Thyroid and annual gonad development, body weight, plumage pigmentation, and bill color cycles of lal munia, Estrilda amandava.

Long-term administration of L-thyroxine (T4) in the wax bill (lal munia) inhibited the current gonad development cycle, and the increase in gonadal volume in the second cycle was suppressed only by higher concentrations. Gonadal inhibition by high but not by low doses of L-T4 was associated with a failure in the development of LH-dependent pigmented nuptial plumage. This suggests that L-T4 depresses FSH secretion more readily than LH secretion. The results indicate that the effects of L-T4 administration on the body weight cycle depends upon the physiological and/or gonadal status of the bird at the start of the experiment and that the juvenile hypothalamo-hypophysial-gonadal axis is more sensitive to L-T4 than that of the adult birds. The bill color, like plumage pigmentation, appears to be LH controlled. It is suggested that while thyroid hormone, acting by way of the hypothalamo-hypophysial complex and/or by direct action on the feather papillae, modulates the annual cycles of plumage pigmentation, bill color, gonad development, and body weight of lal munia, the finch has an endogenous mechanism which enables it to adapt to chronic hyperthyroidism in such a way that breeding occurs at the normal time of the year.