Nocturnal hypothermia and metabolism in the Willow TitParus montanus at 63° N

Summary

• 1. The dynamics of temperature regulation on a diurnal and annual basis were studied in seasonally acclimatized Willow Tits. The diurnal thermoregulation pattern of the tits was the same at all seasons of the year, i.e. at roosting time the body temperature gradually fell until it levelled off at some time between 2100 and 2200h. Then the body temperature remained more or less constant until two to three hours before sunrise, when it gradually rose to the normal daytime level by the start of the daytime activity period (Fig. 1 and 3 A-B).

• 2. Body temperature and O₂-consumption rates were recorded on winter acclimatized Willow Tits, exposed to a series of different but constant ambient temperatures during night. The values at which the body temperature levelled off at night was significantly correlated with that of the ambient temperature (Fig. 2). Energy expenditure on an overnight basis was lowered considerably when the tits entered nocturnal hypothermia (Fig. 5).

• 3. The Willow Tits showed a marked seasonal variation with regard to the depth of hypothermia achieved when they were exposed to different ambient temperatures. The maximum response was observed in mid-winter acclimatized birds, whereas mid-summer acclimatized birds showed almost no response. Between these two extremes, a steady gradient was found (Fig. 4).

• 4. The observed nocturnal hypothermia was a controlled physiological response to low ambient temperature and long winter nights. All the birds in the experiment arose to the normal diurnal body temperature in the morning.