The skin of birds is thin, dry and yellow-white in colour, with few vessels and nerve endings; so it tears easily without apparent haemorrhage or pain.
Subcutaneous injections can be administered at the level of the axillary and inguinal skin folds, or in the dorsal region of the neck.
Although the epidermis is thin in all feathered areas, it is condensed and keratinised in certain places, forming structures such as the rhamphothecae of the beak,
the claws or talons
and the spur
which are present in some species on the medial aspect of the tarsometatarsal region. At this level the epidermis is also modified and forms scales,
similar to those in reptiles. However, without doubt the most characteristic development of the skin is the presence of feathers.
Feathers are specialised epidermal structures, without no living cells and very little keratinisation or mineralisation.
Feathers have a diverse range of functions:
helping to control body temperature; acting as an aerodynamic force during
flight; colouring allows camouflage or communication between different individuals
etc. In the adult bird there are three main types of feathers:
a) Contour feather: remiges, rectrices, coverts and tectrices,
remiges (primary and secondary)
b) Down feathers: Small feathers overlaid by the contour feathers.
Psittacines have a special kind of down feather; dusty down feathers,
c) Filoplume: related to proprioception.
Feathers are distributed over the body and are arranged into specific areas called pterylae, in between which there are bare areas or apteria.
Recognising the pterylae and apteria is important in the case of any surgical intervention in order to avoid damage to the feather follicles. A typical feather
is composed of the calamus (proximal part implanted in the follicle) and rachis (distal part). On either side of the rachis is a row of parallel barbes, which form the vane.
A distal umbilicus is found on the proximal end of the calamus, the area where the papilla of the feather is enclosed by blood vessels and nerves.
Both smooth and skeletal muscles (patagial tensor muscles, tail muscles) control feather movement. Birds shed their feathers once a year, often in summer and autumn. Some species lose all their feathers at the same time, while others do it gradually. Parrots usually change their plumage throughout the year. The immune response is reduced during shedding;
feathers which are accidentally pulled out,
with the exception if primary and secondary remiges, usually re-grow in 2-4
weeks if the follicle isn’t damaged. However, cut feathers do not regrow
until the next shedding.
Grafting of essential feathers for flight (remiges and rectrices) is possible because of the internal structure of the rachis.
The plantar surface of the digits and metatarsophalangeal joints is covered by elevated pads (poorly vascularised adipose tissue).
Comb, wattles, ricti and ear lobes are ornamental appendages.
In these areas the dermis is thickened and highly vascularised, whilst the epidermis is very thin and easily damaged, which can lead to large bleeds. The skin does not contain sebaceous or sweat glands, with the exception of those present in the external ear canal (sebaceous)
and the so called uropygeal gland.
This consists of a body with two lobes, situated underneath the skin, adjacent to the pygostyle.
The gland secretion is a sebum rich in wax and oils which the bird smears over the feathers cleaning them and waterproofing them. Some components of the sebum are transformed into activated vitamin D3 upon exposure to sunlight, which is then ingested by the bird whilst preening. The uropygial gland is not found in all birds; it is absent in certain strains of pigeon, many parrots, emus, ostriches and woodpeckers. Subcutaneous connective tissue is scarce and this means that the accumulation of fat in certain regions (thorax and abdomen) is common.
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