NUTRITION ▶▶▶
two series of parallel barbs, collectively called the feather vane, are attached. The barbs themselves have rows of interlocking bar- bules that give the feather its shape and rigidity. The vanes pro- vide its bulk, radiating out on each side of the central shaft. The vanes lock together with hooklets or barbules, and effectively maintain the characteristic shape of the feather. Abnormalities in barbule development can lead to a frizzled or ruffled appearance. The hair-like feathers easily seen on the bird following conven- tional plucking are known as filoplumes. It is thought that the filoplumes act as a sensory system to inform the bird of the gen- eral positioning of the body feathers. Preening is likely initiated by signals produced by the filoplumes.
Feather generations and moulting Four generations of feathers grow out of the same follicle, each successive feather pushing out the previous juvenile generation. The earliest juvenile feathers to appear are usually the wing feathers. The second moult usually begins at around 4-5 weeks and for the modern broiler chicken, this is the only moult of sig- nificance. Even at this time there will still be natal down in the head region and perhaps some around the abdomen. The head and neck regions are usually the last to receive each successive generation of feathers. In most birds, therefore, a ‘poorly’ feath- ered head and neck region relative to most other body parts is quite normal.
such as glucosinolates in rapeseed meal, has not been stud- ied. Castration or ovariectomy of any bird results in continu- ous growth and renewal of the entire plumage rather than an annual moult. Feather formation will be delayed by high levels of oestro- gen. In adult birds, the initial stages of feather growth are in- fluenced by thyroxine levels, as previously described, al- though development of the emerging feather is greatly influenced by oestrogen. In most birds the presence of oes- trogen results in the typical female feather shape, while an absence of oestrogen causes typical male feathering. Testos- terone per se usually has little effect on feathering and so it is the absence of oestrogen, rather than the presence of testos- terone, that causes typical adult male feathering, especially around the neck.
Feather characteristics Estimates of feather number and weight vary considerably since counting the number of feathers on a bird is obviously a tedious undertaking. By 14 days (d) of age the broiler has around 5g of feathers and by 24d this increases to 10g, with a further additional 10g per week through to 70d. There are three main types of feather that vary in structure, size and ap- pearance. Visible to the naked eye are the contour feathers that protect the bird against physical injury, although the wing and tail feathers of modern commercial strains of poul- try are now less well-developed since there is no need for flight. These contour feathers consist of a shaft onto which
Feather colour and attachment Most broilers have feathers that are white or brown or a mixture of these two colours. White feathering predominated from the 1960s due to problems with processing coloured-feathered birds due to observable residual pin feathers, etc. Feather colour results from an interaction between the skin cells that form the feather and a source of pigment which is usually melanin. Deposition of pigment can be continuous or discontinuous, leading to solid col- ours or various colour patterns. Melanin is the most common feather pigment, although combinations with carotenoids and porphyrins (shell pigments) lead to a vast range of colour poten- tial. Melanin is most often derived from the amino acid tyrosine which is abundant in most diets. The copper-containing enzyme tyrosinase is critical in melanin formation which is why copper deficiency often influences plummage colour or pattern. It is not clearly understood why feathers are so difficult to remove from the live bird since there is little evidence of direct muscular contraction around the base of the feather. Without some means of enhancing feather release, such as hot-water scald or electrical stimulation, the force required to remove individual feathers can be equivalent to 4 kilograms for the wing primaries. When birds are stressed they may shed some of their feathers. This is some- times referred to as a ‘fright-moult’ and occurs very quickly, sug- gesting a nervous reaction rather than one influenced by a hor- monal change such as would occur with natural moult. The ‘fright-moult’ may be an evolutionary response to escape from predators.
▶ POULTRY WORLD | No. 8, 2021 33
PHOTO: THEO GALEMA
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