THE EFFECT OF SHORT-TIME FASTING ON METABOLISM AND NOCTURNAL HYPOTHERMIA IN THE WILLOW TIT PARUS-MONTANUS

The effect of body weight on O2 consumption rate and nocturnal hypothermia was studied in winter acclimatized willow tits P. montanus. During the fasting experiments, the birds were kept at a constant ambient temperature of 0.degree. C, and a photoperiod of 8L[light]:16D [dark]. O2 consumption rate and deep body temperature were recorded continuously throughout the night. The O2 consumption rate 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 the body weight. Results indicate that depending on season, ambient temperature and energy reserves, the willow tit regulates its metabolic rate so as to keep it at a level which corresponds fairly well to the maximum affordable energy expenditure.