Winter is here, and whether you live in the north or the south, you are apt to see a duck if you are anywhere near a body of water. How is that? Many ducks migrate south for the winter, but not all do, some remain in the frigid arctic waters, and this leaves me contemplating two issues that ducks face in the wintertime that affects their decision to migrate to a warmer climate or not.
The first question to ponder is, don’t ducks fly south to escape the cold weather? As a matter of fact, not exactly. Ducks are actually equipped with an absolutely amazing adaptation that helps them to survive freezing air temperatures, while also submerging their feet in freezing water as well. Feathers keep their bodies warm, and by fluffing them up and keeping them waterproof with the greasy substance that they produce in a gland under their tail and rub on their feathers while preening, they keep their bodies dry and encased in what is almost a bubble of warm air. Down feathers, which are mostly responsible for keeping the bird warm, are short, plumulaceous feathers that provide a layer of insulation underneath the outer contour (wing and body) feathers of the bird. Some bird species possess more down than others, for example ducks have more than songbirds, thus songbirds do migrate partly due to weather temperatures.
Underneath their warm plumage, one thing that jumps out at me when I see a duck on the winter ice is their naked feet. Shouldn’t naked feet pose a problem for ducks in the winter? Ducks and some other kinds of birds that inhabit winter climates, like penguins, have evolved with a particularly amazing adaptation that allows their feet to remain featherless, yet not freeze nor become damaged by frostbite.
If you were to walk outside on a snowy day, and then submerge your hand into a nearly frozen puddle on the ground, your body would lose heat into the puddle out of your hand. The same should happen to a duck when its feet are submerged in the freezing waters of northern or arctic latitudes, but it doesn’t. Ducks have a specialized system of heat exchange in their legs that cools the warm blood drastically before entering their feet, thus preventing loss of body heat in the cold water. This system is called counter current exchange, and effectually, what happens is that the blood headed towards the feet passes it’s warmth to the blood heading back to the body from the feet and never makes it out to the water to be lost into the environment. The foot of a duck is kept at about five degrees Ferenheight, which is high enough to prevent damage to the cells, but low enough to not cause extreme heat loss for the duck. If ducks were not equipped with this amazing adaptation, upon submerging a foot into the icy waters, the blood that was circulating in the foot would be cooled, and when circulated back to the body, would cause the core body temperature to drop, causing the duck to become cold, hypothermic, and eventually die as the cycle continued.
In addition to the counter current exchange, low surface area of the thin foot helps prevent heat loss, as well as the design of muscle placement in the leg. Most of the muscles that control a duck’s foot are in the upper leg, close to the body, where they can be kept warmer with less risk. The duck’s plumage also helps keep the feet warm, as they will tuck one under a wing for a period of time, or sit on them to warm them up.
This is not to say that a duck cannot get frostbite. I found a few sources that sited cases of duck frostbite, particularly if a duck is kept inside where it is warm and then turned out. Apparently the heating and cooling mechanism doesn’t work like an on-off switch, good to remember if you have domestic ducks.
So, if ducks are physiologically adapted to being able to survive freezing winter temperatures, our second question arises: why don’t they stay up north all winter? This answer is in the their stomachs: the availability of food drastically reduces in the north in the winter months, and so the time and energy spent on procuring a meal is too high compared to the amount of calories gained, and most birds would not find enough to survive if they had to compete with the same density of individuals as that is present in the summer months. Simply, birds fly south to stay fed.
Why bother expend all that energy to fly north again in the spring? The answer again is in the food sources. As numbers of individuals of a given species have increased in overwintering areas, the population is also competing with other species for food sources, and by flying north, the population becomes more dispersed and competition decreases. As well, southern food species tend to have more natural defenses (think cacti and rattle snakes) than northern counterparts, and meals may be less mortally dangerous to come by in the north. Lastly and most importantly, northern cycles of prey species like insects experience a major boom in the spring and summer, and meals are easier to procure, particularly important for a parent who needs to feed a nest full of chicks.
Interestingly, there are a few kinds of ducks that stay in the arctic region during the winter because of the food that is there. King eiders and Goldeneyes remain in the Arctic Sea amongst the sea ice because of the rich source of mollusks on the sea bed.
So, naked duck feet are amazing, don’t you think? I do. And though their naked feet allow ducks to exploit winter habitats in ways that other birds cannot, survival strategies are eventually limited by the need to feed. Luckily for those of us in the north, some ducks stick around during the winter. I think I might go birding at the lake soon…
Some other places to find information:
PBS.org: Nature episode: An Original DUCKumentary. This link will allow you to watch the full episode.
Today I Found Out: Why Don’t Birds’ Legs Freeze?
Ask a Naturalist.com: Why don’t ducks’ feet freeze?
Quarks, Quirks and Quips: Why don’t ducks get frostbite?
Cornell Lab of Ornithology: King Eider. This site is full of information, spend some time navigating to find out even more!
Photos via photopin.com. Click on photo to be redirected to photographer’s Flickr page for more info, and have access to the Creative Commons license.