Colours of feathers and their structural causes in varieties of the budgerigar, Melopsittacus undulatus (Shaw)
Several papers on the microscopical structure and the pigmentation of the feathers of the Budgerigar have already been published. This bird which is so easily bred and which, under selection, has developed a great number of varieties in two or three decades and practically from the beginning under scientific control, is an object of intense interest for the geneticists. Its varieties differ predominantly by their shades of colour. The subjective impression of a colour, however, is not a criterion sufficiently reliable for distinguishing the varieties. Partly for this reason, but chiefly in order to establish the effects of the genetical factors on both structure and pigment, the researchers included the microscopical examination of feathers and feather-germs in their field of work. Nevertheless, further research along these lines has been found desirable, chiefly for two reasons. First, since the publication of the papers referred to a number of new varieties has appeared with shades of colour very different from those known hitherto. These shades suggest that there may be very interesting changes in the structural and pigmentary factors which are responsible for the impression of colour. Secondly, these authors did not undertake a histological investigation of the feathers and feather-germs which could be considered exhaustive from a purely morphological point of view, as they were chiefly concerned with genetical problems. Moreover, these papers reveal differences in the histological interpretation of the examined material.