trees and shrubs. Some species, such as wrens, huddle together in protected roosts to stay warm. Believe it or not the snow can actually help keep birds warm as well. Some will burrow into loose snow to take advantage of its natural insulation. In a sheltered roost beneath 8 to 10 inches of snow, where the temperature can remain as much as 40 degrees warmer than outside air, energy loss can be reduced by half.
Small birds such as black-capped chickadees have so little body mass that they cool off much faster than larger birds. So, how do they stay warm? They must take every possible precaution to maintain their 104-degree Fahrenheit body temperature. First, they need to reduce the rate of heat loss. Like most birds they employ a tactic known as pilorection, fluffing their feathers to trap air and enhance insulation value. They also constrict surface blood vessels to reduce their circulation of blood in extremities like feet and legs, and take shelter whenever possible. On the coldest days, they hide in dense vegetation or in the hollows formed in snow covered shrubs. At night, they are able to descend into a state of mild hypothermia, or torpor, lowering their body temperature gradually by as much as 20 degrees Fahrenheit, reducing the heat flow of outside air and saving about 20 percent of the energy they would otherwise burn. They also shiver. Shivering increases the rate of metabolism as much as five-fold, making it possible to generate high levels of heat for long periods of time.
So, while your stocked bird feeders may not be terribly important to the survival of wintering birds, you are doing your small part to help. Also, on the next cold day when you're shivering, know that the birds are shivering as well and we're all just trying to stay warm.