Timing of moult in male and female shelducks Tadorna tadorna: Effects of androgens and mates

We investigated effects of testosterone treatment on timing and rate of mou lt of wing-feathers in male Shelducks Tadorno tadorna, and whether this tre atment influences the timing of moult in untreated females. Three experimen tal treatments were conducted, each with two groups of sexual inactive male Shelducks, which were kept under natural photoperiods in The Netherlands. Each group consisted of two pairs and a single male. Males received implant s with testosterone-proprionate for seven weeks shortly before the natural moult onset of wing-feathers. Start of treatment was 27 June (treatment I), 5 July (treatment II), and 18 July (treatment III). Males delayed their mo ult until the end of hormone treatment. The degree of delay in start of mou lt within the same treatment was related to androgen blood levels. The untr eated females of the testosterone-treated males delayed also their onset of moult. The timing of the end of moult was unaffected by the hormonal treat ment. Males that moulted late did not speed up the rate of moult, or change d the pattern of moult, but some of these birds stopped moult before comple tion. The data indicate that under natural photoperiods the start of moult is quite flexible, requires low levels of androgens, and is under the influ ence of social factors, while the end of moult is fixed.