While many birds alleviate seasonal food shortages by migrating elsewhere, many other animals cope with seasonally difficult periods by hibernating, remaining dormant for up to several months at a time. They survive at much reduced metabolic rate on body reserves, and emerge when conditions improve. At one time, the disappearance of most birds from high latitudes for the winter was attributed to hibernation rather than migration. In fact, at least one species of bird does hibernate in winter. This was discovered in 1946, when a Common Poorwill Phalaenoptilus nuttallii (a sort of nightjar) was found in a torpid state in a rock crevice in a California desert (Jaeger 1949). The bird was inert, its respiration and heart rate were barely detectable, and its body temperature was 18-20°C, about half the usual level for birds. The individual was ringed, and in subsequent winters it was found hibernating again in the same crevice. Since then other poorwills have been found in similar sites in the same condition, and their physiology has been studied in laboratory conditions (Withers 1977). The energy consumption of torpid birds was so low that they could live off their body fat for more than three months. Other kinds of birds can also become torpid but remain so only overnight (hummingbirds) or for at most a few days at a time (swifts and colies). Evidently, long-term hibernation is at best extremely rare among birds, most escaping difficult conditions by migration instead.

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