. The biology of birds. Birds. 20 THE BIOLOGY OF BIRDS In powder-downs the calamus forms no rhachis, but splits into barbs and barbules which disintegrate into ap- parently greasy powder. They occur in many birds, some- times in patches as in herons and bitterns, sometimes in scattered tracts and tufts as in parrots, the roller, and the immature Lammergeier (Gypaetus). In the bittern the tufted calamus is about half an inch long ; in the more-pork. Fig. 3.—Different kinds of feathers down-feather, with aftershaft () ; 3, feather with free barbs : shaft ()- I, filoplume ; 2, a permanent
. The biology of birds. Birds. 20 THE BIOLOGY OF BIRDS In powder-downs the calamus forms no rhachis, but splits into barbs and barbules which disintegrate into ap- parently greasy powder. They occur in many birds, some- times in patches as in herons and bitterns, sometimes in scattered tracts and tufts as in parrots, the roller, and the immature Lammergeier (Gypaetus). In the bittern the tufted calamus is about half an inch long ; in the more-pork. Fig. 3.—Different kinds of feathers down-feather, with aftershaft () ; 3, feather with free barbs : shaft ()- I, filoplume ; 2, a permanent nestling down-feather; 4, 5, an emeu's feather with very long after- (Podargus) the maximum of two inches is reached ; but in most cases the powder-downs are very small. Their use is obscure. It has been suggested that they help to keep the plumage clean and in good condition—a sort of feather- powder. Powder-down feathers are peculiarly specialised down- feathers, but they do not seem to be the same in every case. The powder is formed by the degeneration of the cellular sheath covering the barbs, and Schaub has shown (1907). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Thomson, J. Arthur (John Arthur), 1861-1933. New York : Macmillan
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbirds, bookyear1923
