THE DIURNAL BODY-WEIGHT CYCLE IN TITMICE PARUS SPP

The diurnal weight cycle of five species of tits (Parus montanus, P. palustris, P. major, P. caeruleus and P. ater) was studied in Norway. The daily weight increase (DWI) tended to rise to a maximum in mid-winter and thereafter decline. The hourly weight increase (HWI) showed a marked seasonal fluctuation and was about twice as high in mid-winter as in autumn and spring. Generally HWI was highest in the morning. but tended to vary with rank order during mid-winter, i. e. being highest in the dominants. A relatively high evening weight was followed by a large nightly weight decrease, and vice versa. Thus the nightly metabolism seems to be adjusted according to the evening weight, as previously shown in captive Willow Tits. It is suggested that hypothermia during sleep is a trade-off between energy reserves and predation risk. The air temperature apparently had only a negligible effect on the weight fluctuations, perhaps because of the relatively mild winters during the study period.