Changes

==Introduction==

==Introduction==

Feathers are one of the epidermal growths that form the distinctive outer covering, or '''plumage''' of birds.  They are the outstanding characteristic that distinguishes the Class '''Aves''' from all other living groups.  Feathers ar grouped into feather ''tracts''.

Feathers are one of the epidermal growths that form the distinctive outer covering, or '''plumage''' of birds.  They are the outstanding characteristic that distinguishes the Class '''Aves''' from all other living groups.   

==Functions of the Feather==

==Structure & Function==

'''Principal functions:'''

'''Principal functions:'''

*Flight

*Flight

'''Additional functions'''

'''Additional functions'''

*Courtship displays

*Courtship displays

*Defence

*Defence

Cryptic colouring (camouflage)

*Cryptic colouring (camouflage)

The portions of the feather can be described as follows:

 

 

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.

*'''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.

*'''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.

*'''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.

*'''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===

===Types of Feather===

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

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.

*'''Contour feathers''': these are the predominant feather covering the body.

*'''Coverts''': are small contour feathers of the wing and the tail.

*'''Coverts''': are small contour feathers of the wing and the tail.

*'''Remiges''': are large stiff feathers of the wing.

*'''Remiges''': are large stiff flight feathers of the wing.

*'''Retrices''': are large stiff flight feathers of the tail.

*'''Retrices''': are large stiff flight feathers of the tail.

*'''Specialised adapted feathers''': include semiplumes, hypopnea, filiplumes and bristles.

*'''Specialised adapted feathers''': include semiplumes (enhance thermal isulation, enable courtship), hypopnea, filoplumes (hairless feathers than grow along down feathers and monitor the position of adjacent feathers) and bristles (stiff feathers with sensory and protective functions).

*'''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.

*'''Down feathers''': help insulate the bird and contributes to buoyancy. They can be subcategorised into body, 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.

==Physiology of the Feather==

==Physiology of the Feather==

===Feather growth===

===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 - Anatomy & Physiology#Hair Types|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.  [[Skin - Anatomy & Physiology#Beak|Herbst's corpuscles]] at the base of the feather follicles are believed to detect subtle ground vibrations and changes in air current.

Feathers arise from feather follicles.  These are formed by invaginations of the skin and closely fit the calamus.  The feather follicle is analogous with the [[Hair - Anatomy & Physiology#Hair Types|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.  [[Skin - Anatomy & Physiology#Beak|Herbst's corpuscles]] at the base of the feather follicles are believed to detect subtle ground vibrations and changes in air current.

===Feather colour===

===Feather colour===

Feather colour is determined by two factors:

Feather colour is determined by two factors:

*'''Biochromes''':  

*'''Biochromes''':  

Colour pigments that are deposited at the time of feather development.  These can be naturally occurring, e.g. food derived carotenoids in unaltered or modifed states, or synthetically produced, e.g. melanins (browns, greens and black).

Colour pigments that are deposited at the time of feather development.  These can be naturally occurring, or synthetically produced. Black, brown and grey colours come from melanin, whereas carotenoids in the diet create red, orange and yellow and porphyrins synthesised by the bird form green, red and some browns.

*'''Structural properties''':

*'''Structural properties''':

These structural features of the feather can be inherent in the development of the feather.  They can also be induced by materials that are placed on the feathers after development, that alter the absorption or reflection of light.

These structural features of the feather can be inherent in the development of the feather.  They can also be induced by materials that are placed on the feathers after development, that alter the absorption or reflection of light.

If a feather reflects all light, it appears white; if it absorbs all wavelengths, it appears black.  The capacity of the barbs and barbules to scatter and reflect varying wavelengths of light causes and irridescent glow of the feathers.  Blue colours are created by the barbs interacting to reflect blue light, while allowing other wavelenghts of light to be absorbed by darker melanin granules.  Green colours may be created by pigments, or more commonly, through a combination of blue and yellow pigments.

If a feather reflects all light, it appears white; if it absorbs all wavelengths, it appears black.  The capacity of the barbs and barbules to scatter and reflect varying wavelengths of light causes an irridescent glow of the feathers.  Blue colours are created by the barbs interacting to reflect blue light, while allowing other wavelengths of light to be absorbed by darker melanin granules.  Green colours may be created by pigments, or more commonly, through a combination of blue and yellow pigments.

===Colour mutations===

===Colour mutations===

*Species

*Species

*Sex

*Sex

*Time of year

*Time of year (including temperature and hours of light)

*Breeding patterns

*Breeding patterns

The moult may be:  

The moult may be:  

'''Fret marks''' (stress lines) appear as clear breaks in the feather colour as a result of interruption in feather growth and malformation of the feather barbs.  Nutritional, environmental, disease or drug stresses can result in the production of 'fret marks'.

'''Fret marks''' (stress lines) appear as clear breaks in the feather colour as a result of interruption in feather growth and malformation of the feather barbs.  Nutritional, environmental, disease or drug stresses can result in the production of 'fret marks'.