The annual moult of some 20 species of birds inhabiting the dry western section of New South Wales is studied, emphasis being on time of commencement, duration, finishing time, and the interrelationship of breeding and moulting. Primary feather replacement is taken as the main criterion.
The moult begins in most species in November and is completed in February or March. The duration ranges from about three to five months, being four to four and a half months in most species. This is materially longer than the six to eight weeks commonly taken by small passerines in the Northern Hemisphere. A longer, and more protracted moult is considered to be of adaptive value in a dry and somewhat unpredictable environment, reducing the physiological stress on the bird.
Migratory species have later, and somewhat shorter moults, than resident ones. Those of three aerial-feeding muscicapids are relatively short. Small birds commonly have shorter, and larger birds more protracted, moults.
The moult is much more regular in its occurrence than breeding, which can only occur in those springs when seasonal conditions are reasonably good. Thus the “postnuptial” moult takes place whether breeding has occurred or not. This suggests a measure of direct photoperiodic control. On the other hand there are instances of the moult starting up to two or three weeks earlier than usual when (a) there is an early termination to successful spring breeding and, (b) severe drought conditions render spring breeding impossible. The onset of the moult may apparently be delayed by up to a couple of weeks when (a) spring breeding is protracted or (b) it is delayed. This indicates a measure of antagonism between the “moulting” and reproductive hormones in some species.
Good rain in mid-summer will induce out-of-season breeding in many species, notwithstanding that they may be in full moult at the time. This particularly applies in years when spring breeding was impossible. No data is available as to whether or not this causes a slowing-down, or interruption, to the moult although, it will be noted, Carter (1923) suggests this to be the case. In some species “mid-moult” breeding did not mean any delay in the completion of the moult but, in others, it apparently did so. The ability to moult and breed simultaneously is obviously an important adaptation, permitting the birds to make maximum use of a time of food abundance.
Attention is drawn to adaptational parallels in the moult of certain dry country birds of Australia, Africa, and South America.