Why don’t ducks’ feet freeze?
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Here in Florida, I sometimes forget about the icy conditions northern birds (and people) must endure and I’ve wondered, why don’t ducks’ feet freeze? But when I think about my Niagara Falls upbringing, one of the most remarkable adaptations I can recall is the canvasbacks, mergansers, goldeneyes, and other diving ducks living in and on the ice-cold Niagara River. The nearly 20 species of gulls that migrate from Greenland and Siberia to the Niagara region in winter is also astonishing. Imagine how hard those conditions are for them to favor the average January high temperature of 32.2 degrees F in Niagara Falls. In addition to these birds, our domestic geese and ducks are well equipped to deal with the freezing temperatures.
Waterfowl, including penguins and flamingoes, have countercurrent heat exchange systems in their legs. This enables them to keep those feet submerged into icy cold water or to stand on ice for hours without the consequences of frostbite. In addition to cold water, flamingoes adapted to stand in or drink near-boiling water.
So, why don’t ducks’ feet freeze? Like us, all birds are homeotherms, also known as warm-blooded. Their body temperature keeps the same regardless of the weather. When the birds stand in icy cold conditions, the warm blood from the body goes down into the animal’s legs. This travels next to veins that bring the cold blood from the feet back up to the warm body. Since the arteries and veins are close to one another, the hot blood cools, and the cold blood warms. Since the cold blood heats up, it doesn’t bring down the body core temperature as severely as it would in a chicken or us, for example. The warm blood is cooler when it reaches the feet extremities compared to the body temperature.
“There is a lot about the countercurrent exchange system we don’t know, particularly when it comes to interspecific differences,” Dr. Julia Ryeland says. Dr. Ryeland is a professor at Western Sydney University in the Centre for Integrative Ecology. “There is good evidence, however, that morphology plays a big part in the ability of different species to withstand extreme heat and extreme cold. Our work is based on Allen’s Rule, an extension of Bergman’s theory. Together these suggest that animals evolve to cope with extreme colds by being large in size with smaller appendages (and vice versa for extreme heat), which has been tested and confirmed for a number of taxa.”
“There are obviously a number of different factors that might also influence this, including other mechanisms for coping with extremes in temperature — for example, migration,” Dr. Ryeland says. “We showed that birds can lessen the impact of heat loss or gain by making postural adjustments, but this is likely only effective to a certain degree, and as such, you get evolutionary pressure for differing morphologies under different climates.”
Since heat exchange happens when there is a difference between objects, the bigger the temperature difference, the quicker the exchange happens. If there isn’t a big difference, the heat exchange is slow.
Vasoconstriction is when the blood vessels are restricted. This allows oxygenated blood to still go to the wings and feet without losing a lot of heat. In animals where frostbite occurs, this restriction is so extreme that it causes the fluid in the tissue to freeze into ice crystals. This allows the blood flow to be redirected from the extremities and focus on the vital organs.
In addition to countercurrent heat exchange, birds have several other adaptations to help them get through the cold. Their preen gland helps waterproof their feathers. Standing on one foot reduces the heat exchange from their warm bodies to the cold environment, so it is more energy-efficient. The scaly skin also limits heat loss. While some birds tuck their foot into the warm plumage, others crouch down to cover both feet. Some birds eat more in the fall to build up fat layers. The birds will also rouse their feathers, which act as insulation, or they may huddle together. Because of these adaptations, only 5% of heat loss occurs through their feet and the rest through their feathered bodies! Now you, too, know the answer to why don’t ducks’ feet freeze?
Originally published in the December 2021/January 2022 issue of Backyard Poultry and regularly vetted for accuracy.