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.
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…]
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.
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.
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.