THYROID-HORMONES IN NESTLING GREAT TITS (PARUS-MAJOR)

Plasma concentrations of thyroid hormones (T-4 and T-3) were examined in nestlings of the altricial great tit (Parus major), from hatching t o when young birds left the nest at 16 days of age. T-4 levels were ap proximately 1-2 ng/ml during the first 2 days after hatching and incre ased to reach maximal levels (9-10 ng/ml) 12 days after hatching, a ti me at which nestlings reach maximum body weight. Both sexes showed the same pattern. T-3 levels showed a quite different pattern. Levels wer e at all times low (1-3 ng/ml). Both sexes showed a transitory, but si gnificant, increase in circulating levels of T-3 during the early nest ling period. (C) 1996 Academic Press, Inc.