Wednesday, February 9, 2011

How Do Birds Keep Their Feet From Freezing?

Wild Turkey in Arkansaw, Wisconsin
Today's high temperature was 9º F with an overnight low of -8.  The sun was out - so it didn't feel so cold - inside my Prius.  Outside was another story altogether.

The chickadees at my feeders were all puffed out.  Their little bodies were almost as wide as they were long.   The eagles were perched in their usual trees back in the coulees, soaking up the rays.  But there weren't many birds along the roadsides, even though the snow plows had been by to clear the shoulders of the road - exposing grit and weed seeds.

The windchill advisory warned of temps in the double digits below zero.

We headed out to Arkansaw, Wisconsin.  I wanted to see how liverworts deal with winter.  I'd staked out a roadside colony of these primitive plants last fall.  We found them and I was surprised to see that they stay alive - and green - over the winter.

Mosses & Liverworts (green) and Ferns (brown)

On the way back to town, a Wild Turkey trotted out of the woods in front of the Prius.   I stopped the car, rolled down my window (the blast of cold nearly took my breath away) and pointed my camera at the hen.

She trotted off into the snow - sinking up to her feathered tibiotarsus.  I expected her to fly, but she pulled herself out of the deep snow and walked up on the crust.  Brrr...

Even in my insulated boots, I have trouble keeping my feet warm.  So how do birds keep their naked feet from freezing?

It's all about controlling heat loss in their feet.

Avian feet are comprised primarily of tendons, ligaments and bone - and not much muscle, nerves and blood.  They don't need to keep their feet as warm as we do.  Just above freezing will do.

   -  Their feet are covered with scales which are less susceptible to freezing.

   -  The circulatory system in avian feet - the proximity of their veins and arteries - creates a biological "countercurrent" heat exchange.  Waterfowl have a more complex net of arteries and veins - a "rete mirabile" - that helps keep the temperature in their feet just above freezing.

    -  Birds can constrict the muscles in their legs to actually pump warmer arterial blood into their feet.

And when that's not enough, birds can stand on one foot.  That way they can heat up their cold feet in warm downy feathers - one foot at a time.

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