I’m currently visiting Chicago, relishing the finger-stiffening, face-numbing cold and wind that make up a proper midwest winter. Whenever I look out from the warmth of my big puffy coat and see a bird, I feel a little bad for enjoying the weather so much. I can go home and make myself hot tea; they can’t.
Very cold Tree Swallows (up in the Yukon, not Chicago!). Photo by Keith Williams
Like mammals, birds are endothermic (“warm-blooded”), meaning that they maintain their body temperature independent of the outside environment. This almost always means keeping themselves warmer than the outside air. Birds have quite high natural body temperatures, even higher than ours, so any given outside temperature seems even colder to them than it does to us.
Birds are also smaller than we are (well, omitting the ostrich), which means that they have a higher surface-area-to-volume ratio than we do. This is a problem because the volume (inside) of an animal is where heat is produced and stored, while the surface (skin) of the animal is where heat is lost to the environment. Imagine holding your hand in a bitter wind: how would you keep it warm? By making a fist. Making a fist reduces the surface-area-to-volume ratio of your hand, and lets it keep warm longer. In contrast, if you hold your hand out flat with all the fingers spread, your surface-area-to-volume ratio is larger, and your hand will get cold very quickly. Because birds have higher surface-area-to-volume ratios than we do, keeping warm is harder for them. How do they do it?
Feathers: There is a reason why we fill our best coats with goose down. Feathers are fantastic insulation. Downy feathers trap tiny pockets of air next to the bird, allowing the bird to warm those pockets of air and hold that warm air around itself, preventing cold air from touching its skin. The more air trapped, the warmer the bird. Birds fluff up (the technical term for fluffing up is “ptiloerection”) in the cold to trap as much air in their feathers as possible.
Goldfinch fluffed up against the cold. Photo by Jen Goellnitz
The legs and feet of almost all birds are thin and lack feathers, and so are vulnerable to rapid heat loss. Some birds handle this the obvious way:
Dark-eyed Junco at zero degrees Fahrenheit, standing on one leg to tuck the other into her down. Photo by Pete Zarria
Why doesn’t the junco in the photo just sit down on both of her feet to keep them warm? Possibly because she wants to be able to escape predators. Juncos that are fluffed up and sitting on their feet in order to keep warm take more time to fly when startled, compared to sleek standing juncos, making them vulnerable to predators (Carr & Lima 2011). Carr & Lima didn’t study fluffy standing-on-one-foot juncos, but it’s possible that standing on one foot is a more flight-ready position than sitting.
Counter-current exchange: What if you can’t sit on your feet? What if, instead, you have to swish them around in frigid water?
Male Mallard in Chicago in December. Cold feet!
Ducks and gulls can have such cold feet that they do not leave melted footprints in snow. The tissues in their feet are adapted to very cold conditions, and can still function close to freezing (Steen & Steen 1965). Yet if their feet are near freezing, and the blood that circulates through their feet then enters their bodies… those warm insulated bodies are going to cool off very quickly.
They deal with this by using counter-current heat exchange. Veins and arteries in the leg are close to each other, and as warm blood leaves the body, it heats up the cold blood returning to the body.
Crude diagram of counter-current exchange in a duck foot. Red blood is warm, blue is cold; arrows indicate direction of blood flow.
Heat loss is minimized and the duck doesn’t freeze, even if its feet do.
(And if you’ve ever wondered, as I have, whether ducks are warmer on land or in the water: it appears that they lose 22% more heat while swimming than while standing on land in wind. So swimming is colder, but not by as much as you might expect (Van Sant & Bakken 2006).)
Mallards on land in Chicago in December: 22% less cold than the other Mallard.
Huddling: Many birds, especially small birds, huddle together to conserve warmth (Gilbert et al. 2010). Huddling reduces the birds’ surface-area-to-volume ratio, since it turns many small birds into a single big group, and larger objects have higher surface-area-to-volume ratios than smaller objects.
Long-tailed Tits form single-file roosting “huddles” (lines, really) to keep warm at night. They are tiny, weighing 7-9 grams, and on average lose 9% of their body mass overnight. Energy conservation is crucial for them. The birds on the ends of the huddle lose more mass overnight than those in the middle, so the birds jockey for position, all trying not to be the cold one on the end (Hatchwell et al. 2009).
Long-tailed Tit. Photo by Rich Mooney
Long-tailed Tit. Photo by Sergey Yeliseev
Okay, last Long-tailed Tit photo. I just love these little guys. Photo by Sergey Yeliseev
I once saw a photo of Long-tailed Tits huddling, but I can’t find it now, so here is my (very accurate, really!) recreation of the photo in Powerpoint.
Long-tailed Tits huddling at night for warmth
Carr JM, Lima SL. 2011. Heat-conserving postures hinder escape: a thermoregulation-predation trade-off in wintering birds. Behavioral Ecology 23:434-441.
Gilbert C, McCafferty D, Le Maho Y, Martrette J, Giroud S, Blanc S, Ancel A. 2010. One for all and all for one: the energetic benefits of huddling in endotherms. Biological Reviews 85:545-569.
Hatchwell BJ, Sharp SP, Simeoni M, McGowan A. 2009. Factors influencing overnight loss of body mass in the communal roosts of a social bird. Functional Ecology 23:367-372.
Steen I, Steen JB. 1965. The importance of legs in the thermoregulation of birds. Acta Physiologica Scandinavica 63:285-291.
Van Sant MJ, Bakken GS. 2006. Thermoregulation on the air-water interface – II: Foot conductance, activity metabolism and a two-dimensional heat transfer model. Journal of Thermal Biology 31:491-500.
How do birds keep warm?