Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [45]
Golden-crowned kinglets, like humans, are colloquially called “warm-blooded” animals. For us, the problem of surviving winter is similar: how to keep from freezing and have enough energy left after paying heating costs. But for the kinglet, the problem is much more severe, because the greater the difference between body and air temperature, the more energy must be expended to keep warm. Furthermore, the smaller the animal, the higher the energy cost per given body mass.
Good insulation reduces energy costs, but there are limits on the amount of insulation that a small creature can carry. Large mammals, such as musk oxen, wolves, and arctic foxes, put on thick winter coats that insulate them so well that they may not need to shiver on the coldest nights. Snowshoe hares and red squirrels also put on a thicker, though comparably modest, insulating coat in winter (underground hibernators do not have seasonal changes of fur thickness), while still smaller animals generally do not become better insulated in winter.
Birds vary their insulation less by exchanging their garb than by changing how they use it. To conserve heat they fluff out, thereby increasing the depth of the insulating air layer that surrounds them. Foot and leg temperatures stay low, regulated just above the freezing point. (We try to keep our feet warm, and pay a high energy cost for doing so.) When sleeping, kinglets insulate themselves even more by tucking their heads and feet into their inch-thick layer of feathers which, from inside to outside, can maintain the astounding difference between body and air temperature of up to 78°C.
To find out how quickly a fully feathered kinglet loses body heat, I experimentally heated a dead kinglet and then measured its cooling rate. In still air the body temperature of the heated bird dropped 0.037°C per minute for every 1°C difference between body and air temperature. Thus at an air temperature of -34°C a kinglet that maintains a steady 78°C difference between air and body temperature at its normal body temperature of 44°C during activity would have a passive cooling rate of 78 × 0.037°C/min. = 2.89°C/minute. The heat production in a live bird that is required to oppose such cooling can be calculated by multiplying cooling rate by body weight and by the specific heat of flesh (0.8 calories/gram/°C). This calculation showed that a kinglet (with feathers) must expend at least 13 calories per minute to stay warm at -34°C. This is a conservative estimate because a normally active bird would experience moving air, or wind, that would greatly increase the rate of heat loss. Given the above data I could now find out how large a role the kinglet’s feathers served in insulation—how much energy they save the bird.
I measured the plucked kinglet’s cooling rate. My naked kinglet had a 250 percent greater rapid cooling rate than fully feathered ones. That is, a naked bird would experience air temperature at least two to three times colder than a feathered one. Due to its small size, a kinglet would also cool approximately sixty times faster than a naked 150-pound pig. (I was once asked to testify in a murder case—involving a naked human body that was found chilled—on how long the victim could have been dead. Not wishing to experiment on the human body, I used an appropriate animal substitute. I bought a freshly killed, still-warm pig, shaved it, and cooled it to get the appropriate data. Perhaps not coincidentally, I was not called in as a witness when I, an empiricist, told the lawyer my results and conclusions.)
Because of their habit of fluffing out in the winter, kinglets look especially plump. But a plucked one looks like a pink cherry on spindly legs. The naked female that I examined weighed 5.43 grams, and she had