On Being a Turkey (In Winter)

By Bob Engel
Marlboro College Professor Emeritus of Biology and Environmental Science

The American turkey fascinates me.  They’re big and clunky, yet they fly fairly gracefully onto the branches of tall trees where they roost.  When they’re wandering around here they see me through our house windows with ease —their beady little eyes taking in my movements.

Coyotes and foxes, trading some eyesight for smell, are much less observant.  I can see why Benjamin Franklin promoted turkeys as the National bird. They’re wily, surprisingly hard to see (unless they move), they use a wide range of habitats, and they’re numerically successful. Around here, some of the hens have stared down our cat as they wander around the place with their chicks, eating grasshoppers and new grass seed.

But when you look at the range of these birds, it’s as if someone snapped a chalk line at the U.S./Canadian border.  These are patriotic birds; they don’t do Canada.  Why not?

I used to teach a course on wildlife management that was really an excuse to teach the ecology of big, charismatic animals.  As a result, I came across a lot of research papers of “game” animals, including turkeys.  Some of articles surprised me.

Maybe not so surprising is the fact that turkeys have trouble finding food in deeper snow.  A study done in Pennsylvania found that turkeys are unable to get to bare ground — and food — with as little as a foot of fresh snow.  What, then, do they eat?  They’re big birds and they have to shovel in quite a bit.  Buds?  A few withered fruits?  The occasional big insect pupa?  It must be tough.

But a second paper was even more of a shock.  It was about their thermal competences and what they were able to navigate in terms of winter temperatures.  A bit of background first.

When a “warm blooded” animal is warmer than its environment it loses heat to that environment.  The bigger the temperature difference between body and ambient temperatures, the greater the heat loss (we think birds and mammals here, but some bigger marine fish, and even large insects like bumblebees and sphinx moths can elevate their body temperatures to permit activity in colder environments).

There are a few ways for winter animals to deal with this heat loss (other than eating enough to compensate for the loss….food becomes heat, after all).  One is to be large like a cow or a horse.  The deal here is that heat is lost across the surface area of the hot-blooded animal.  The smaller the ratio of the surface area to the volume that produces that heat, the lower the rate of heat loss.  So a good evolutionary solution to losing heat and having to replace it with food, is to be large and as spherical as possible.  A sphere is the shape that minimizes the surface/volume ratio, and, therefore, attendant heat loss. That’s why a dog or cat curls up like s/he does.  Because they live in heat-sucking cold water, all marine mammals are large and chunky; ditto for bears, deer, elk, moose and so on.  Those ungulates are just barrels on sticks.

Small mammals and birds are in a real bind here.  To survive the cold, unless they hibernate, they must eat incessantly.  A turkey is not a small bird, so that’s a plus.  But, as the temperature drops, all warm blooded animals eventually have to increase heat production.  The outside temperature where the metabolic rate starts to increase is called the lower critical temperature, and depends on size, shape, insulation, and so on.   It is astoundingly low in a husky dog: minus 40 degrees!  Is a turkey similarly endowed?  It does have insulation in the form of feathers, but the answer is no.

The lower critical temperature for a turkey is a balmy 52 degrees F.  Thus, anything colder than fifty and a turkey has to amp up its metabolic rate.  That can only occur with adequate food. With three feet of snow on the ground, where is that food?  Starvation has to be a constant threat.  No wonder they can’t make it in Canada.  I’m amazed they can survive here.

Mr. Franklin was right.  They are magnificent animals.