EFFECTS OF ANDROGENS ON THE INDUCTION OF ECLIPSE COLORATION IN CASTRATED WILD MALLARD DRAKES (ANAS-PLATYRHYNCHOS)

Effects of androgens on the induction of eclipse colouration in castra ted wild Mallard drakes (Anas platyrhynchos). - According to the liter ature the eclipse colouration in wild Mallard drakes seems to depend o n estrogens. I studied the effects of the 2 androgens testosterone (T) and 5alpha-dihydrotestosterone (DHT) on the induction of eclipse colo uration in 15 wild Mallard drakes which were castrated since several y ears. From the beginning of March to the beginning of May 5 castrates were daily injected with 5 mg T, 5 birds received daily injections of 5 mg DHT, and the remaining served as controls. In the second half of July, near the end of the postnuptial moult, the 5 controls (like all castrates in previous years) exhibited the nuptial colouration. The T treated group had moulted into an almost complete eclipse plumage. The DHT injected birds showed a mixed colouration with distinct individua l variation, the estimated average being about 70 % eclipse and 30 % n uptial plumage. The weaker effect of DHT, compared with T, on the indu ction of the eclipse colouration is discussed with regard to an aromat ization of the androgens to estrogens.