THE PROBLEM OF THE MOULT IN THE CASTRATED BROWN LEGHORN FOWL

¹ Institute of Animal Genetics, University of Edinburgh

From the observations that have been recorded here on the shedding of feathers from the plumage of fowls it can be concluded that capons do not behave in the same way as normal males and females. They exhibit a tendency, to an appreciable extent, to cast feathers in the winter period, a season of the year when normal birds retain their feathering.

With the onset of spring an intensification of the moulting process sets in and feather loss reaches its maximum in the May-June period, and this may be taken as the appearance of a true moult in such birds. Females, on the other hand, characteristically enter the moult much later in the year—autumn and early winter—as determined by the cessation of egg production and coincident casting of feathers. In normal males, although no accurate determination of moulting phenomena has so far been made, casual observations over a number of years suggest that they tend to moult at a time intermediate between that shown by castrates and females respectively.

The data discussed lend support to the hypothesis previously put forward by Greenwood that in the fowl reproductive activity inhibits the onset of the moult, and only when this activity ceases, or continues at a lower level, can the casting and renewing of feathers occur.

The investigation failed to provide evidence in support of the conclusion of Benoit [1929] that the capon does not exhibit a true annual moult, although agreement is reached with him on the point that feathers continue to grow and are replaced throughout the whole year. The contention of Pézard [1918] that capons behave as normal males received no confirmation, and the view of Zawadowsky [1922] that castrated males moult twice a year was not borne out, neither by reference to the total number of feathers shed by these birds in the course of a full year nor by the rate of feather shedding. His explanation of the phenomena would only be valid if confusion had arisen with regard to the moult as it affected one particular region only, the neck, an area which, even in normal birds, may exhibit a double moult annually.

A further point of interest which was brought out in this study is the appearance of distinct and characteristic rhythms in feather casting in various regions of the plumage. It has been suggested tentatively that an explanation of this may be found by attributing to the expression of a causal relationship the known sensitivity of certain feather types to thyroid stimulation with the variable character of the moult as it affects certain regions. Among many workers Giacomini [1924] and Zawadowsky [1925] have shown that a generalised moult may be obtained in birds by feeding a heavy single dose of thyroid substance. Moulting of the plumage follows as a direct result of hypophysectomy in birds [Hill, Corkhill and Parkes, 1924], but while the primary factor here may be the removal of the pituitary gland it is significant that the new feathers subsequently growing in indicated by their colour and structure a marked inactivity of the thyroid glands.