White Blackbirds

New Zealand Herald, Volume LXXVI, Issue 23381, 24 June 1939, Page 10 (Supplement)

White Blackbirds

By A. G. STEVENSON

SOME time ago I saw- a blackbird that had departed somewhat from the usual sombre colour pattern peculiar to its kind. Several of its feathers were snowwhite, and in addition it was irregularly sprinkled in parts with small white dots, giving it a most unusual appearance. This is by no means the first one I have seen, and as a matter of fact blackbirds rather commonly exhibit this tendency to a I greater or lesser degree. Sonic of them even achieve a purewhite effect and all intermediate stages are to bo met with. But pure-white blackbirds arc rare and the pied or speckled form is the one usually seen. Some years ago we had one which could be seen season after season regularly frequenting the same corner of our garden, and easily identified by a large patch of white upon one wing. Starling in Museum Among the other introduced birds white starlings are sometimes met with and a perfect example may be seen in the Auckland War Memorial Museum. Its plumage shows no traco of colour, its eyes are pink and its bill and legs are yellow. Pheasants, too, may exhibit all

stages from tho natural colour down to a pure-white. Sometimes a small patch of feathers will remain quite unaffected, while all the rest have lost their colour completely. The red area seems invariably to remain around the eye, however, and indeed red appears to be a colour much less subject to change than any other. Our native birds seem to show a general tendency to produce occasional whito forms, but on tho other hand there appear to be certain species which never do so. Several white kiwis have been recorded, and visitors to our museum may seo also a whito kingfisher, whito crows and even a whito penguin. Watering Down Process It is a peculiar fact that most purewhite forms are produced by birds that are normally black or grey, and are almost unknown among the green birds. Every shade from black to white may be met with, according to the varying degree of shortage of black pigment, which perforce has to be distributed more sparingly. In England starlings and rooks often exhibit this watering down process. We commonly call the white forms albinos, but in most cases they are really only white varieties, as evidenced by the fact that they retain the normal colouration of the eyes, the bill and tho feet. Just why white varieties should be produced is not yet fully understood, but an intensive study of the subject has brought to light many interesting facts. The white colour is usually due to the absence of pigment in the feathers. The small spaces which it occupied are filled with minute air-bubbles which produce a whito effect by reflection of the light, just as they do in soap lather or sea foam. Absence of colour does not usually affect the pattern of tho plumage, and in white peacocks, for instance, all the characteristic markings of tho feathers are still discernible. Sometimes a bird with whito plumage will achieve its normal colouration after a moult, but in most cases the tendency toward whiteness or partial whiteness seems to be congenital, and individuals crop lip hero and there among the olfspring of apparently normal parents

What Causes

In these ordinary white varieties, as shown by the blackbird, for instance, the eyes and some other parts usually retain their proper. colouring, but in true albinos even these parts are devoid of pigment. The eyes are pink, the absence of pigment in the iris allowing the blood to show through. Without the protection of the lightabsorbing pigment, the eyes are generally weak, and this puts the albino at a distinct disadvantage, especially if it lives in strong sunlight. Albinism is liable to appear in most animals and plants, as well as in man. White rats or rabbits, with their pink eyes, are familiar to all, and partial albinism may be seen in such animals as the arctic fox and in some birds. The summer coat or plumage of these is normally* pigmented, but in the winter it is replaced by white. Apparently changes of temperature have something to do with this and, in fact, experiments in exposing young Himalayan rabbits to varying degrees of cold have resulted, in "the production of black or white fur as required. Glandular Activity In the case of some birds it is thought possible that seasonal changes affect glandular activity, which appears to have some influence on the amount of pigment in the feathers. These seasonal changes to whiteness do not constitute true albinism, however, for there is always some colour remain-

ing. We might rather consider them as examples of concealing colouration, enabling the creatures to efface themselves in thp snow. True albinos are animals, or birds or plants, which for some reason or other lack one or both of the two factors essential to the production of colour. If we breed albinos together, the offspring will be albino, because parents lacking the essential factors cannot transmit these to the next generation. By crossing any of these albinos with a normal individual we will introduce the colour developing factor and so produce a generation of coloured individuals. But breeding from these latter gives a rather surprising result. Threequarters of the next generation will bo coloured, but the remainder will be albino again. This points to a congenital lack of certain factors, and to understand it properly we must refer to the work of Mendel, a monk who made extensive experiments with the groupings of colour and other factors in common peas. Mendelian Theory From his results he was able to build up what we now call the Mendelian theory of hereditj', a theory which with certain extensions is commonly accepted to-day. Experiments with animals only, helped to prove his theory that in order to produce a given characteristic the individual must be possessed of certain given factors. Breeding from two individuals brings about a collecting and resorting of their various factors and results in each of the offspring displaying characteristics according to the factors received from its parents and at the same time lacking in others. it is all very complicated, but most interesting, and the results work out with something like mathematical precision. To-day breeders of birds, . animals or plants are able to forecast fairly accurately what will be the result of any given cross. A strain of white rabbits or white peacocks may be perpetuated by careful selection, but in nature true albinos are rare, because it would be very seldom indeed that two individuals both lacking the colour-developing factor would unite.