“Baby, It’s Cold Outside …”
Although Frank Loesser’s well-known Christmas song was not specifically about Christmas, its lyrics certainly captured the winter season with references to ice, blizzards, and freezing! Like humans, all birds are warm-blooded, so how do Saskatoon’s avian winter residents tolerate and survive our winters outside 24h per day? It turns out that they have multiple strategies to survive in the cold.
Feathers are their first line of defence. The outer layer of stiff and durable flight feathers provides an initial layer of insulation from the cold and moisture. Because air is an extremely poor conductor of heat (this is why you have to hold your hands close to a campfire to keep warm), feathers prevent heat from escaping by creating still pockets of air. This is why birds will often fluff their feathers in the cold, increasing the thickness of their “coat” and trapping as much air as possible under their feathers. Hidden under this layer of flight feathers, however, birds have another layer fine, fluffy feathers known as down. Down feathers form soft plumes that create even more pockets (e.g. millions of tiny pockets) of still air. Indeed, down is such an excellent insulator that it still outstrips the best synthetic materials made to date and is why it is used frequently in high-quality outdoor gear. We also know that many birds produce a winter plumage, which typically has more feathers to better insulate their bodies.
Of course, down loses its insulating power when it gets wet. Water collapses those fluffy down plumes and correspondingly, collapses all of those insulating pockets of air. This is why birds also preen themselves. Most birds have a special gland that secretes oil (preen oil). By rubbing its beak or head against the gland, a bird can transfer the oil to its body, rubbing the oil throughout its feathers and on its legs and feet. The oil maintains the integrity of the feathers and effectively waterproofs them, leading to the familiar phrase “like water off a duck’s back”.
Birds can also maintain their body temperature at a somewhat lower level during periods of inactivity, thus requiring less energy. For example, red-tailed hawks can drop their nocturnal body temperature by 3-4 ˚C below their daytime temperature; black-capped chickadees can reduce theirs by as much as 12 ˚C. This “regulated hypothermia” mechanism achieves significant energy savings. Hummingbirds, swifts, and goatsuckers (like poorwills) enter a deeper state of inactivity called torpor (kind of a “mini-hibernation”) in which their body temperature might drop as much as 28 ˚C for several hours at night or for days. While the energy savings are very large, there is also increased risk for these birds because of slower reaction times (in response to threats) and the extra energy required to return back to a normal state.
Birds that spend a lot of time on ice often stand on one leg and tuck the other among breast feathers, or sit down completely, covering both legs to minimize heat loss. These winter birds also have special adaptations to keep their legs and feet warm. They have a unique vascular system in which the arteries and veins in their legs and feet are adjacent to each other, allowing circulating blood to be warmed and cooled quickly. As warm oxygenated blood leaves the heart, it moves through the arteries towards the bird’s legs and feet. In doing so, it passes the veins, which are returning cooler oxygen-depleted blood from the bird’s extremities back to the heart. However, because the arteries and veins are in contact, the warm blood in the arteries is used to reheat the cooler blood in the veins, effectively creating an efficient mechanism of continuous heat exchange. Interestingly, this mechanism isn’t just used by birds and has been found in squirrels and other mammals that spend a lot of time in the snow.
Finally, behaviour also helps. Birds will seek out shelter in dense foliage or in cavities to avoid the elements. On sunny days, many birds will turn their backs to the sun, exposing their largest body surface to warm via solar heating. Some birds tuck their bills into their shoulder feathers to breathe warmed air. Other birds roost in groups, bunching together to share warmth. Some species, like finches and chickadees, add fat in the winter to better insulate themselves and provide an energy source. Many small birds, however, can’t afford to put on too much weight, because that would affect their ability to fly. For these birds, shivering provides another way to stay warm. Although we might shiver when stepping out of a swimming pool into cool air, these birds have a more efficient form of shivering, in which they can activate opposing muscle groups that cause muscle contractions, allowing them to better maintain their body heat.
So the next time you find yourself in your warm and cozy home staring outside at a cold winter wonderland, you can be content in knowing that our feathered friends are probably doing just fine. Happy holidays to everyone!
Saskatoon Nature Society
Connecting People and Nature
Saskatoon Nature Society
Box 27013 Grosvenor Park
Saskatoon, SK S7H 5N9