Feather Tracts (Pterylae and Apteria) in birds: Explained With Diagrams

  

Feather Tracts (Pterylae and Apteria) in birds: Major Types and Significance With Diagrams 

In birds, the sites on the skin, from where the feathers are attached and development of feathers occurs are known as feather follicles.They are just like small indentations. 

In most birds, the feathers are not attached uniformly over the body, but are grouped into feather tracts called pterylae ( singular: pteryla). Between the tracts are regions of bare or less feathered skin called apteria ( singular: apterium). 

This arrangement and distribution of feathers in specific tracts over the birds body is called pterylosis.

The contour feathers ( pennae) form the general covering of the body. The flightless birds or Ratitae and screamers have feathers uniformly distributed over the body.  Ostriches (Family Struthionidae), rheas( Family, Rheidae),emus, cassowaries, penguins ( Family, Spheniscidae)and screamers have feathers uniformly distributed over the body. 

Superficially, the same will look true for the common pigeon, English sparrow, and other birds but when closely inspected it become clear that the follicles of these contour feathers are concentrated in dense specialized tracts called pterylae and inbetween are apteria.

There is considerable variation amongst species of birds with respect to the size and shape  and the density of feathers on certain pterylae.

Within a species, however, the pterylae and apteria show generally constant characteristics.

The study and description of pterylae and their intervening apteria is called pterylography 

( Pterylography is sometimes very useful in taxonomic work. By comparing the pterylosis of species apparently similar, it is often possible to determine whether or not a close relationship actually exist )

 Arrangement of feather tracts  and bare patches, varies from one taxonomic group to another and some groups have unique patterns for example the corvid family which includes ravens, crows, jays, nutcrackers and magpies. They have a characteristic apterium in the middle line of the pteryla located on the back. 

Historically, such pattern have been important in classifying the main groups of birds. The feather patterns are symmetrical from one side of the body to the other and are shared by all members of a species.

Types:The major pterylae or feather tracts 

Diagram of Pterylosis depicting ventral and dorsal view in Columba livia and Passerines 

Capital tract 

It extends,

• Entire dorsal surface of the head (the pileum) from the base of the upper mandible (upper bill),
• Down the back of the skull to the nape or occipital region (where the skull meets the spinal column),

It is bounded laterally by an imaginary line passing from the mandibular ramus to the angle of the jaw.

This capital tract includes, 

• auricular feathers,
• rimal feathers (around the eyelids),
• rictal bristles,
• and all other specialized feather regions of the head.

Diagram of Pterylosis in lateral view in Columba livia and Passerines with magnified view of section of tract 

Spinal tract

It is a lump of highly variable series of tracts and apteria placed into one group 

It extends, 

Posteriorly from the capital tract to the upper tail coverts.

Runs down the dorsal midline of the body from the base of the skull to the pygostyle at the posterior end of vertebral column

The spinal tract is bordered along its length by 

• the cervical apteria of the lateral neck in most birds (but not the rock dove),
•  the scapular apteria at the shoulders, 
• and broad lateral apteria along the lateral walls of the abdomen.

( Note: how the posterior end of the spinal tract broadens over the  synsacrum and pelvis).

The spinal tract is divisible into four regions identified mainly by their shape and locations.

•  the narrow cervical region extends from the head to the trunk, 
• the narrow interscapular region extends posteriorly between the shoulder blades (scapulae);
• the saddle-shaped dorsal region extends from the shoulder blades to a point approximately halfway to the tail;
•  the broad pelvic region, lying between the hips and extending from the dorsal region to the tail coverts.

 This completes the spinal tract.

Ventral tract 

It covers the ventral neck, breast, and abdominal regions. 

It extends,

Posteriorly, along the ventral midline of the body as a single tract. 

It begins at the junction of the mandibular rami and extends posteriorly to the circlet of feathers surrounding the anus. 

It enclose a longitudinal apterium, called the median apterium . It is bounded by the capital tract and the cervical and lateral apteria.

Caudal tract 

It includes the major flight feathers of the tail rectrices ( vary in number depending on the taxonomic group) 

(Pigeon there are 12 rectrices, six on each side of the tail)

(Most passerines have 12 rectrices)

The caudal tract also includes the upper tale coverts of the dorsal caudal tract and lower tail coverts of the ventral caudal tract.

* (The oil gland or uropygial gland, which is located dorsally at the base of the tail is not feathered in sparrow. When feathers do occur on it, as in certain kinds of birds, the feathers are considered part of the caudal tract)

Humeral tract 

The humeral tract is a band of contour feathers on the shoulder area of the bird. It overlie the humerus bone (upper arm area) on the dorsal side of the wing.

The feathers arising from this pterylae are the scapulars.

Alar tract 

It comprises of a series of smaller pterylae covering both dorsal and ventral surfaces of the outer wing. 

It also includes all the feathers arising on the wing except those of the humeral tract from which it is separated by a narrow apterium.

It includes,

•  the primary, secondary, and if present the tertial remiges which are the major flight feathers of the wing. 
• Also the alula feathers of the second digit. 

The smaller alar tract contain all the rows of greater, middle, and lesser covert feathers covering the wing both dorsally and ventrally,accept those coverts within the humeral tracts.

Femoral tract 

This is another narrow pterylae which extends along the outer surface of each thigh from a point near the knee to the anus. 

when the leg is drawn up  this tract is almost parallel to the spinal tract.

Crural tract 

The remaining leg control feathers are included within the crural tract. 

It is separated from the femoral tract by a narrow apterium.

( In some large birds with heavily feathered legs, an additional metatarsal tract is identified, covering the tarso-metatarsal area of the lower leg).

Brood Patches 

( a special form of Apteria)

Brood patches are specialised temporary apteria that develop on the abdominal and breast feathers of female and some male birds during the incubation of eggs. 

Due to increase in hormonal levels of estrogen, some contour and down feathers of the ventral abdominal tract loosen and fall out, living a large apteria or a number of smaller patches on the brooding parents abdomen. 

This area of bare skin contains many blood vessels and when placed in contact with the incubating eggs, it transfer heat from the parent to the eggs. For example, in the shore birds and gulls (Order Ciconiiformes) show typical clutch.

Significance of pterylosis 

 a.

To reduce the overall weight during flight, feathers from the feather track cover the apteria forming flight surfaces and providing insulation without requiring a solid mass of feathers.

  b.

Grouping of feathers into tracts also may allow the muscles that move them to be smaller and more localised lightening the bird even more.

 c.

In addition, apteria may aid loss heat, as many birds raise their feathers to expose their skin when becoming overheated.

Many birds like penguins have a continuous pterylosis with no apteria preventing water from penetrating to the penguin skin and chilling it. 

Also the skin of penguin is so good at trapping and insulating layer of air near the body, that the skin stays dry even while the bird is swimming and diving. 

Adult Ostriches from Africa also lack apteria, but their embryo's have them. Many researchers believe that the stages that the developing embryos passes through indicate the stages their ancestors evolved through. 

The lack of apteria in adult Ostriches is probably a secondary condition. The ancestors of ostriches and penguins undoubtedly had a more typical feathering.

 Feathers spread on the body and wings of the bird with impressive neatness. Such orderliness helps to produce the tidy streamline cover of feathers that is crucial to a bird survival.