Baby, it’s really cold out there…

It wouldn’t be winter in Minnesota without a week or two of sub-zero (in F degrees) weather (-2F this morning and that’s -19 degrees C). And the little birds have been hitting the feeders pretty hard lately, especially the peanuts.

Chickadees never seem to slow down, regardless of the weather. But they were slow to get up this morning and didn’t make an appearance in the backyard until the sun was well up.

Now here’s the problem with being a small bird trying to survive in this winter climate. Heat will be lost from a body surface when the body is warmer than the environment, and in the case of the chickadee, whose body temperature is normally about 107 F, that is a 109 degree difference between its internal core and the air temperature hitting its feathers this morning.

In addition to this huge thermal gradient for heat loss, small birds like Chickadees have a very high ratio of surface area to their heat-producing body volume — thus accelerating the rate of heat loss. [If you’re interested in why this is, we can talk about the mathematical basis of surface to volume ratios in the comments…]

The chickadee looks like a sphere at rest, because that’s the shape that conserves heat the best. Maximal fluffing and tucking exposed parts in is essential in these subzero temperatures.

Chickadees are metabolic marvels, being able to harvest enough energy from their foraging efforts all day to last them overnight and part way into the next morning, before they can visit their stash of seeds or a bird feeder. BUT…they are economical with their overnight energy expenditures (because you never know what the next morning’s weather will be like), and they make their fat reserves last by lowering their body temperature as much as 20 degrees F (from daytime highs of 107 F to night-time lows of 86 F).*

Hypothermia makes it possible for Black-capped Chickadees and most other titmice members of the Paridae family to survive winter cold in northern latitudes. Not all birds can or do utilize this strategy. Some species, like Common Redpolls, eat a more fat-rich diet to have a larger overnight energy reserve. Some species, like the Common Bushtits that weigh half as much as a Chickadee, share body heat with each other by roosting together communally, packed tightly side by side on a branch. And some species, like Eastern Bluebirds do a little of both the Chickadee and the Bushtit strategy to make ends meet energetically.

I was intrigued the other day when I found Eastern Bluebirds going in and out of the bluebird boxes at Como Park golf course. I knew they weren’t setting up nest sites at this time of year, so they must be taking refuge from the cold in the boxes. It’s unusual for these Bluebirds to stay this far north in the winter, and I wondered how they were managing it.

Multiple birds in the same box together would be ideal for conserving some body heat on a cold day, in the same way we use the shelters at the bus stop and (used to) stand close to friends and family to retain heat.

Groups of six to eight Eastern Bluebirds in Indiana were observed to roost together in a nest box overnight, forming a circle with their heads toward the center of the box and pointed downward below their bodies so the heat from their exhaled breaths was directed toward the other birds.** It’s possible they might be using this heat conservation strategy during the daytime as well here in Minnesota, when multiple birds enter the same box.

In addition, Eastern Bluebirds forage in small flocks during the winter, using the strategy of more eyes to find food. And when they take a break from foraging intensively, they often huddle tightly together on a branch close to the trunk of a large tree that provides some protection from heat loss from the wind.

Scott Mohn found just such a collection of male Eastern Bluebirds huddling together on a tree limb at Como Park golf course on this cold morning and graciously allowed me to use his image. (Click on the image to see it full screen). Notice these are all male bluebirds, which would not tolerate each other’s presence during the breeding season. But for the purpose of winter survival, they are bosom buddies.

It must be tough to be an Eastern Bluebird that depends on a fruit and insect diet to find enough to sustain itself overnight in Minnesota’s subzero climate. What do they do if they can’t find sufficient food to last them overnight? Bluebirds in Indiana were lethargic and immobile when researchers opened the nest box after a cold night; it’s possible they might also be lowering their body temperatures overnight to conserve energy like the Chickadees do. But there are no published data on this — at least that I can find.

*The data on hypothermia in Chickadees overnight was part of my Ph.D. thesis at Cornell University in 1973.
**The data on communal roosts of Eastern Bluebirds in Indiana was published by Frazier and Nolan in 1959 in Bird Banding.

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?


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.


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).


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…

Down by the bay…

Well, down by the side of the lake anyway.  No watermelons there (as in the song), but instead an unusual (deciduous) conifer that drops its needles in the winter.  It’s a Tamarack (Larix laricina), or Larch, as it is called up north.

Tamarack are nothing but bare scraggly branches and cones during the winter, having dropped their needles in the fall.

Tamarack are nothing but bare scraggly branches and cones during the winter, having dropped their needles in the fall.

tamarack cones

Pure stands of Tamarack Larch can be found throughout parts of Canada and extreme northern U.S.  The color change produces a glorious scene in the fall when the short needles turn bright yellow.

Flaming Larch trees in the Enchantments, Washington state.

Flaming Larch trees in the Enchantments, Washington state in October.  Photo from

The tree is quite tolerant of a wide variety of environments, but favors wet swampy places.  It  can tolerate cold temperatures down to -60 F, making it one of the few trees to be found on the edge of the arctic or alpine tundra.

Like the Bald Cypress (also a deciduous conifer), Tamarack Larch have to make adjustments to keep their root tissue oxygenated in the stagnant swampy water.  Where Bald Cypress send up root knobs (knees) above ground, Tamarack Larch instead spread their root mass widely through a thin layer of topsoil.  However, one problem with this solution is that without a deep taproot anchor, they become vulnerable to damage from high winds.

This side view of a downed tamarack shows how shallow the root ball can be in a particularly swampy area.

This side view of a downed tamarack shows how shallow the root ball can be in a particularly wet, swampy area.

The wood is tough and durable, but flexible.  Algonquian Indians made their snowshoes from larch strips.  Log roads over swampy areas and corner posts marking property were often constructed from tamarack because of its high resistance to rot.

It’s curious why so few conifers are deciduous (in fact, most of the examples are species of Larch), even though you might think it would be more advantageous to lose needles in the winter when the tree can’t use them for photosynthesis anyway.  Bare branches can also shed snow more easily, and are less likely to break under the weight of a snow pack, as well.   Perhaps the answer lies in where these species grow best.

Most evergreen conifers are adapted to dry, cold climates, where decomposition is very slow and the nutrients necessary for production of new needles every year are in short supply.  In that case, it makes sense to hang on to the needles and replace them slowly over several growing seasons, as nutrients become available.

On the other hand, swamps are nutrient-rich places full of lots of decomposition products, and Larch species have specialized on this rapid turnover, allowing them to rebuild a crop of needles each growing season.

Ah, the myriad ways of nature…