Nocturnal hypothermia and its energetic significance for small birds living in the arctic and subarctic regions. A review

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Abstract
Nocturnal hypothermia is demonstrated in a number of small northern birds. These birds utilize hypothermia among other energy-saving mechanisms and lower the body temperature by some 10°C. In most bird species hypothermia is utilized only in conjunction with a state of inanition. However, hypothermia has also been demonstrated in birds with satisfactory feeding conditions and body weight. For none of the small northern birds utilizing nocturnal hypothermia. is inanition necessary for the induction of a state of hypothermia. A seasonal effect on the hypothermic response has been demonstrated for two species of tits, the black-capped chickadee Parus atricapillus and the willow tit Parus monfanus, and also for an Andean hummingbird. The depth of hypothermia achieved significantly and linearly was correlated with the ambient temperature for the same two species of tits. By the use of nocturnal hypothermia, birds living in temperate zones can save as much as 75% of their energetic costs, compared with their energy consumption at normal body temperature. The reduction in the nightly expenditure of energy is considerable also in small-sized arctic and subarctic birds that utilize nocturnal hypothermia. The saving of energetic costs may easily represent the margin between life and death for such small birds living under the combined stresses of hunger, cold and long nights.