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.