Naturally Curious: How Birds Cope in Winter
Blue Jays put on their own down jackets when the temperature drops significantly. Mary Holland photograph
The Pileated Woodpecker protects its relatively lightly-feathered head from the cold by tucking it down into its back feathers. (Mary Holland photo)
White-breasted Nuthatches are among the species of birds known to cuddle together to keep warm on cold winter nights.(Mary Holland photo)
Common Redpolls sustain themselves through cold winter nights by digesting seeds they have stored in a special pouch off their esophagus.
(Mary Holland photo)
There are very good reasons why many birds choose to leave for a warmer climate in the fall, despite the risks of migration. The food supply is greatly reduced — insects and other invertebrates which many migrants such as swallows, warblers and flycatchers depend on practically disappear. The daylight hours, when many species of birds forage for food, are reduced. Shortened days mean longer nights (14-15 hours) — hours and hours during which birds aren’t feeding. Finally, the temperature is much colder, forcing birds to use more energy to keep warm. Even with these obstacles to overcome, some species do remain in the Northeast year round and have adapted in a variety of ways to these winter challenges.
The structure of feathers is one of the biggest factors that allow birds to stay warm and thereby reduce their need for food in the winter. A typical wing or tail feather consists of a central shaft, with rows of interlocking barbs down either side of the shaft. There are different kinds of feathers, however, designed for different functions. The feathers that cover a bird’s body (contour feathers) have interlocking barbs, but only at their tips, where they aren’t covered by an overlapping feather. The rest of the barbs on the feather are not connected — they are loose and fluffy, similar to down feathers, and create many air pockets between the bird’s skin and its feathers. This layer of air provides excellent insulation, preventing much of the bird’s 110-112 degree F. body heat from escaping into the air.
Bird feet and legs are usually featherless and thus, exposed to the cold, but they consist largely of tough tendons, not fleshy muscles that would be susceptible to freezing. Even so, birds take measures to reduce heat loss in their extremities, primarily by reducing the flow of warm blood through the arteries into their legs and feet. Heat loss is reduced by up to 90 percent in a bird’s extremities by reducing the diameter of the arteries in their legs and feet and through an exchange system. The arteries and veins in a bird’s legs and feet are adjacent to each other. The heat of the warm, oxygenated blood in the arteries is transferred to the cooler blood in the adjacent vein before the blood re-enters the body.
When temperatures drop significantly, resting birds will often shiver for several minutes at a time, as shivering converts muscular energy into heat. They also will tuck their exposed heads into their feathers, and sit on their feet.
If the temperature is so cold that birds cannot maintain their body temperature at its normal high level through shivering, puffing out feathers and other behavioral tactics, some may go into a state of torpor for several hours, allowing their body temperature to drop 30 or 40 degrees lower than normal. A few birds, hummingbirds and nighthawks among them, are known to do this. Their metabolism drops dramatically — their heart beat slows down, their breathing rate slows and their body temperature drops. This change in metabolism conserves energy, often enough to allow the bird to survive a frigid night when otherwise it would expire. When a bird is in a state of torpor, it is unresponsive to its surroundings, but when the temperature rises, the bird is capable of raising its body temperature back to normal, and resumes normal activities (unlike humans who experience severe hypothermia). There is one species of bird, the Common Poorwill (an insect eater) which researchers feel is a true hibernator. The same bird was found in a torpid state for weeks at a time, in the same location in California, over a period of four winters. This little-known phenomenon was discovered long ago by Native Hopi Indians who referred to this bird as “holchko,” which means “the sleeping one.”
Night is the biggest challenge of all, as small birds do not feed at night and have to go many hours without food. Many save body heat by roosting in locations where they can get out of the wind. Grosbeaks, cardinals and crossbills are known to head for the thick branches of conifers, which act as a wind break. Some birds, mostly cavity-nesting birds such as chickadees, nuthatches and woodpeckers, roost at night in tree cavities for this same reason. On especially cold nights, grouse will fly headfirst into a soft snowbank, tunneling their way into short, well-insulated burrows.
Another strategy employed by certain birds is huddling. As many as 29 White-breasted Nuthatches have been found clustering together in a large tree cavity so as to minimize heat loss.
One last example of the ingenuity of avian winter adaptations is overnight food storage. Evening grosbeaks and some other northern finches are able to store relatively large amounts of seeds in their well-developed crops, and this supply helps them get through the night. Common and Hoary Redpolls can survive colder temperatures than any other songbird due to a special storage pouch in their esophagus which they fill with seeds (often birch) just before dark and throughout the night they digest these high-calorie seeds.
Fluffy feathers next to the skin, sinewy legs and feet, shivering, torpidity, cavity-roosting, cuddling and specialized anatomy that accommodates extra food storage — an impressive array of both physical and behavioral adaptations that allow some birds to withstand the rigors of a New England winter. Chances are great they’ve all come in handy this winter.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England,” “Milkweed Visitors,” and “Ferdinand Fox’s First Summer.” She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com .