Problem of moult

New Zealand Herald, Volume LXXV, Issue 23169, 15 October 1938, Page 10 (Supplement)

Problem of moult

By R. A. FALLA, M.A.

IT is easy to accept a plausible theory, based on a certain amount of observation, and to be so satisfied with it that further observation of the facts or testing of tho theory is deemed unnecessary. An example is the general acceptance of the theory of "eclipse" plumage in duck species of which the drakes have gaily-coloured plumage. It can be readily observed that a mallard drake, in mid-summer, begins to shed the feathers that have given him a bright green head, chestnut breast and curly black tail. In their place grow others of dull brown and buff, and the resulting plumage is much like that of the duck. Flightless Period At the same time the annual moult of wing quills begins, and at one stago of it both clucks and drakes may bo almost flightless. From the fact that tho period of inconspicuous plumage coincides with the comparatively helpless period of wing-moult, it has been suggested that the change of bodyplumage has survival value and has been preserved by natural selection. Now, it need not bo doubted that the possession of dull plumage when wings are moulting is advantageous, but it is not a necessary corollary to hold that the phenomenon of double body-moult had its origin in such advantage, nor that it persists because of it. New Zealand "Greys"

Yet so strong has been the influence of this attractive theory that it has been assumed that double moult does not occur in females, nor in either sex when both alike ha\e

dull plumage; the New Zealand grcv duck is an example in the latter class, tn nearly a century of writings about New Zealand ducks there is no mention of eclipse plumage, even though the drakes of such native species as the shoveller have a bright and striking plumage. Australian records are equally silent on the point, and ns recently as four years ago a world authority stated that the phenomenon of "eclipse" plumage was unknown in the southern hemisphere. Mr. Stead's Observations Actually the occurrence of n decided "eclipse'' moult in southern hemisphere species has been casually noticed in recent years by one or two observer#, but it has been left for Mr. E. F. Stead, by simple experiment and continuous observation over a period, to show thftt a double moult occurs annually in every native duck, in imported geese and swans as well, and is probably universal in the family. The "eclipse" of brightlyplumaged drakes therefore takes its place as part of a family tradition of double moult. It is the development of striking plumage in males of somo species that is the specialised feature, not tho change to ancestral dull pattern. Important Work

In spite of an uncomfortable feeling that these simple and important facts should have been discovered long ago, there is room for satisfaction that an independent and straightforward piece

Ducks And Their Plumage

of work in New Zenland should have such an important bearing on accepted theories. Mr. Stead's experiments were of such a nature that they can be repeated in any part of the world, and the validity of his conclusions thus tested. The}' are described in two papers recently published in the Transactions of tho Royal Society of New Zealand, but may be briefly summarised here. Careful Watching The preliminary work was done with grey duck kept for breeding purposes by tho Auckland Acclimatisation Society. On January 18 some pairs had just completed a post nuptial moult, and Mr. Stead clipped the ends of new feathers on several parts of the bodies, including wings, of two drakes and a duck. These birds then had their legs ringed, were carefully watched by Mr. F. G. Newell, and examined again by Mr. Stead on August 11. New feathers began to replace some of the clipped ones within six weeks, and by the final date of observation all had been replaced except the clipped wing feathers. An extensive body-moult had therefore taken place in both drakes and ducks, the new feathers having the same colour and pattern as the old. Male and Female During the following shooting season in Canterbury Mr. Stead Mas able to examine about 30 specimens, male and female, of Paradise duck, and found that every one of them had new contour feathers growing during the month of May. It is interesting to note that in this species the female has a more conspicuous colour pattern than the male. She has a white head and bright

chestnut body-plumage, and in the postnuptial moult the latter is, to some extent, replaced by speckled grey. The so-called "eclipse" is here a feature of the female moult sequence. Wild White Swans A tame Canada goose in Mr. Stead'x garden was next submitted to the clipping experiment and showed clear evidence of a double moult, but again without change of pattern. At the end of April wild white swans were caught and examined, and again new growing feathers were found. Of moulted feathers floating or lying about some were frayed and some not; the former no doubt shed after months of wear at the post-nuptial moult and the latter after only a few weeks weai at the pre-nuptial, which was in progress. Wading Birds Mr. Stead puts forward no theory to account for double moult. He hasshown it to be probably universal in the duck family, and it is alreadv known to be a characteristic of wading birds like the godwit, and of terns. It is significant that these three groups appear to be of Arctic origin, and artall notable as long-distance migrants. It is tempting to correlate a double moult annually with tho habit ol transequatorial migration, and to assume that all the sedentary specie* were originally migratory and have retained the ancestral moult sequence although they no longer migrate.