When it’s too cold to grab a peanut

The porch thermometer read -10 F (-23 C) and there was bright sun in the backyard, but there were no birds and no squirrels present at or under the feeders.  Finally about 10 a.m., the first foragers appeared.  The thermometer had worked its way up to just under 0 degrees F (-18 C), but the wind was blowing, so windchill temps would be well below 0 F.

male northern cardinal-winter plumage

Mr. Cardinal monitored the surroundings while Mrs. Cardinal foraged at the bird feeder.

female northern cardinal

She is finding the discarded sunflower seeds left by the chickadees and nuthatches who pitch them out of the feeder in search of peanuts instead.  The wind is fluffing out her feathers, but also carrying away her body heat.

White-breasted Nuthatch stashing a peanut

This White-breasted Nuthatch is stashing a peanut in a bark crevice, where it will hammer at it until it breaks off pieces it can eat.  I wonder if the crevices are more apparent when they are upside down, or it’s easier to break them up when they come at it upside down????

White-breasted Nuthatch stashing a peanut

After much hammering away, the Nuthatch has reduced the peanut (directly under its beak) to just a small chunk.

White-breasted Nuthatch basking on a cold morning

You know it’s cold when you see a Nuthatch pause for several minutes in its foraging efforts to bask in the sunlight. The bird’s back was directly facing the morning sun.

Black-capped Chickadee

Peanuts are the preferred high-energy foods on subzero days.

Black-capped Chickadee drilling a peanut

The Chickadee’s method of eating the peanut is much different — grasping the nut with its foot, the bird drilled into it to break off small chunks.  But peanuts at -10F must be brick hard, because this bird drilled it over and over and was only able to break off small bits.  After much effort it finally flew off with the nut, perhaps to its roost hole.

American Goldfinch and Black-capped Chickadee

The Goldfinch watched the Chickadee eating a peanut but made no attempt to go find some food for itself at the feeder.

Two birds exhibiting contrasting strategies for surviving harsh winters in the northern temperate climates:  Goldfinches turn up their metabolic furnace to keep their body temperatures stable on extremely cold winter nights; they do this by harvesting fat-rich seeds (like sunflower seeds) and keeping their fat reserves high.  Chickadees are primarily insectivores that do eat seeds in the winter, but typically conserve their internal fuel resources by lowering their body temperatures at night — just in case they need it to find food the next day.

Black-capped Chickadee

The heartiest of little winter birds…Chickadees are survivors!

By noon, the wind had picked up, and the temperature had dropped again, on its way down to an overnight low of -27 F (-32 C), and I didn’t see birds at the feeder for the rest of the afternoon.  I hope they survived overnight!

Why don’t trees freeze solid in the winter?

A bright, sunny day in mid-winter in the northern U.S. makes it look like a good time for a walk in the backyard — but, one step out the door and I know it won’t be fun at all. The air temperature is -7 F, and there is a stiff wind blowing.  This makes for a very short walk, snap a couple of photos, retreat indoors again.  Why bother?  Because I got to wondering how trees manage these sub-freezing conditions.  Obviously, standing still in this kind of weather would be lethal for any warm-blooded creature, so how can trees withstand freezing solid for six months of the year?  Or do they?

oak-tree-in-snow-

And what happens on warm sunny days when half of the tree is subjected to bright sunlight while the other side remains in the dark?  Is there freeze and thaw going on?

The short answer is that they don’t actually “freeze solid”, because the same changing light (decreased photoperiod) and fluctuating temperature conditions in the fall that bring on that wonderful display of fall color also induce physiological changes in plants called “cold hardening” that prevent freezing.

snowfall

The strikingly white bark of birch trees serves a useful purpose in the winter by reflecting a lot of the sun’s radiation on bright, sunny days, and thus preventing the unequal heating of the exposed and unexposed sides of the tree.

Photoperiod and temperature signals in the fall cause plant cell membranes to become more permeable and flexible.  Sugars produced by the leaves move down to storage in the roots, and water follows the sugar movement, so cellular contents become much more concentrated.  So concentrated in fact, that they lower the threshold for freezing dramatically, to -30 F or more.  In addition, cells produce protective cryoproteins that act like potent antifreeze agents.   Residual water trapped between cells may freeze, but the now shrunken and flexible cells remain uninjured, and ready to restart their metabolic engines when spring weather thaws the ground, the roots take up water from the soil, and the sap rises in the plants’ fluid transport vessels (xylem and phloem).

basking-squirrel-

Rough textured bark also serves a useful purpose, beyond providing a foot-hold for the squirrels. It can’t serve as an insulative blanket like a warm coat of feathers or fur does, but corrugations of bark absorb the radiant heat of winter sun, expanding and contracting in heat and cold, without affecting the underlying layers that might crack with exposure to the sun.

In effect, trees and other plants that survive the sub-freezing conditions of northern winters are in a static state of super-cooled dormancy, still liquid and viable, although metabolically quiescent.  Waiting…