A frequent question among readers in the aftermath of last winter’s deep freeze was, “How do birds survive overnight low temperatures that plunge below zero degrees Fahrenheit?”
The answer is a fascinating story of behavior and physiology. I’ll use chickadees as an example because they’re one of the few wild birds whose adaptations to cold have been well studied.
Behavior and physiology
Weighing in at just 10 to 12 grams — about a third of an ounce — black-capped and Carolina chickadees are familiar backyard birds. They cope with severe winter weather in three ways: physically, physiologically, and behaviorally.
They physically combat the worst of winter with a dense insulating coat of feathers. At the end of summer, chickadees molt — they grow a new, denser set of feathers. Therefore they enter fall and winter with the best insulation nature can provide.
Many animals, especially mammals, take the physiological route to protect themselves from winter cold. Bears and ground hogs, for example, add layers of body fat. Chickadees do too, but each night they burn all the fat they stored the day before; chickadees cannot rely on fat to insulate their body cores. They do, however, use it to fuel their metabolic furnaces. Each day chickadees double their early morning fat reserves. That fat then burns off during the night.
Chickadees make most efficient use of their daily fat stores by reducing their metabolic needs to match the amount of energy available. They cannot consume enough food each day to maintain their normal body temperature of 42°C (107.6° F). Instead, on cold nights, chickadees enter a controlled state of hypothermia — they lower the body’s internal thermostat.
Their body temperature drops 10 to 12 °C below the day time temperature of 42°C.Consequently, chickadees can make it through the night on far fewer calories than they’d need to maintain their normal body temperature.
Chickadees also counter intense cold with a variety of adaptive behaviors. For example, they escape wind, rain, sleet and snow by sleeping in dense vegetation such as conifers or in small cavities. When cavities are used, they are much smaller than nesting cavities. In fact, they are usually just big enough for the chickadee to get inside. The empty space in such small cavities is quickly warmed by heat loss from the bird, and the cavity becomes a cozy little bedroom. In such small cavities total heat loss is reduced 60 to 90 percent.
Though not as effective as cavities, dense evergreen vegetation also offers protection from the elements. Golden-crowned kinglets, which at six grams are even smaller than chickadees, huddle beneath snow covered evergreen branches to make it through long winter nights in northern Maine.
Chickadees enhance the protective value of a roost cavity further simply by adjusting their posture. They fluff their feathers to cover their naked legs and feet, and they tuck their beak and part of their face into the shoulder feathers on the wing. This posture reduces the heat loss from naked body parts.
Reduce energy use
Another simple way that chickadees change their behavior to reduce their energy needs is to slow down. On cold windy days, they fly less frequently, stay closer to the ground, and spend more time at any given food source. Watch your own feeders. Compare how long chickadees stay at feeders on cold, windy days to their visits on calm, mild days.
The greatest dangers chickadees face in winter are periodic blizzards and ice storms that make natural foods almost impossible to find. That’s where their food storing habit comes in handy. Throughout the fall and into early winter, chickadees scatter small caches of food throughout their winter territory. Those that roost in cavities store food there, too. Thus during winter storms chickadees have a safety net of stored food that can see them through a stressful day or two until natural foods (or feeders) become available.
Despite their small size and fragile appearance, chickadees (and other winter birds, too) cope remarkably well with severe winter weather. Thanks to dense plumage, nightly hypothermia and a variety of adaptive behaviors, they thrive when most other creatures either hibernate or migrate.
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