While leading a field trip to Edwin B. Forsythe National Wildlife Refuge, one of our participants observing the waterfowl queried, “How do they keep their feet warm?” I’d read an excellent explanation in David Sibley’s book What It’s Like to Be a Bird, but was having trouble recalling the particulars. I offered, “It has something to do with warm blood vessels being adjacent to cold ones.”
Then attendee Dr. Coifman offered, “Countercurrent circulation transfers heat from outflow to cooler inflow blood.” Admittedly not a direct quote; he was more eloquent and knowledgeable.
You may recall that arteries carry oxygen-rich blood away from the heart and veins carry oxygen-depleted cooler blood back to the heart for reoxygenation.
I assume this question comes up rather often when people view a duck in its downy coat of water-repellent feathers perched upon exposed, bare legs. Sibley gives greater detail to what happens in the bird’s more muscle-bound thigh surrounding the tibia bone. Here the arteries and veins actually split into many multiple smaller vessels and become intertwined, allowing for greater heat transfer between the two, before flowing to the skin-covered tarsus. The tarsus is the portion of the leg that is not covered by feathers, but is just leathery skin. This leathery skin is also scaly which further conserves heat.
Birds are not the only species that use countercurrent circulation. In humans the arteries of our arms and legs run parallel to a set of deep veins. The warmer blood running in the arteries gives off some heat to the colder blood returning from our extremities in the veins, thus keeping us warmer by minimizing heat loss.
Strategies for keeping warm vary from one species of bird to another. Some birds migrate to warmer locales. Those that don’t employ bodily defenses and conscious strategies; we will discuss a few of them. This is not intended to imply that birds in warmer climates don’t regulate temperatures using strategies, but rather that those who remain in cold climates must employ these techniques to avoid freezing!
Some birds, when they molt or replace seasonal feathers, grow more down for the winter. These are the fluffy light feathers, which lie under the stiffer sturdier feathers that repel water and wind. The looser soft down traps air that is heated by the bird’s body. If you have ever worn a down vest or slept under a down comforter you have experienced the luxury of warmth that these feathers offer.
Feathers are an incredible aspect of a bird’s anatomy; they have an enormous amount of muscle control over them. You surely have seen birds near a feeder in winter “fluffed up” to stay warm. Many birds puff up or raise feathers to make space for trapping and heating air, essentially creating their own insulation.
For feathers to do their job properly they must be kept interlocked in an orderly fashion. Feathers are much more complex than you might notice at first blush. Each feather has a central shaft or rachis that is also arched. Off this main shaft are barbs, like mini shafts, and then there are barbules with fine hair-like hooks that act like velcro, holding each neighboring barbule in place. For all this to work a bird has to be well-groomed. This is why birds shake and continually comb their feathers with their beaks, a maintenance behavior called preening. Part of the process involves moving oils from a gland at the base of their tail and redirecting it to their feathers, which helps in weatherproofing.
Birds may place their beak within the feathers of their chest or back, or put their head under a wing to keep warmer.
A less noticeable aspect is shivering. Birds do not shiver like mammals; you don’t see a tremble like what you see in people or our dogs. Instead their muscle groups involuntarily contract and expand to create body heat. When you see a bird that appears to be shivering it is actually quaking—normally a sign of excitement, fear, grooming, pleasure, sickness, or overheating. Overnight many small winter birds can lose all their body fat through shivering, making adequate food resources essential for survival.
Possibly if you are a hiker or camper you have read that hypothermia can be prevented by getting in a sleeping bag with a partner; I’ll spare you the details. Well, birds will cuddle up to stay warm, much like the intertwining of arteries and veins. Bobwhite quail sleep in a wagon-trail formation, all facing outward to watch for intruders. The colder the weather the more closely they will huddle.
Male emperor penguins gather in Antarctica in a massive incubation event. For two months they carry an egg over the tops of their feet within a brood pouch, enduring temperatures as low as minus 80°F. During incubation they rely on accumulated body fat and a huge huddle with other incubating males. They shuffle frequently, taking two-inch to four-inch steps every minute or so. They stay packed tightly while rotating placement from outside to inside the flock to stay warm. This is one of the most selfless, often poignant child-rearing events on the planet. If you haven’t seen the acclaimed movie March of the Penguins (2005), watch it for a truly awe-inspiring portrayal of dedication. The winter wonderland setting and devotion to raising young makes it the perfect family movie.
Shelter is another common way to keep warm. Birds make use of cavities in trees excavated by woodpeckers. They can also gather and cuddle. University of Vermont biology professor Bernd Heinrich relays his escapades of climbing trees to look in cavities. Once he found a hairy and a downy woodpecker sharing the same tree hole. He also discovered four kinglets huddled together in a massive pine, heads first with tails protruding.
Birds, especially turkey vultures, often perch on the peaks of roofs to take advantage of heat escaping from poorly insulated houses. Black shingles absorb heat from the sun, providing additional warmth. Also, blacktop roads absorb warmth from the sun during daylight hours, and for this reason, on cold winter mornings a number of ground birds can be seen on road surfaces, notably mourning doves and woodcocks.
When it is cold but clear animals seek shelter from the wind, allowing the sun’s rays to warm them. Often on a hike I will seek a sunny spot surrounded by red cedars. An isolated niche is amazingly cozy compared to an open area just a few feet away.
Adding people to this discussion may help readers better relate to birds’ winter strategies. In grade school I learned that the Alaskan Inuit people eat whale blubber before winter to insulate their bodies from the cold. In the early 2000s scientists studied the Denisovan genome, the DNA of a group of archaic humans from Arctic climes where temperatures are freezing for at least half the year. This archaic DNA mixed with Neanderthal, and the genome persists in Arctic peoples today. Scientists found that the people with this genetic make-up have biological adaptations that help them tolerate cold by promoting heat-generating fat.
Fattening up is an important aspect of staying warm in winter. Fat is also an important fuel for migration. Birds can’t afford to burn precious muscle needed for flight; this is why birds build up fat resources prior to their long journeys.
In these winter months you may want to consider a migration of your own—what Australians call a walk-about. I hope to see you on a trail in the great-out-of-doors.
How Do Birds Survive Winter? By B. Heinrich, All About Birds Cornell.
How Do Birds Keep Warm In Winter? By Tina Shaw., USFW.
How Do Birds Cope In Winter, Audubon
Archaic Adaptive Introgression in TBX15/WARS2, Fernando Racimo, David Gokhman, .Et al., Molecular Biology and Evolution
Emperor Penguins Rotate Through Giant Huddle for Warmth, www.wired.com