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It was about 10 degrees with a frigid wind blowing across the river on Tuesday afternoon in Hudson.

The waterfowl were hunkered down. A group of swans, geese, and ducks hanging out near the small patch of water where the Willow River/Lake Mallalieu pours under a narrow bridge and into the St. Croix seemed to be sleeping. Their heads were tucked under their wings, but regularly raised to preen feathers, which improves the insulation, and to look around for possible predators.

The guy freezing his face off to record them seemed to pose little threat. They weren’t doing much that made exciting video, except a swan making a goose slip and fall, but that’s how they survive cold weather.

After several minutes of filming, I hustled back to the car. I felt cold to my core. The swans stayed out there, pointed into the wind, waiting out winter.

I started to wonder, which is always a good excuse to learn more.

Why don’t swan’s feet freeze to the ice?

Like many species of waterfowl, trumpeter swans have unique adaptations to survive cold weather.

For one thing, trumpeter swans are big. They are the largest and heaviest waterfowl native to North America. This means they can have ample fat stores to burn during cold periods, when it’s harder to find a lot to eat. Their heft also provides extra insulation.

Another asset is something familiar to anyone who lives in a cold-weather state. The warmest jacket you can buy is probably filled with goose down. The light feathers hold heat extremely well. Swans can have almost two inches of down on their body.

Finally, blood circulation to the swan’s feet is adapted to keep them safe. Warm blood flowing to the feet passes close by the veins carrying cold blood leaving the feet. The warm blood loses much of its warmth to the cold blood, which means the feet stay pretty cool all the time. This reduces heat loss.

“They have a counter-current heat exchange system in their legs: Veins and arteries in the leg are close to each other, and as warm blood leaves the body, it heats up the cold blood returning to the body,” Peter Marra, head of the Smithsonian Migratory Bird Center at the National Zoo, told the Smithsonian Insider. “It’s brilliant!”

What’s that band around the swan’s neck?

One of the birds was sporting a bright yellow necklace. These bands are put there by biologists to help track numbers, migration, and what habitat the birds are using.

Since 1960, 36,627 swans have been banded in North America, by many different government agencies and other organizations. Each band is registered in a federal database.

Luckily, I was able to get a photograph in which the band’s code was visible. That meant I could report the sighting, and learn more about the swan.

So I went to the U.S. Geological Service’s Bird Banding Laboratory website and filed a report. It was pretty easy, walking me through several questions about how I saw the bird, where, when, its status, and other basics.

All banded waterfowl have a long code on a band on their leg. Those letters and numbers are usually only visible if the bird is captured or dead. But just based on the one letter and two numbers on the neck band, as well as the color of the band and the lettering, the bird’s record could be found in the database.

Two days later, I received a certificate of my sighting, with information about the bird’s history.

• Age of Bird: Hatched in 2009
• Banded: July 15, 2010
• Location: Clear Lake, Polk County, Wisc.
• Bander: Brian Dhuey, Wisconsin Department of Natural Resources

This spring, when swan cygnets are hatching in ponds and wetlands across the St. Croix River region, Swan F11 will celebrate its 10th birthday.

That means it is probably already paired up, perhaps with one of the other swans nearby, and probably a parent.

Not only do swans mate for life, they get to know each other before leaping into parenting. Trumpeter swans usually find their partner between two and four years of age, but don’t start reproducing until about seven-years-old.

Trumpeter swans hang out all winter almost anywhere there is open water — usually areas with lots of springs, or where a stream enters the river. I highly suggest going out and finding some, but you might want to wear a down jacket.

References:

1. Trumpeter swan, BirdWeb
2. USGS Patuxent Wildlife Research Center, Bird Banding Laboratory
3. Trumpeter swan, Toronto Zoo
4. Keeping Warm in Winter is for the Birds, Smithsonian Insider
5. Trumpeter swan, Cornell Lab of Ornithology