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.

Eat to stay warm

On these intensely cold days, the birds hit the feeders before sunrise to replenish their energy for the day.  I found this cardinal digging around at the base of one of the feeders and realized it must be mostly empty.

A bunch of sunflower seeds had piled up underneath the feeder, and I assume the cardinal dug through the snow to find them.

A bunch of sunflower seeds had piled up underneath the feeder, and I assume the cardinal dug through the snow to find them.  Smart bird!

Sub-zero temperatures make the suet cakes concrete-hard so that the woodpeckers have to drill them to get a bite.

This stuff is as hard as tree bark at -12F.

“This stuff is as harder than tree bark at -12F.”  Red-bellied Woodpecker

Peanut butter suet seems more attractive to the Downy Woodpecker and easier to extract.

Peanut butter suet seems more attractive to the Downy Woodpecker and easier to extract.

Animals convert their daily food intake to fat stores, which are then metabolized overnight at a prodigious rate.  Black-capped Chickadees, for example, can turn-over 25% of their body weight overnight on a very cold night.

But energy costs (metabolic rate on the Y-axis below) go up as the temperatures go down (on the X-axis below), and the smaller the animal, the higher that rate of fuel consumption and energy burn, as shown in the graph below.

Avian Energy Balance and Thermoregulation (

Avian Energy Balance and Thermoregulation (

At some point, there is a limit to how much heat a small animal like a Downy Woodpecker can produce in order to stay warm, and still make it through the night revving its furnace to that high level.  A friend of mine found that 80% of the Downy Woodpeckers they tested in the winter in South Dakota became hypothermic (3-5 degrees below normal) after 30 min exposure to 5F, which leads me to wonder how these birds are faring at our current below 0F temperatures.

After the long winter’s nap

Chipmunks in the backyard have decided that winter is over, and it’s time to come out of hibernation and do some spring cleaning.  This little guy sat in a bush by one of the bird feeders, grooming itself, basking, and generally looking a little groggy from all that sleeping.

Eastern chipmunk

Eastern chipmunk

Unlike other hibernators (woodchucks, ground squirrels, even bears), chipmunks don’t lay on a big surplus of fat before entering hibernation in the fall.  Instead they arouse every couple of weeks, eat a little of the nuts, seeds, fruit, etc they have cached in their burrow, and then go back into a torpid state where their body temperature is about the same as that of their burrow.

NASA scientists have been fascinated by the special physiology that allows hibernators to lower their core body temperature almost to freezing and still recover organ function after rewarming.  Although adult humans can become incapacitated with as little as a 3.5 degree drop in core temperature, some young children can recover from severe hypothermia (more than a 10 degree drop) following drowning in extremely cold water without loss of function.  Perhaps we do have some physiological ability to withstand a core temperature drop, but have simply lost it as adults.

Will our great-grandchildren be doing this to cross galactic distances?

From the movie, Alien (1979)

From the movie, Alien (1979)