Seasonal changes in body weight, fat depots, and plasma levels of thyroxine and growth hormone in free-living great tits (Parus major) and willow tits (P. montanus).

Annual changes in body weight, fat depots, and plasma levels of thyroxine (T4) and growth hormone (GH) were studied in free-living great tits and willow tits. Birds were collected during six ecologically well-defined periods of the year. Special attention was given to the nonreproductive part of the year. T4 showed simple unimodal cycles in both species and both sexes, with high levels during the warmer part of the year, and low levels during the winter and spring periods. Although increasing levels were temporarily separated between the two species, they were in both cases correlated with the onset of gonadal regression and moult. Plasma levels of GH fluctuated in a much more complex pattern, and no obvious and consistent correlation to any extrinsic or intrinsic factor was found. Body weights and fat depots both showed seasonal variations that varied slightly between the two species. Values, with the exception for breeding females, were generally the highest during the autumn, winter, and spring periods.