Classification of Animal Chordates of Class 11
Shape : The streamlined body of a bird is designed to offer minimum resistance to the wind, and hence easily propelled through the air.
Compact body : The compact body, light anteriorly and heavier posteriorly, helps in maintaining balance in the air.
Body-covering of feathers : The smooth, closely fitting and backwardly-directed contour feathers make the body streamlined and further help its passage through the air by reducing friction to minimum. The
non-conducting covering of feathers insulates the body perfectly and prevents loss of heat.
Forelimbs modified into wings.
Short tail : The short muscular tail bears a series of long, but light caudal feathers. It serves as a rudder for steering during flight, to suddenly check flight, and as a counterbalance in perching.
Beak : The mouth is drawn out into the horny beak which is used as a pair of forceps in picking up things. Besides procurement of food, the beak is also used for nest-building.
Integument : The loose skin is a modification for flight.
Large muscles of flight : The flight muscles on the breast are greatly developed, wings are depressed by pectoralis major elevated by pectoralis minor.
Endoskeleton : Most of the bones are pneumatic and light. The rigidity of the dorsal part of vertebral column, due to fusion of vertebrae, provides a firm fulcrum for the action of wings. The shortening of the
caudal vertebrae and formation of pygostyle has assisted stability in the air. The fusion of pelvis with synsacrum provides firm attachment to the legs, supports the weight of the body when the bird is walking.
The sternum is greatly expanded and bears a large mid-ventral ridge or keel for the attachment of flight muscles.
Air-sacs and respiration : The inelastic lungs of birds are supplemented by a system of air-sacs, which grow out from lungs. The air-sacs secure more perfect aeration of lungs and help in internal respiration.
Warm-bloodedness : Birds are warm-blooded animals.
Circulatory system : The avian heart is relatively large and completely divided into four chambers. It functions very efficiently with a double circulation of blood.
Uricotelic excretion : Birds do not have a urinary bladder. The insoluble uric acid and urates are voided at once. These features help in reducing the unnecessary weight of body.
Brain and sense organs : The eyes are large and the large optic lobes correspond to the great development of sight. The developed cerebellum indicates the delicate sense of equilibrium and the great power of muscular co-ordination belonging to birds.
Single ovary : Presence of a single functional ovary on the left side in the female bird also leads to reduction of weight. Right ovary is rudimentary or absent.
Types of Feathers
The arrangement of feathers in the body is called Pterylosis.
Feathers are arranged in definite feather tracts the pterylae, separated by featherless areas the apteria, which bear only a few filoplumes.
The feathers are of various types : quill feathers, coverts, contour feathers, filoplumes or hair feathers and down feathers. Some birds have special modified feathers called the bristles.
Fig. Pigeon : Kinds of feathers—A. Remex, B. Rectrix, C. Contour, D. Filoplume, E. Down,
F. Rictal bristle (not found in pigeon)
Large feathers found in wings (Remiges) and tail (Rectrices).
Has a central axis called shaft.
Small proximal part of the shaft is hollow, translucent and cylindrical and is termed as calamus.
The long distal, solid and opaque part of the stem is known as rachis.
An umbilical groove extends all along the ventral side of rachis.
Small hole on the proximal end of calamus is known as inferior umbilicus, the hole on the distal end of calamus is known as superior umbilicus.
Each vane is composed of parallel filaments, the barbs. Each bears barbules.
The distal barbules of each barb bear small hooklets.
Coverts are small feathers found in the wings and tail. They fill up the gaps left between the bases of the quills. They have short calamus in comparison to quills.
Contours : Small feathers that cover the body and give it its shape. They resemble the quills but their barbs are not so strongly joined and can be separated easily.
Filoplumes : Occur beneath contour feathers, very small in size, consists of long calamus bearing at the tip a few weak free barbs with barbules.
Down feathers : They cover the newly hatched bird. These are called the nestling downs. They consist of short calamus, reduced rachis bearing flexible barbs with short barbules. They are found beneath contours.
Bristles : Short calamus, a long rachis bearing a few vestigeal barbs at its base. Bristles occur near the mouth in fly-catchers.
The first digit of the hand (thumb or pollex), bears a tuft of small feathers known as bustard wing or alaspuria or false wing.
Types of beaks
Seed-eating beak : Short peg-like and conical beaks are characteristic of small seed-eating birds, such as sparrows, finches.
Cutting beak : Long and slender beaks with cutting edges e.g., crows.
Fruit-eating beak : The beak is sharp, massive, deeply hooked and extremely strong. It is well adapted for gnawing e.g., parrots.
Insectivorous beak : In swallows and swifts, the beak is small, wide and delicate to scoop up their living insect prey while on wing.
Wood-chiselling beak : Woodpeckers have elongated, straight and stout chisel-like beaks for drilling into the barks or wood for insect larvae or for nest construction.
Tearing and piercing beak : Flesh-eating birds, such as vultures, hawks, eagles, owls, kites, etc., have short, pointed, sharp-edged and powerful, hooked beaks for tearing flesh.
Mud-probing beak : In sand-piper, Jacana beaks are extremely long and slender and are used as a probe for thrusting far down into water and mud in search of worms and larvae.
Water and mud-straining beak : In ducks and geese the beak is broad and flat. The edges of the jaws are furnished with horny serrations or transverse lamellae, which act as a sieve. In flamingos, the beak is distally curved downwards.
Fish-catching beak : Storks and kingfishers have long, powerful and sharply pointed beaks to capture fish, frogs, tadpoles and similar aquatic animals.
Spatulate beak : The spoon-bill possesses a very specialized form of beak. It is flattened throughout its length but terminates in a broad, spoon-like expansion meant for search of insects, worms, fish, etc. in water & mud.
Flower-probing beak : The long, pointed, probing beak e.g. humming bird.
Types of Feet
Cursorial or running feet : e.g., bustards, cassowary, ostrich.
Perching feet : e.g., sparrows, crows, bulbuls.
Scratching feet : e.g., fowls, quails, pheasants.
Raptorial feet : e.g., eagles, kites, vultures, owls, etc.
Wading feet : e.g., herons, jacana.
Swimming feet : e.g., ducks, pelican, cormorant.
Climbing feet : e.g., parrots and woodpeckers.
Clinging feet : e.g., swifts, and humming-bird.