Mallards rely on ‘wonderful net’ to keep their feet from freezing

by Emily Stone
Naturalist/Educator Cable Natural History Museum

The bitterly cold wind numbed my cheeks, stung my eyes, and blasted my half inch of exposed forehead with an ice cream headache. Happily, the sun was shining and the rest of my body— entombed in layers of wool and down—remained a comfortable temperature.

As they say, there is no bad weather, only bad gear. I felt adventurous to be out walking on this sub-zero day, even though the temperature had risen a full 14 degrees from -17 to -3 degrees Fahrenheit.

With the wind at my back, an ethereal sunset glowing on the horizon, and a warm house waiting less than two miles ahead, I decided to take Duluth’s Lakewalk along the shore of Lake Superior. Ice and snow mingled with rocks on the beach, and although no ice floated in the lake, the water looked frigid just the same. NOAA data shows that the temperature in this corner of the lake is about 40.5 degrees, which is just a tad warm for late December.

As I rounded the corner by Endion Station, I was surprised to see life. Dozens, maybe hundreds, of mallard ducks huddled in rafts along the leeward shore, the iridescent green heads of the drakes shimmering handsomely in the last rays of sun. They looked especially dapper against the pure white patterns of ice-draped riprap. The presence of this concentration of life somehow made the afternoon feel warmer.

Don’t ducks migrate south for the winter, though? Well, as with so much in nature, it depends. Mallards are medium-distance migrants, who only go as far south as needed. Some do fly all the way down to Mexico and the Caribbean, but many seem to prefer a “staycation.” About 1,500 mallards even make Anchorage, Alaska, their year-round home! As long as the ducks can find sufficient food, they can withstand some pretty brutal winter weather.

In species of songbirds who are medium-distance migrants, such as red-winged blackbirds, and juncos, it tends to be mostly males who overwinter farther north. For them, being the first back on their summer breeding territory is of utmost importance.

That’s not the case for mallards. Although this flock seemed to have a few more males, that’s likely due to a male-heavy skew in the overall population. Male and female ducks need to overwinter in mixed flocks, because that’s when they choose a mate and form pair bonds. In spring they arrive on their breeding territory together. Once copulation is complete, though, the males disperse. The female incubates the eggs and protects her clutch of fuzzy nuggets all by herself. (Insert your favorite joke about lazy males here.)

Continuing up the ramp to where the path parallels the railroad tracks, I looked down into a small pool—hidden in the riprap—that seemed to be at the outlet of a culvert. Here, out of the wind and in water that was potentially slightly warmer, ducks carpeted every surface. Pulling my scarf up over my windward cheek, I stopped to watch.

Some ducks floated with their beaks and heads tucked backward under their wings. My rosy nose was envious. Other ducks paddled idly in circles, perhaps a little off-kilter because one foot was being warmed up in their feathers. Occasionally one or two ducks would start dipping their heads quickly and letting the water slide cleanly off their backs. It didn’t match the feeding behavior I’ve come to expect from dabbling ducks: pointy tail sticking straight up in the air for several seconds while their beak probes the bottom. Mallards use a similar dipping motion in their courtship rituals, but that looks more like a head bob than this dolphin dive. It reminded me of how I feel in an outdoor hot tub, constantly submerging myself in the water to stay warm. Could it be that the ducks are using the “warm” water (more than 40 degrees warmer than the air!) to help maintain their body temperatures?

Plenty of ducks weren’t even in the “warm” water; instead they huddled on a small shelf of ice at one end of the pool. They looked cold. But looks can be deceiving. Mallards are big-bodied ducks, and well able to maintain their 100-degree body temperature.

All of our winter parkas imitate their fluffy, warm, down feathers protected by a waterproof shell. We can’t even come close to imitating their feet, though. Not only are webbed duck feet adapted to swimming, they are adapted to manage heat loss as well.

Mallards have blessedly few nerves in their feet. They don’t seem to feel the excruciating pain of too-cold toes warming up. The discomfort that humans feel is actually a helpful adaptation, though, and inspires us to warm up our feet or hands before they are damaged by the cold. Ducks don’t need that motivating pain to keep their feet safe. They have a net-like pattern of veins and arteries in their feet, called “rete mirabile,” which is Latin for “wonderful net”.

This wonderful net allows cold blood returning from the feet to be warmed up by outgoing blood before returning to the body. Ducks will increase or decrease blood flow to protect against tissue damage while losing as little heat as possible to the environment. They may not feel as cold as they look!

Flocks of mallards in the open water near Duluth are a common winter phenomenon, at least in recent memory, and they seem to have the adaptations to handle an Arctic blast.

I think they probably would agree that there is no bad weather if you have the right gear.

Special Note: Emily’s book, Natural Connections: Exploring Northwoods Nature through Science and Your Senses is here! Order your copy at http://cablemuseum.org/natural-connectionsbook/.


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