COLOURS AND PATTERNS OF BIRDS 105 



common moorhen has a greenish lustre. The down of young owls 

 is usually pure white. In most of the birds-of-prey the down is 

 uniform ; in ospreys it is clay-coloured ; in the condor the head is 

 naked, but the body is covered after a few days with a thin coat 

 of white down. In vultures it is usually white, but may be yel- 

 lowish or picked out with black on the wings as in the American 

 black vulture, whilst in most of the eagles it is dirty yellow. In 

 birds which never acquire a real coat of down, but which are naked 

 except for a few hair-like tufts, these remnants are white or pale. 



The down covering of many young birds shows a conspicuous 

 pattern of either spots or stripes, spots elongating to form stripes, or 

 stripes breaking up to form spots. These patterns resemble in a 

 most striking way the simple growth patterns to which I called 

 attention in the case of young mammals. The arrangement is 

 usually one that recalls the simple kind of bilateral patterns made by 

 squeezing ink marks in folded paper (see Fig. 21, p. 65), and although 

 the result may sometimes be of use in helping to make the young 

 birds less conspicuous against the background, when they are squat- 

 ting in the sunlight amidst reeds and other vegetation, or on a 

 pebbly beach, they occur so constantly in many different groups of 

 different habits that I find it difficult to think of them as special 

 adaptations. They appear to be the more or less inevitable result 

 of the particulate character of the skin and of the mode of growth. 

 They have been retained in cases where they are either useful or 

 harmless, but they are survivals of an ancestral or primitive con- 

 dition which have been preserved, rather than new creations for the 

 special benefit of their possessors. Moreover, in many of the self- 

 coloured chicks there are faint indications of obliterated stripes 

 which would seem to show that the plain-coloured chicks have more 

 modern coverings than the striped and spotted forms. 



The nestling rhea or South American ostrich (Fig. 25, p. 107, right- 

 hand figure) is covered with a thick coat of long down feathers, dirty 

 grey on the head and under surface, but with a long dark brown patch 

 on the neck which forks over the wings, and is continued along the 

 middle line of the back as a diamond-shaped mark tapering off towards 

 the region of the tail. On each side of this a broad brown stripe runs 

 backwards from the wings towards the tail, whilst a second stripe at 

 each side runs along the outer surface of the thigh and leg. These 

 brown stripes leave the pale grey background between them as 

 narrow bands. In the young emu, the same general arrangement 

 of dark stripes on a light background is present, but the stripes are 



