Feather: Development and Uses

Development of feather

Feathers are supposed to be modified reptilian scales such that a bird is often described as a feathered biped. 

The body of the bird is invested with the close covering of feathers constituting the plumage. This covering of feather is arranged and designed in a way, giving birds a streamlined body and making feathers a major feature of morphological flight adaptation in birds.

Feathers are an outgrowth of special feather follicles, which are derived from the bird's epithelium and dermis layers of the skin. The development of feathers occurs out of the bird's skin very much in the same way as hairs or scales do in mammals and reptiles.

Development of feather diagram
Stages of development of contour feather: A- dermal papilla, B- feather papilla in V.S, C- feather germ elongated, D- stereogram of a later stage in V.S, E- mature contour feather.

The skin or integument of birds ( and other vertebrates ) consists of two parts, the dermis and the epidermis. The dermis ( inner part ) is the nutritive tissue carrying blood vessels. The epidermis ( outer part ) consists of numerous cell layers :

• innermost layers - live stratum germinativum, producing new cells outwards,
• outermost layers - dead stratum corneum, consisting of horny flattened dead cells.

In the formation of a feather, the dermis becomes the pulp that supplies the nutrition and pigment materials, 

• the stratum corneum forms the feather structure and encasing sheath called the periderm and, 
• the stratum germinativum forms a ring of cells, the germinating ring at the base of the feather immediately surrounding the inferior umbilical.

Development of a down feather ( Neossoptile )

The development starts about the fifth or 6th day of incubation when the feather rises on the delicate and still semi-transparent skin of the embryo as tiny papillae from which feather will develop are called dermal papilla .

Epidermal cells in the outside layers of the papilla then begin to differentiate into structures that become the protective sheath surrounding the growing feather and into the feather itself. 

This layer of epidermis is called the malpighian layer consisting of stratum corneum, and stratum malpighii. This growing papilla quickly increases in size and takes the shape of a cone with an apex directed backwards. 

The base of the papilla then sinks into a gradually increasing maot or feather follicle from which it projects as an elongated feather germ.

The distal part of the germ is covered by a malpighian layer which forms a series of vertical radiating ridges forming the barbs or rami. 

The lower part of the germ remains intact and thickens forming the quill.

The stratum corneum ( horny epidermal layer ) forms the transparent periderm enclosing the growing feather.

The dermal mass forms the pulp.

The pulp provides nutrients and colour to the growing feather as it contains blood vessels.

The temporary horny sheath splits and peels off liberating the barbs which are almost parallel yet slightly divergent.

Next, by secondary splitting, the barbules are formed. This process forms the rudiments of the down feather or the plumule having several barbs at the distal end of the quill.

   By the time the growth of the feather is completed the entire periderm above the follicle has disintegrated into a powdery material the barbs have spread out from their base on the calamus; the pulp in the calamus has dried up and the inferior umbilicus no longer functions as an entrance for nutrition. 

Deep in the follicle below the base of the calamus, there remains a small papilla that will give rise to the teleoptile.

Development of the contour feather  

The contour feather develops from a deeper level papilla, such that the earlier down feather drops off from the apex of the later.

The development of contour feathers is the same as down feathers, but here the ridges of the Malpighian layer do not remain of one size, instead two ridges in the middle of the dorsal side fuse to form the rachis. They are longer due to differential growth. 

The rachis during elongation,  carries with it the neighbouring barbs, which then become the lateral barbs.

In a similar way, the fusion of two ventral barbs with differential growth results in the formation of the after-shaft or hyporachis.

The umbilical groove is thus the fusion of two sides of the collar of the malpighian layer.

By moulting, the new feather papilla pushes up into the base of the older feather, eventually forcing the older feather out of the follicle altogether. The birds periodically shed out the old feathers and by the growth of new feathers, the bird acquires new plumage.

The different colours are imparted by the deposition of pigments. The colours may change, fade out or de-colour during its life.

In pennaceous feathers, the bases of the barbs migrate towards the central rachis of the feather. If this does not happen, the barbs remain attached only to the calamus at the base of the feather and a plumulaceous or downy feather results. As the feather develops the vane rolls within the feather sheath, and the whole feather sheath pushes up through the skin.

*Note :

The barbules, flanges, barbicels and hooklets of the teleoptile develop by fraying out of the barbs after the feather opens up. If a developing feather is to possess an after shaft, a process extends outwards from the ring on the mid-ventral side of the papilla, opposite the growing rachis and carries adjacent barbs with it. The subsequent growth of the after shaft is identical to that of the main feather.

Uses of feathers 

( Related question: Give an account of the functions of feathers in birds. )

Some of the most important functions served by a feather are,

1. 

Protection  

Feathers serve as a lightweight, impervious, flexible, durable and resistant body covering. The tender skin is protected from any type of mechanical or physiological wear and tear by the feathers.

2. 

Heat retention  

The most important physiological function of feathers is heat retention. Birds body temperature ranges from 104°F to 112°F, even in subzero weather.

Dead air spaces of the plumage form an efficient non-conducting covering giving insulation. 

During cold weather, fluffing out of feathers reduces heat loss to a minimum. This happens, as the process of fluffing increases the depth of insulating material by adding air spaces within the feathery layers.

Conversely, in warm weather, the feathers are often depressed or held close to the body to allow some escape from body heat.

3.

For flight 

Feathers are organs of flight, as they furnish the bird with admirably lightweight, elastic and horny structures. 

The wing and the tail feathers are thin, flat and overlapping, forming a surface almost airtight by the linkages of barbules and the hooklets to support the bird in flight.

4.

To camouflage ( protective colouration ) 

 Camouflaging with the habitual surroundings renders the bird to remain inconspicuous and guards them against their natural predators.

5.

For ornamentation, sex determination and courtship

The feathers are modified on some parts of the body and serve as decorative or ornamental functions, utilized for courtship displays. Usually, male birds have more such feathers than females which helps them attract the females. 

6.

Natal covering   

Down feathers, especially in some birds like pheasants, ducklings have special importance as natal covering in an advanced state of development. Also, it acts as a dense undercoat in many aquatic birds.

7.

Formation of Nest 

Feathers are mostly used in the formation of nests as they conserve heat around the nestling also making it comfortable for the brooding parent.

8.

Additional uses 

a. According to some ornithologists, the peculiar powder produced by the disintegration of the powder down in herons, bitterns and other birds is useful in maintaining the feather and checking the multiplication of ectoparasites.

b. Other feathers  ( filoplumes ) form eyelashes and some are present near the base of the beak. The spiny rectrices of chimney swifts and woodpeckers facilitate clinging to vertical surfaces.

Feathers are used by humans in many ways some countries export millions of pounds from all over the world every year. They are used for,

• As pens for writing and drawing. ( The word pen means " feather " in Latin. )
• For making darts for games.
• In wars, feathers are used to camouflage.
• After, their usage in industries, the dirt of feathers makes fertilizer for crops.