too cold…

It’s -13 F right now, and with a moderate wind blowing, the wind chill makes it -36 F.  That’s too cold for me, but not too cold for the hungry birds and squirrels in the backyard to come into the bird feeders for a meal.

Gray squirrels don’t venture out until the sun is well up on these really cold days. And before they cross snow patches to get to the feeders, they bask on the trees for a while to warm up, orienting the darker fur on their backs directly toward the sun.

Even the chickadees take a few minutes between trips to the feeder to bask a little, fluffing their feathers out to make their tiny bodies into an almost spherical shape.

The bluejay must have been too cold to move — it just sat there looking around, squinting into the sun.  I could see the wind ruffling up its feathers — brrrrrrr.

Basking to gain what little radiant heat the sun provides at this time of year can be effective in warming up, but wind currents that penetrate fur and feathers carry that precious heat away.  What can a small animal do to cope with this intense winter cold?  Activity helps, as muscles generate heat, but that comes at a cost to be repaid by eating more.  Shivering helps, and when animals are not active, muscles engage in isometric tremors that generate heat, which is also expensive, but not as much as flying or running across the snow and climbing trees.   The only animals that seem unperturbed by this weather are the feisty little red squirrels.

Red Squirrels must generate a lot of heat dashing around, because they spend almost no time basking and a lot of time digging into the snow looking for buried treasures (food) and running up and down trees to stash whatever they find away.

Cold weather and one-legged birds

Do you find that ice cubes sometimes stick to your warm fingers as that slight moisture in your fingertips instantly freezes on contact with the ice surface?  So then, why don’t the webbed feet of ducks, geese, swans, gulls, etc. freeze to the ice?

Standing with her bare webbed feet on snow and ice doesn’t faze this mallard hen. Why not?

A wonderful adaptation of the circulatory system in the legs, called a counter-current heat exchanger, warms the cold venous blood returning from the foot while it cools the arterial blood flowing to that site.  The “exchanger” is actually a network (rete) of arterial blood vessels wrapped around a central vein, and the counterflow between the two sets of blood vessels allows a steep gradient of temperature to be created between the body core and the webbed foot.

With no heat exchange (left), the foot stays warm and loses heat to the ice. A warm web would also stick to the ice! With heat exchange (right), arterial blood going to the foot is gradually cooled by giving up its heat to the vein bringing blood back from the foot. Thus, with heat exchange, foot temperature is about the same temperature as the ice it is resting on. Voila! Less heat loss! [Figures from (Ask a Naturalist)]

This anatomical adaptation for reducing foot temperature has important implications for staying warm in cold weather.  Even with reduced blood flow to the legs, the bare areas of the foot and leg are potential sites of great heat loss in extreme cold weather.  So, the less leg and foot surface area exposed, the better.

Birds have perfected the one-legged stance of the yoga “tree pose” and adopt it frequently when temperatures dip, placing their center of mass directly over one leg and tucking exposed limbs into their plumage.

Trumpeter Swans maintain perfect balance, with their head tucked under a wing for additional protection from heat loss.

On first glance, you might think this was simply an unfortunate, one-legged Ring-billed Gull, But the “tree pose” is used in all activities — whether resting or eating.

At rest, long-legged herons and egrets often tuck one leg up into their belly plumage to reduce heat loss while standing in cold water.  Even shorter-legged Black-crowned Night Herons take advantage of this behavioral strategy.

A Broad-winged Hawk sat on a cold steel pole in my backyard one fall morning, and fluffed its belly feathers over its legs as it perched on just one foot (right photo).  A few minutes later, when the sun had reached that spot, the bird let the sun do a little warming of its bare legs and feet (left).

Would the one-legged, yoga tree pose work for us, while we wait at a cold bus stop?

Humans posing like trees.
It’s unlikely this would promote heat conservation for humans in cold weather, though.

 

Basking squirrels

Even though the air temperature was well below 0F for most of the day, the squirrels were out looking for goodies.  Unlike their usual ground foraging activity, today their efforts seemed to be restricted to the trees.  Something I have never seen these squirrels do before:  flatten themselves against the tree and sit still for several minutes at a time.  I think they must have been trying to pick up some heat from solar radiation.

This squirrel stayed in this position for more than 10 minutes.

I couldn’t find much in the science literature about basking behavior in gray squirrels, but there are other species of squirrels that definitely use this strategy to reduce metabolic costs in extreme cold weather.  Rather than remain inactive in their nest, the squirrels have opted to utilize whatever warmth the weak sunlight provides and hope they can remember where they stashed a few of their nuts for a quick snack.