Thyroid and Gonad as Factors in the Production of Plumage Melanins in the Domestic Fowl

The results of thyroid feeding on five breeds of domestic fowl are recorded. They vary with dosage and with sex. When the daily ration is compatible with good health, the males of pigmented breeds, both normal and castrated, tend to darken conspicuously owing chiefly to an increased production of plumage melanins. Normal females are affected, if at all, to a much slighter degree. Castrated females, however, resemble males in this respect as in others. An antagonism thus appears between ovary and thyroid as indicated by their relation to the formation of plumage melanins. The blanching of plumage observed after excessive doses of thyroid is attributed to a toxic effect, non-specific in character. A correlation between the effect of thyroid feeding on melanin production and feather structure is pointed out. White Leghorns show the latter but not the former, being dominant whites.