Dealing With the Cold

Dark-eyed Junco, Junco hyemalis

People in cold environments take advantage of a variety of clothing to keep them warm – mukluks, parkas, sweaters, and thermal underwear. Birds can’t add layers of clothing but it seems they are more physically, physiologically, and behaviorally suited to avoid hypothermia better than hyperthermia, and not just because some of them migrate to the tropics for the winter. I’ve watched geese sitting on frozen ponds, shorebirds pecking away on mud flats while a biting wind blew through, and gulls flying in heavy rain over winter ocean swells. Makes me cold just watching them.

George Murray Levick was a surgeon and zoologist on Robert Scott’s disastrous expedition to the Antarctic in 1910-13. Scott did not survive but Levick did and so did  his notebooks, which described the sexual behavior of Adelie Penguins. He was so taken aback by the “astonishing depravity” of the birds’ behavior – homosexuality, coercive sex, and sex with dead females, that he wrote his observations in Greek so that only learned gentlemen could understand it. Depraved or not, penguins are fascinating. You may have seen March of the Penguins or a similar nature film depicting fuzzy young penguins being whipped by the blistering winds of the Antarctic and wonder “how do they do it?” For the first 15 or so days after hatching, the young of Gentoo and Chinstrap Penguins have yet to develop a down layer sufficient to insulate them so they are brooded by their parents. By the time they are 25 days old their downy coat is quite adequate for the task. In fact, when the winds bluster over the colony, the birds are very comfortable because the insulating efficiency of the fledglings’ coat is not only sufficient but 136 to 178 percent higher than in calm winds! The feathers are stiff and short and not arranged in tracts like other birds but cover the entire body and overlap like roof shingles. The wind, instead of ruffling the feathers, compresses them, increasing insulation for the young birds. The contour feathers of the adults are compressed when the birds are swimming, increasing both insulation and aerodynamics. Also, the deeper feather layers are composed of ever smaller feathers that produce insulating pockets of air.

As winter approaches, many birds increase their body fat for energy storage and insulation, and thicken their plumage with down feathers. The Common Eider, a northern sea duck, increases the insulation of its feathers by 25 percent for the winter. When molting in the breeding season the eider uses its down feathers for lining its nest. Vikings made down comforters out of eiderdown for their long voyages, but today eiderdown is rare. The only down harvested from wild birds is from Iceland, which exports a small truckload, 5500 pounds (2500 kg), of eiderdown each year. This truckload essentially comprises the entire market for down, which is why an eiderdown comforter will cost you nearly $4000 (€3300). The Rock Ptarmigan puts on extra fat, comprising 32 percent of its winter body mass and almost doubles its weight; that fat plus down make its body 8- 30 percent more insulation-efficient. For the same reason, American Robins might increase the number of their feathers by 50 percent as winter approaches. But not all birds. A study of Bullfinches revealed that they did not add any extra feathers for cold weather; instead the birds increase the amount of fat ingested, not for insulation but for food reserves to allow them to survive the night.

I was once asked about pregnant birds. After I thought a bit, I realized the inquirer was looking at a bird that had fluffed its feathers, giving it an apparently larger body.  Ptiloerection, also occasionally used for cooling by exposing the skin, is perhaps the most obvious sign to us that a bird is cold and trying to reduce heat loss by fluffing its feathers just enough to trap air. How many holiday greeting cards have you seen with a fluffed up bird pictured against a snowy background reminding you of the holiday season? Generally, a moderate amount of fluffing traps sufficient air to form layers and increase insulation value by 30-50 percent, but laboratory studies of American Kestrels demonstrated that fluffing during a rainstorm allows water to penetrate to the skin and lower the body temperature of the birds, so the birds slicked down their plumage instead.


Birds need to eat enough food during the day to put on enough fat reserves to get them through the night. The colder the temperature, the more foraging occurs and the more fat is stored, but not up to the maximum level a bird can hold because there appears to be a tradeoff between putting on fat reserves, expending the energy necessary to forage for and being more exposed to predators while feeding. Evidence also indicates that heavier birds are more at risk from aerial predators. Every day is different because of variation in the weather, competitors, predators, and the food supply.

European Robins sing more when they greet the dawn with a higher body mass than a lower one. After a cold night, which required them to use their fat stores, the birds were less apt to sing at sunrise. The amount of time nightingales spend singing during the night is directly related to their body mass and the ambient temperature in the evening; the heavier the birds were, the more they sang and the colder the night the less they sang. Male Black-capped Chickadees, experimentally fed supplementary food, sang more at dawn than other male chickadees.