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 torpor, saving energy by allowing their body temperatures to decline drastically. Grosbeaks stay close to trees whose seeds contain sufficient fat to fuel 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, for example―and so would have to consume 75 percent more food per unit of body mass than the larger birds to maintain the same body temperature. The insulation provided by feathers, which, similarly to other northern birds, make up about 25 percent of the kinglet's mass, accounts for some of its heat-retaining capability but does not explain how kinglets manage to survive as well in cold climates as do the larger birds. Because smaller birds have a lesser absolute amount 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 consume the necessary amount of insects each short winter day. The question is more vexing considering that a kinglet's stomach when filled 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 torpor in kinglets. Another theory, which is still untested but which may be borne out by a recent study of gold crests, a related species, is that kinglets cluster together at night. Kinglets flock in groups of twos and threes during the day; while such small groups alone could not maintain such high body temperatures, it is hypothesized that after nightfall several groups in a region may find each other by means of calling and consolidate in a central location.