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By Root 1345 0

how such an individual could survive the energy crunch on a cold, sixteen-hour-long winter night is an unimaginable marvel from our human perspective—it defies physics and physiology. We don’t know for sure how they do it, so we search for explanations offered by animals, and especially other birds. Their example suggests that the kinglet’s winter survival will not likely involve a new biological phenomenon or new laws of physics and physiology; I suspect, rather, it will depend on a species-specific balance created by precisely juggling a set of conflicting benefits and their costs and doing everything just right with little margin for error. There is no magic. It’s a matter of details—of getting everything just right.

I was attracted to them in part because I understood so little, and to some extent I don’t mind keeping it that way, to preserve the mystery. My pursuit of hard fact is not for the sake of facts. It’s to “capture” the story behind them.

Thaler (1990) presumed that the European goldcrest survives in winter by huddling at night to save energy, fueling itself on an energy base of springtails (Collembola), and by its ability to seek microclimates such as under cushions of powdery snow and other places. However, there is no energy balance sheet to determine the limits of these strategies, and although other birds tell us where to look, what they do does not necessarily apply to the golden-crowned kinglet which, given its North American haunts, faces even lower temperatures than its European relative. Our studies of foraging behavior of kinglets in the Maine woods (discussed in Chapter 9) showed, surprisingly, that they either did not have or did not use springtails as their main energy source. Instead, stomach contents showed that they subsisted on tiny frozen caterpillars of the moth family Geometridae. Caterpillars are not of high caloric content, as seeds are, so these little insectivorous birds would like many others drop their body temperature at night to become torpid. Nevertheless, it was presumed that reduction of body temperature at night is not necessary for winter survival in goldcrests (Reinetsen and Thaler 1988). This assessment was based on an overnight weight loss of 1.3 to 1.5 grams that was assumed to be fat. However, the birds could have a much lesser fat cushion than that, since much of the overnight weight loss could have been gut contents. Charles R. Blem and J.F. Pagels examined fat, specifically, and showed that in midwinter the extractable fat reserves accumulated by the North American golden-crowned kinglet over a day were about five times less. These authors calculated that this amount of fat (0.3 gram) would contain insufficient calories for a kinglet to maintain a high body temperature all night.

I think we vastly underestimate these, and many other birds, when we expect them to follow simple rules. Kinglets probably won’t go torpid unless they have to. As can be inferred from innumerable studies of other animals, what happens in captivity (when the animals are well fed and not stressed) may be a pale reflection of what they confront, and perform, under field conditions.

The kinglets’ main adaptations for keeping warm (and conserving energy) include those also found in most other birds. They fluff out their feathers to trap air, creating an ever-greater insulating air space around themselves. The main avenue of heat loss is then through the uninsulated bill, eyes, and the feet. During sleep, however, the first two avenues are greatly reduced as the birds tuck their heads deep into their back feathers. Reductions of body heat loss through the feet is accomplished by countercurrent heat exchange and/or reduction in blood flow, to keep leg and foot temperatures as low as possible, probably just above the freezing point of water, near 0°C. Conversely, the kinglet’s legs and feet can also be used to shunt heat from the body. For example, Thaler (1990) observed that kinglets normally have light brown legs, but when incubating their eggs, females’ legs blush to pink and red as blood is