What a blizzard we had recently. I’ve never seen so much snow in my life (which may tell you something about where I have lived). Walking in the frozen stuff can give me a cold nose, fingers, and toes, but I have a scarf, gloves, and wool socks and get to go back to my warm house. How can birds survive the cold?
It’s amazing what adaptations birds have to deal with the cold. I’m going to talk about feathers first. It is truly remarkable what you can do with a little bit of protein. Keratin is the same material (a protein) that makes up our hair and fingernails, but it is structured differently in birds to produce feathers. It also covers birds’ beaks and feet. Feathers all have the same basic features- a sturdy hollow shaft with the soft part, called the vane, on both sides. The vane has branches called barbs, which have branches called barbules. These barbules have hooks and edges called barbicels or hamuli. These structures zip together like Velcro, creating the neat vanes which you can pull apart by running your finger down a feather. You can then zip it right back up by smoothing the feather again. This basic pattern is varied for feathers of different functions.
The most common feathers you will see are contour feathers and down feathers. The body feathers and flight feathers (those on the wings and tail) are contour feathers. They have the full set of barbs, barbules and barbicels creating a smooth vane, which provides a protective shell for the bird from wind and water. Down feathers lack the barbicels that hook contour feathers together, creating a fluffy insulative feather. Birds must carefully maintain their feathers by bathing and preening to keep them working well.
You’ve probably noticed how fluffed-up birds get in the winter. This is them using their feathers for insulation. Different substances, such as water, fat and air, conduct heat differently. Of these, air is actually the best insulation because it transfers heat most slowly. Thus, a layer of air will prevent more heat loss than a layer of fat or a layer of water. By fluffing its feathers, a bird traps a layer of air that keeps its body warm. There are several hypotheses about why birds have feathers, such as increased gliding abilities for their early ancestors, but one is that feathers evolved for insulation. They are certainly used for this now, but it may or may not be the original function.
So a bird has a feather-covered body – what about its exposed feet? First, they can tuck them in. You will often see cold birds standing on one foot and holding the other close to its body. In addition, some birds that are often standing on cold ice or snow, such as some gulls, have a modified circulatory system in their feet, called countercurrent heat exchange. Instead of circulating warm blood to the foot and having cold blood sent back into the main body (chilling the bird), the outgoing artery and incoming vein pass close by one another so that incoming blood is warmed by outgoing blood (which then has less heat to lose to the environment), reducing the chilling.
What about the size of a bird? This is a strange problem of geometry. If you take two objects of the same shape but different size, for simplicity let’s say two cubes, the smaller one will have a larger ratio of surface area to volume. The small one still has a smaller absolute surface area, but its surface area is greater when you account for the decreased volume. That is, there is less surface area per unit volume. If you take two birds, the smaller one will have a larger surface area from which it loses heat. Thus, smaller birds will lose heat faster and need more energy to keep alive. This oddity of surface area has caused what we call Bergmann’s rule. Animals of the same species tend to be larger in cold climates. Because they deal with less cold weather, Downy Woodpeckers here in Texas will usually be smaller than Downy Woodpeckers in Canada.
The adaptations that our birds have for the cold are remarkable and wondrous to think about. Next time you are watching a fluffed up goldfinch huddling at your feeder, think about how it is able to survive. Then go outside and fill up the feeders. Life is everywhere and it’s amazing what wonders nature has hiding right in front of our eyes.
The next monthly field trips on the Lyndon B. Johnson National Grasslands will be Feb. 3. We will depart at 9 a.m. from the Forest Service District Office in Decatur. For more information, please contact Mary Curry (see below) or the Forest Service District Office, (940) 627-5475.
Claire and Mary Curry are nature enthusiasts based in Greenwood. If you would like to contact them, please e-mail them at firstname.lastname@example.org, or call them at (940) 466-3299.