Different metabolic strategies of northern birds for nocturnal survival

Summary

• 1. The regulation of O₂-consumption and body temperature, especially the occurrence and utilization of nocturnal hypothermia, was studied in three small-sized northern species of birds occupying different ecological niches. Being sympatric, the Willow titParus montanus, the Great titP. major and the Common redpollAcanthis flammea encounter the same environmental conditions during the winter-time, i.e. long nights, low ambient temperatures, a snow-cover, and the possibility of periods of food shortage.

• 2. When fed ad libitum, neither the Great tit nor the Common redpoll utilized nocturnal hypothermia. The Willow tit lowered the body temperature to a value which was significantly correlated with that of the ambient temperature.

• 3. During fasting experiments the evening body weights of the birds were depleted 10–20% below the normal value. In all three bird species the O₂-consumption during the night was positively correlated to the body weight of the birds. Consequently, the depth of hypothermia achieved at thermal homeostasis was inversely correlated to body weight.

• 4. When energy reserves in terms of body weight, are less than normal, both Willow tit, Great tit and Common redpoll utilize nocturnal hypothermia in order to make the energy reserves last throughout the night. The reason why the latter two species, in contrast to the Willow tit, do not use nocturnal hypothermia under normal conditions, may be due to differences in body size and also feeding strategies.