Pigmentation and Growth Rate in Gray Breast Feathers of Adult Mallard Drakes

Abstract

Before the physiology of sexual dimorphism in birds can be completely understood the normal change from the male winter plumage to the female-like summer plumage in the Mallard drake must be explained. This report embodies certain facts regarding feather pigmentation and growth rate in Mallard drakes which must be considered in future studies of the physiological basis for this “eclipse” or “summer plumage” change of the adult male duck.

The males used in these experiments are semi-domesticated Mallards, the majority of which are genetically “dark-phase mallard” (M Li). 1 These drakes are very uniform in breeding, and in adult body weight, between 980 and 1160 gm. The males are left in outside yards until they are to be used in an experiment, when they are brought indoors and kept in artificially heated and lighted pens, the hours of illumination being approximately the same in all trials. At the beginning of the experiment, 50 to 60 feathers are plucked from the middle of the ventral surface on either side of the keel, the plucked area being in the same location on each drake. All trials were conducted between the months of September and the following April. At any time within this period the growing feathers normally show no detectable variation in pigmentation or growth rate.

The depth of color in the growing gray breast feathers may be increased artificially by 2 different treatments, oral administration of desiccated sheep thyroids (Armour & Co. preparation, 0.2% iodine), or subcutaneous injection of an extract of the urine of young men, prepared according to the method of Funk, Harrow and Lewja 2 for male hormone. The thyroid increases the pigmentation and the growth rate of the feathers, and the urine extract augments the pigment increase stimulated by the thyroid. The increase of pigmentation in the partially-grown gray breast feathers is recorded in the tables by direct comparison of the normal and modified feathers of each drake. Each plus symbol indicates a definite intensification of the melanic pigment. Feathers with 5 plus symbols, in addition to being very dark in color, have a definite edging of light brown and in this respect resemble the feathers of the eclipse (summer) plumage. Each drake was killed at the termination of the experiment and, in parts B and C of Table I, a certain number of growing feathers were removed for measurement.