Thyroid hormones in nestling great tits (Parus major)

Plasma concentrations of thyroid hormones (T4 and T3) were examined in nestlings of the altricial great tit (Parus major), from hatching to when young birds left the nest at 16 days of age. T4 levels were approximately 1-2 ng/ml during the first 2 days after hatching and increased to reach maximal levels (9-10 ng/ml) 12 days after hatching, a time at which nestlings reach maximum body weight. Both sexes showed the same pattern. T3 levels showed a quite different pattern. Levels were at all times low (1-3 ng/ml). Both sexes showed a transitory, but significant, increase in circulating levels of T3 during the early nestling period.