Studies on plumage in the male brown Leghorn fowl

Abstract

Apart from the tips of a few wing flight feathers the entire feathering of the newly hatched chick consists of down.This is structuralli similar on all regions, but the .colour differences exhibited result in a characteristic pattern. This is Trite unrelated to the regional pattern developed in the adult bird.The replacement of down by definitive chick feathers, begins immediately and is initiated in the different body regions in a definite sequence. Within these' regions a constant .order of emergence also obtains. Before the down has been completely replaced chick feathering gives way to the juvenile type and this appears in a continuance of the serial order until the pterylae have developed their full complement of feathers.After this, moulting of the chick feathers and their replacement by juvenile ones begins, generally in the same orderly manner.The chick plumage is characterised by a black and drab colouration, and the full development of barbules along the barbs (except in the feathers of the thigh where semiplunnes are present). The red pigmentation, typically exhibited in the feathers of both juvenile and adult males is never visible at this stage.The onset of juvenile feathering is first visible at about six weeks of age; this phase, arbitrarily defined to include all feather types between the chick plumage and the adult form, comprises a varying number of generations due to variations in moulting rate and the seriation of feather replacement. The generations do not form discrete types but represent intermediate points in the continuous gradually changing character of the feathers between first juvenile and adult plumage.The feathers of this group are characterised by a black and red pigmentation, and by .the gradual appearance of,fringing in some regions in successive generations. The proportion of melanin, at first extremely high, becomes progressively reduced as maturity is approached. Variations ih the degree to which these changes occur in the different regions of the body lead to the assumption of distinctive feather types in the various areas. This regionalisation also becomes more pronounced with age.Consideration of these phenomena, led to the conclusion that either (1) some stimulus influencing feather morphology was undergoing a progressive change throughout the immature life of the bird, or (2) there was a.' continuous alteration in feather response to a. constant stimulus.From the reports of earlier workers it ap ,eared possible that the thyroid hormone was the stimulus involved, and a histological study of ti' -is gland in growing chicks at various ages disclosed appearances which have been interpreted as indicating 4 decreasing activity between the - ages of 8 weeks and maturity. This is in line with a suggestion made that the juvenile plumage reflected a decreasing level of thyroid activity.In order to investigate this question further a series of experiments on thyroidectomy and thyroid feeding were undertaken.In juvenile and adult males thyroidectomy tends to reduce the contour feathers to a uniform type of red barbuleless feathers with pointed tips. The number of feather barbs is also decreased, and the time-required for completion of growth greatly prolonged. The non -viability of thyroidectomised baby chicks rendered impossible the determination of the effect of the operation on the first definitive plumage.Thyroid administration caused no change in the first definitive plumage or in -the first generation of juvenile plumage. In the second and third juvenile stages feathers similar to the first generation resulted. In the adult,plumage developed following this treatment, was also juvenile in type: the tendency was towards a black unfringed type of feathering; the number of barbs per feather appeared unaltered but feather growth was completed more rapidly than normal.By administration of graded doses of thyroid substance it was possible to reproduce in Mature cocks plumages resembling those of the sewn and third juvenile generation. Similar treatment of six -week old males which had previously had their thyroid glands removed resulted in the exhibition of plumage of the adult type .The results support the hypothesis that that sequence of juvenile and adult feather types exhibited by the growing male express a gradually lessening degree of hyperthyroidism, due to a gradual decrease in the level of thyroid activity or to changing response in the feathers. It is uncertain from the experiments whether the first definitive plumage fits into this series, or whether thyroid is involved at all in its expressionThe variations in feather reaction in the different body regions are consistent with the view of earlier workers that they represent varying degrees .of response to thyroid. An attempt to correlate them with the relative rates of feather growth in these regions proved abortive.