In the aftermath of last week’s winter storm, many readers wondered how birds survive snowstorms and frigid temperatures. Birds that winter in mid and northern latitudes are naturally adapted physically, physiologically, and behaviorally to cope with winter’s worst.
Small bodies lose heat more rapidly than larger ones, so let’s begin with chickadees as an example. They weigh about a third of an ounce.
Feathers are birds’ first line of defense. They fluff their feathers and trap air to form one of nature’s finest insulators. Soft down and contour feathers closest to the body are extremely effective at trapping air.
It’s not an exaggeration to say that birds’ feathers function as down jackets. Birds also have a high metabolic rate to maintain their body temperature. Unlike small mammals, birds cannot protect themselves with layers of body fat.
Burning energy
Each day birds burn virtually all the energy they consume. They eat enough each day to get them through frigid nights. That’s why feeders are so busy on cold, snowy days. Furthermore, on very cold nights, chickadees become hypothermic by lowering their body temperature. Their body temperature can drop as much as 10 degrees Celsius on frigid nights. That means they need fewer calories to make it through the night.
Chickadees’ most observable adaptations to survive winter are behavioral. When the wind and snow blows, they seek shelter in dense vegetation such as conifers and thickets. At night, they (and other cavity-nesters such as woodpeckers, bluebirds, and wrens) roost in natural cavities and nest boxes.
Bluebirds
Though chickadees tend to roost solitarily, bluebirds roost in cavities at night in groups of three to ten. At 6 grams, golden-crowned kinglets are even smaller than chickadees, yet they make it through frigid winter nights in Maine by huddling in pairs or threes beneath snow-covered conifer branches. The microclimate created by the miniature snow caves provides just enough protection to get them through the night.
Sleeping birds also enhance the protective value of roost sites further simply by adjusting their posture. They fluff their feathers to cover their naked legs and feet, and they tuck their beak and face into the shoulder feathers on the wing. This posture reduces the heat loss from unfeathered body parts.
Slowing down
Another way birds reduce heat loss during daylight hours while feeding is to slow down. They fly less, spend more time at feeders, and stay closer to ground level where the wind is usually reduced.
Forest birds
Ruffed grouse, which are chicken-like forest birds, use a novel behavior to stay warm during snowstorms. Normally they roost in trees at night. But after a big snow, they dive into snowdrifts at dusk. These snow caves act as an insulating blanket until morning. And even if the snow crusts over, grouse are strong enough to break through anything short of solid ice.
For many birds, however, ice can often be deadly. Blizzards can make natural foods almost impossible to find. Combine heavy snow with a follow-up ice storm, and many birds die. Ice encrusted snow makes food almost impossible for most birds to access. So any ice storm followed by more than two days of subfreezing temperatures can kill many birds.
Rarely, however, do we see evidence of these deaths because predators and scavengers make short work of winter kill. Some birds avoid the possibility of starving to death during winter storms by storing food. Throughout the fall and into early winter, chickadees, nuthatches and blue jays scatter small caches of food throughout their winter territories. Those that roost in cavities store food there. Blue jays scattered their caches on the ground under leaf litter.
Safety net
Thus during winter storms, some birds have a safety net of stored food that can see them through a stressful day or two until natural foods (or feeders) become available.
Though birds do not depend on feeders to make it through the winter, during periods of heavy snow and ice supplemental foods become invaluable. Those are the times when special efforts should be made to keep feeders free of snow and ice.
