The seasonal change of plumages in certain birds has long attracted the attention of ornithologists though until recent years little has been accomplished in the analysis of factors in control of this periodicity. The literature shall be considered in a following paper, which will deal with the many correlated features of seasonal variation in different species. The present paper reports only some experiments on the effects of certain hormones on the breast feathers of the African orange weaver finch (Pyromelana franciscana). The females carry a hen plumage throughout the year. The breast appears white in the mid ventral region, as in Fig. 1, left breast, turning into a light buff laterally. The base of the feathers, always invisible in the well arranged plumage, is of a dark gray, due to the deposit of melanin granules (Fig. 3, top left). The males also have this hen plumage outside of the breeding season. It is identical with the plumage of the female to such an extent that it is impossible to tell the sexes apart, unless an occasional nuptial feather may be found that was not shed during the preceding molt. With the approach of the breeding season the males pass through an incomplete molt, shedding only the “small plumage’ which is replaced by a brilliant cock plumage in orange-red and black colors. The breast feathers are black (Fig. 1, right breast). Along with the color, the size and shape of the feathers also change (Fig. 4, lower row, right). Regenerating feathers assume the cock type during the active breeding season (about June 15 to September 30) and the hen type during the remainder of the year. Castrated males and females show in principle the same seasonal periodicity as normal males (Witschi
1
) though molting and regeneration are more irregular and the plumage of castrates outside the breeding season is sometimes of an intermediate type.
