Feather - Anatomy & Physiology

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Functions of the Feather

Principal functions:

• Flight
• Insulation
• Waterproofing

Additional functions

• Courtship displays
• Defence

Cryptic colouring (camouflage)

Colour mimicking

Dominance/subdominance conflicts

• Aggressive territorial behaviour

Anatomy and Physiology of the Feather

In most birds, the body is divided into areas that contain feather tracts, pterylae and areas that do not, apteriae. The location and the number of tracts varies between species. By originating from tracts rather than being randomly distributed, feathers can smoothly overlap and conform to the natural contours of the body. By laying feathers over apteriae, air can be trapped beneath, providing insulation.

The portions of the feather can be decribed as follows:

• Calamus: The short, hollow, tubular, unpigmented end of the mature feather inserted into the feather follicle and thus present below the skin level.
• Rachis: The long solid, tubular extension of the calamus above the skin. The rachis contains pith, which is composed of air-filled keratinised epithelial cells surrounded by a solid keratinised outer cortex.
• Shaft: The combination of the rachis and the calamus.
• Vane or Vexillum: The portion of the feather that extends to either side of the rachis and is composed of the barbs and their associated structures. The vane is either plumulaceous (soft and downy) or pennaceous (compact and closely knit) depending on the individual type of feather.

Types of Feather

Feathers are characterised and categorised by the structure of the rachis, barbs and barbules. There are 10 types recognised.

• Contour feathers: these are the predominant feather covering the body.
• Coverts: are small contour feathers of the wing and the tail.
• Remiges: are large stiff feathers of the wing.
• Retrices: are large stiff flight feathers of the tail.
• Specialised adapted feathers: include semiplumes, hypopnea, filiplumes and bristles.
• Down feathers: can be subcategorised into adult, natal and powder down. Powder down are specialised down feathers that disintegrate and produce a powder (keratin) that is spread through the feathers during preening. They are found throughout the body among the down and contour feathers.

Feather growth

Feathers arise from feather follicles. These are formes by invaginations of the skin and closely fit the calamus. The feather follicle is analogous with the hair follicle in mammals. As in mammals, the follicle consists of epidermal and dermal cells and is lined with living and cornified epidermal cells. At the umbilicus of the calamus, the dermis carries a hump of pulp into the tip of the calamus. The follicular wall has an abundant supply of sensory nerve fibres, and the papillae, pulp and feather muscles are also well innervated. Smooth muscles at the base of the feather follicles help maintain body temperature by increasing or decreasing the elevation of the feathers from the skin. Herbst's corpuscles at the base of the feather follicles are believed to detect subtle ground vibrations and changes in air current.