THE CAUSE OF THE PRODUCTION OF "DOWN" AND OTHER DOWN-LIKE STRUCTURES IN THE PLUMAGES OF BIRDS

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Abstract
1. The highly modified region of the "down" is in all respects similar to other feather defects or fault-bars which have already been described as occurring at any and all levels in definitive feathers.2. Juvenal feathers can by under-feeding be made to persist (chick) in the "downy" conditions practically without growth for several months if the reduced feeding be begun immediately after hatching.3. Occasionally feathers may be found which have been able to continue their growth proximal to the downy portion despite the inhibitory influences of the lack of food ; such feathers have been found to represent a type of structure intermediate to the downy and pennaceous formations.4. The cause of the modification at the base of the down is to be traced to an interruption, or at any rate to the inadequacy of the nutritive processes of the bird. This interruption is doubtless partly accounted for by the change of source of food from the embryonic to the adult life.5. The apparent absence of such growth-marks in the feathers of the duck and other anserine birds remains unexplained.6. The rate of growth of the two ends of a juvenal, pennaceous feather, and of the proximal ends of all pennaceous feathers which bear a plumulaceous proximal portion, is much slower than the rate of growth in the central pennaceous part of the feather.7. It seems probable that all plumulaceous structures are produced at a relatively slow rate of growth ; and also probable that during their growth they have not enjoyed optimum nutritive conditions.8. The formation of the quill is probably the direct result of a progressive diminution of an already lessened food-supply.

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