Naturalists have long studied the ability of North American forest birds to survive extremely cold overnight temperatures in winter. For example, nuthatches sleep in cavities such as tree hollows or holes dug into snowdrifts, retaining heat closer to the body and thus saving energy by reducing the need for shivering. Chickadees induce shivering. But the survival of one species, the kinglet, remains something of a mystery.
There are two reasons for this. First, although kinglets are tiny—about 9 cm long including the tail—they maintain extremely high body temperatures at conditions well below freezing. According to the physical laws of heating and cooling, kinglets would lose heat at a rate about 75 percent faster than birds twice their mass—chickadees, of insulation than larger birds, they would cool even faster than predicted by body mass alone.
The second reason kinglet survival is so remarkable is that, unlike most bird species that remain in cold climates during winter, their diet consists exclusively of insects. Researchers wonder how it is possible for kinglets, birds that do not cache food and are known not to forage at night, to gather and to capacity contains only enough food to keep it warm for one hour.
A partial explanation is that kinglets store fat; kinglet body fat can triple in the course of a day. Nevertheless, this increase accounts for only about half the energy needed to maintain the kinglet's body temperature overnight. Researchers once theorized that torpor might make up the difference, but found no evidence of a region may find each other by means of calling and consolidate in a central location.
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