Changes

===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===