Thermoregulation. That’s the word of the day and a super fantastic Scrabble word to boot. It refers to the way animals maintain their body temperatures by either cooling off or heating up in response to surrounding air temperatures. In the midst of a winter Scrabble game, it’s the heating up and staying warm part that matters. Little creatures, such as the White-throated Sparrow pictured above, are especially concerned about these matters. This is the little guy that got me thinking about such things.
Little birds happen to be especially hot little packages. On average they have a running body temperature of 104-108 degrees F . Ounce for ounce, birds have a warmer body temperature than mammals of equivalent size. Bigger birds are closer to the mammal model than the smaller ones, however. I’m talking about the wee (“wee-er,” if that’s a word) birds in this case. In an effort to stoke their tiny furnaces they need to take in a lot of calories and burn a lot of oxygen. This means their rate is nothing short of phenomenal. A small fowl’s heart rate can be up to in the 400-1,000 bpm range. When you compare this to a human exercise rate of only 160 bpm, you wonder why these things don’t just blow up! Winter poses a whole set of problems for such hot-headed birds.
As you can see, this sparrow was doing his best to stay warm on a bitter January day where the temperature was about 9 degrees F. To achieve his goal, he basically curled up into a fluffy ball. From a different angle (see here) the thing looked more like a dust bunny than a bird. Every feather was erect, his feet were withdrawn, and his head was tucked around so deeply into the back feathers that it appeared to be lost. He was so involved in his thermoregulatory endeavors that he did loose his head, in a manner of speaking, because he ignored his predatory avoidance endeavors. I, the potential predator, was able to crawl up to within inches of this fellow without alerting his alarm system.
A few days later I came upon another one of these sparrows. The day was “warmer” at a balmy 15 degrees F, so it was puffed out but not balled up. He cracked an eye open just enough to check me out (see below) before resuming his energy conservation tactics. Other birds are doing the same thing as well – like this inflated winter robin (see here). There is more going on with these little peeps than first meets the eye. The small scale winter bird has many metabolic and behavioral tricks hidden under those feathery sleeves.
To stay alive, a bird needs to find a balance between producing heat and conserving it. Different small birds have different responses. Cold weather, especially that rendered brutal by high winds, causes sparrows to increase their rate of heat production. Chickadees can go into a state of controlled hypothermia if necessary and drop their body temperatures by up to 15 degrees F. Redpolls can eat like pigs and keep their furnaces going at their normal rate. The major way for birds to produce the quick heat they need is to shiver. First, by shaking their large flight muscles and then adding in some quivering leg muscles at lower temperatures, heat is generated.
Conserving that precious heat, once produced, is equally as important. Changing shape, like going from an oval to a round shape, decreases the amount of surface area. Erecting feathers traps a surrounding layer of air. The action that brings these feathers up is akin to the muscular action that creates goosebumps on us, by the way. Tiny feet are protected from frostbite through a system known as counter-current blood flow. In short, this means that the warm blood coming from the body into the foot transfers its heat to the cold blood returning from the cold little toes. The cold blood is brought back up to body temperature before it gets back into the body.
On top of all this “body stuff,” the smallish bird has to use that hot little head to make some behavioral choices. They need to choose roosting spots out of the wind and position their bodies into the wind if necessary. For instance, my fluff ball White-throated Sparrow chose his spot away from the wind and next to a building. You’d think standing on a cold slab of cement would be a poor idea, but the fact is that the building and the concrete store some of the sun’s heat and then radiate it out as the temperatures drop. Tree trunks and rocks will do the same thing which is why sheltered birds seek these spots for the few extra degrees of comfort they receive. On another level, one study even showed that many birds choose to feed later in the day so that they can have that additional energy boost to get them through the long cold night.
These are all things you can think about when next you nestle deep into the folds of your down jacket and curse the north wind. Little birds don’t curse – it’s a waste of energy.