Nature – Why Birds’ Feet Don’t Freeze

A quick glance at a winter weather forecast is enough to make most of us grateful for our warm homes and thankful for those who work outdoors in these brutal conditions.

For the wildlife in our communities, there is no escaping the harsh realities of winter. In fact, many of these animals must spend time each day out in the snow and ice to find the food necessary for survival. Take the Canada goose, for instance. They are a familiar sight at our local lakes and ponds, whether they are frozen solid or not. How do they do it?

Like people and all mammals, birds are warm-blooded. Their body temperature remains constant—about 106 degrees. To maintain their body heat in freezing temperatures, their bodies have been designed with several specialized mechanisms.

Birds’ feet are little more than bone, sinew and scale, with very few nerves, and for birds such as geese, ducks, and other waterfowl that spend time on ice or in cold water, keeping their legs and feet warm is essential for survival. Birds rely on a vascular system called rete mirabile designed to keep their feet warm. In this system, the arteries and veins in the legs and feet are close together, allowing the blood to be warmed and cooled quickly.

Here is how it works. As warm, oxygenated blood leaves the heart in arteries moving toward the bird’s feet, it passes in close proximity to veins returning oxygenated blood back to the heart. This allows for a concurrent heat exchange. The exchange allows the warmth from the arterial blood to increase the temperature of the blood returning in the veins, which is colder after circulating to the bird’s extremities. The warmer arteries heat the cooler veins and the veins cool the arteries. And since the temperature of the bird’s feet is closer to the environmental temperature, they don’t lose as much heat as they would if they were at body temperature. Also, birds’ feet lack sweat glands, so they stay dry with no danger of freezing to a perch.

The heat exchange is just one way birds stay warm in winter. Many birds have a winter plumage with more feathers to better insulate their bodies. Some species, such as finches, add fat in winter to both better insulate themselves and provide an energy source.

Many small birds, though, cannot bulk up too much because it would affect how they fly. For these birds, such as chickadees, shivering helps them stay warm. Shivering in birds isn’t quite like it is for humans. When birds shiver, they are able to activate particular muscle groups that work in opposition to one another. This causes muscle contractions that allow the birds to better retain their body heat.

All of the interaction between the several parts of the different systems in a bird’s anatomy only reinforces the certainty that birds did not evolve, but were instead created by a loving God who cares deeply for all of His creation, great and small.

“But now ask the beasts, and they will teach you; and the birds of the air, and they will tell you; or speak to the earth, and it will teach you. Who among all these does not know that the hand of the Lord has done this, in whose hand is the life of every living thing, and the breath of all mankind?” Job 12:7–10

Sources: (including Audubon Society, Smithsonian, Popular Science); Michael Stein for Bird Note