Feathers : Structure and Types

 

Feathers: Structure and Types 

The bodies of the birds are covered by flattened tightly closed feathers that create aerodynamic surfaces such as those of wings and tails. 

All birds have feathers which are Characteristics of Aves and do not occur in any other groups of animals. They are light, elastic, waterproof and designed for flight. They develop from integumentary feather follicles which are confined to specific feather tracts.

The arrangement or distribution of feathers on the body of birds is known as pterylosis. It varies from one taxonomic group to another and some groups have unique patterns. 

In the flightless birds or Ratitae, feathers are uniformly distributed over the body but in Carinatae, they are arranged in distinct patches or feather tracts called pterylae ( singular: pteryla ) and in between them are featherless spaces called apteria ( singular: apterium ).

 

Each feather remains rooted in its follicles by a basal quill until it is moulted and replaced.

Feathers differ morphologically according to function.

Feathers lie all over the body and wings with impressive neatness which helps them produce the tidy streamlined cover of feathers that is crucial to a bird's survival.

Feather Structure 

All the feathers show a typical structure whether they are the, 

• Contour feathers from the general body covering or 
• the flight feathers present on the wings or tail.

Feather structure diagram 

A typical flight feather consists of a supporting central axis or main stem and an expanded distal portion, the vane or vexillum.

I.

Axis 

The axis is divided into a proximal lower portion called the calamus or the quill and a distal upper portion called the rachis or the shaft.

1.

Calamus 

Structure: The calamus is hollow, tubular semi-transparent.

Location: The base of the Calamus is inserted in the epidermal follicle of the skin. The calamus has a small opening at the lower end called an inferior umbilicus.

This opening receives, 

• muscle fibres to the feather affecting individual movements.
• Also receive a small conical nutritive papilla of the dermis, through which blood vessels enter to supply nutrients and pigments to the developing feather.

A second minute opening called the superior umbillicus, occurs at the junction of the quill and the rachis on the inner or the ventral side.

Also in many birds, there is a presence of a tuft of soft feathers called aftershaft or hyporachis near the superior umbillicus to cover it. Examples - are cassowary, emu and extinct Dinonris. 

2.

Rachis 

The rachis or the shaft forms the longitudinal axis of the vane. In the transverse section, it is a quadrangular structure as it has a longitudinal furrow or the umbilical groove running throughout the length. It is stiff, solid, opaque and filled with a closely packed mass of pith cells.

II.

Vane 

The membranous part on either side of the rachis is called the vane or vexillum. The rachis divides it into two unequal lateral halves and also its distal end is narrower than proximal.

Parts of vane:

Feather magnified structure diagram 

1.

Barbs

Barbs or rami,  are closely spaced, delicate, thread-like structures arising from either side of the rachis. The size of the barbs decreases moving towards the ends of the rachis.

2.

Barbules

The barbs give rise to still smaller and extremely delicate double rows of oblique filaments called the barbules or radii. There are two types of barbules,

a. Distal barbules, which are directed towards the tip of the feather and their lower edge produce minute barbicels (or hooklets or hamuli)

b. Proximal barbules are directed towards the base of the feather and their upper edge is deeply curled to form a groove or flange.

Because of the oblique placement, the barbules of adjacent barbs overlap and the hooklets or barbicels of the distal barbules become interlocked with the curved edges of the flanges of the proximal barbules.

This sliding interlocking arrangement of the barbs and barbules forms a flexible, firm, wide,  flat and continuous surface ideal for flight.

If needed this interlocking arrangement can be easily pulled apart. If the vane is stretched, at first it resists but eventually splits. They can again be restored by preening or pulling them by the beak.

This arrangement of feathers helps in flight such that the absence of this interlocking mechanism has resulted in the loss of flight in birds like ostriches and kiwis which have very fluffy feathers.

*Note: Aftershaft never possesses barbicels.

Types of feathers:

Types of feathers diagram 
A-Remex, B-Rectrix, C-Contour, D-Filoplume,
E-Down, F-Rictal bristles, G-Semiplume, H-Powder down 

Depending on form, function and location, feathers are divided into the following types. :

1.

 Quills or flight feathers :

These are typical flight feathers and thus have a strong shaft. Also shows the presence of barbules with interlocking arrangements. They are further divided as,

a.

Remiges ( singular -remex ) 

Remiges ( or pinions or wing quills )are the flight feathers of the wing, including Primaries, secondaries and tertiaries. The inner or the posterior half of the vane is slightly broader than the outer or anterior half. 

They are pennaceous contour feathers with prominent asymmetrical vanes. In ducks gallinaceous birds and owls, the ventral vane surface is partially modified into a Shiny firm structure formed by specialised tegmen feather barbs, which are believed to strengthen the vane and resist the flow of air upward through the vane surface.

b. 

Rectrices

Location :

The rectrices (singular-rectrix) are large, almost equal-sized vaned flight feathers of the tail. They are arranged in a semicircular fan-like manner on the tail. They are similar in structure to the remiges of the wings. In some groups like woodpeckers, the rectrices have been adapted and strengthened to act as props, helping birds remain vertical as they forage on the tree trunks. Swifts use similar stiff rectrices as an aid in perching on vertical surfaces. 

Function: The rectrices help the bird as a break during landing,  and steering the flight. Rectrices can also be the key to a female's heart, as when they are spread into an erect fab during courtship.

c. 

Coverts 

Coverts are smaller in size than quill feathers but are structurally similar.

Basis of the wing quills are covered by upper and under wing-coverts and the basis of tale quills are by upper and under tale-coverts.

Function :

Coverts cover the interstices between quills of flight feathers thus forming a continuous area to oppose the buoyancy of air.

2.

Contour feathers ( pennae ) 

Location :

The contour feathers form the general covering of the body by covering the base of the flight feathers.

They are typical in a structure consisting of a central axis and vane. The smaller contour feathers that cover the body have symmetrical vanes divided between a firm pennaceous distal vane area and a soft plumulaceous inner vane area. They are smaller and woolly feathers, having poorly developed barbules so that the barbs can be easily isolated.

Function : 

Contour feathers provide, 

• A smooth surfacing to the body and reduces the frictional resistance to motion through air.
•  Giving a good shape by overlapping of the vanes.
•  Also, forms a good heat-conserving layer.

* ( In short, they provide waterproofing, insulation and streamlining. )

3. 

Filoplumes ( hair feathers or pin feathers ) 

Location :

Filoplumes are long hair-like with few barbs at their tips, and feathers that are sparsely distributed over the body ( as seen in plucked pigeons ). 

It consists of a short calamus and a long thread-like rachis with a few weak barbs and barbules at the tip.

Function : 

• They monitor the position of the pennaceous feathers such as those of wings and tail.
•  Sensory corpuscles at the base of each filoplume detect the fine movements of the filoplume shaft.
• Filoplumes are often numerous based on wing remiges to monitor the position and movement of the remiges during flight.

In many passerines, they also protrude through the outer contour feathers of the crown and nape, perhaps warning the bird when wind disrupts the smooth outer surface of the plumage.

4. 

Semiplumes 

Semiplumes are intermediate in form between the more pennaceous contour feathers and the strictly plumulaceous down feathers (which lack a central rachis). Semiplumes always have a distinct rachis that is longer than any of the barbs. 

Location :

They are seldom exposed but lie under the surface contour feathers.

Function  :

They help insulate the body and form smooth aerodynamic body contours.

5. 

Down Feathers ( plumules ) 

Down feathers are small, soft and woolly feathers. They either lack a central rachis or sometimes a very short rachis that is shorter than the longest barbs and attached directly to the basal calamus of the feather. The barbs are long flexible and with short barbules without hamuli and arising as a fluffy tuft from the top of the short calamus.

Adult Downs are also called definitive down and Natal Downs are also called neossoptiles. Natal down which covers hatchling Birds is generally simpler in structure than adult down. Often natal down is immediately pushed out of the feather follicle by the emerging juvenile plumage and appears as tufts at the tips of new feathers.

Down is not evenly distributed, and some groups ( sea ducks for example ) have much heavier down coats than other groups ( such as songbirds). They are absent in adult pigeons.

Function :

They provide a layer of insulation underneath the contour feathers. They form natal covering in newly hatched birds.

6. 

Powder down feathers 

These are simply down feathers of which the top never develops beyond the early stage and continually breaks down into a powdery substance. 

They are the only feathers that grow continuously and are never moulted. 

Many species have widely scattered powder feathers within patches of normal down feathers, but herons and bitterns have dense, prominent patches of powder feathers on the breast and belly.

Function :

The fine powder is thought to aid the bird in grooming and waterproofing its feathers.

7. 

Rictal bristles 

Bristles are stiff hair-like contour feathers without vanes consisting of only central rachis almost bare of barbs and barbules. 

Bristles are not found in all birds (for example: Rock dove does not have bristles )

Location :

They are mostly found around the eyes ( for protection ), the lores, the nostrils to guard against dust and the rictus of the mouth ( Rictal bristles ) in birds that catch insects in mid-air.

Function : 

They give protection. Insectivorous birds are thought to use their prominent Rictal bristles as sensory organs much the way mammals use whiskers.

8. 

Tactile feathers or vibrissae 

They are best developed in nocturnal birds.

Location : 

At the root of beaks or around the eyes.

The not-so-familiar but peculiar feathers which may be derived from contour feathers, are also found in birds. For example, 

• Bristles at the gape of night jars.
• Eyelashes of hornbills.
• Wires of Birds of paradise.
• Ornamental plumes of many species 

About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. & M.Phil. in Zoology, with a focus on university-level academic topics. Read the full Author Credentials and Background here.