General Characteristics and Adaptations of Class Aves-Birds:
Skin of birds bear feathers and the legs have scales. Skeletons are bony. There are 2 pairs of pentadactyl limbs, front pair of which make wings. Their visceral clefts never grow gills. They do not have external ear. Fertilized yolky eggs are placed on land, and enclosed with calcareous shells. Fertilization is interior. They are worm blooded. Like reptile described as fish-like vertebrates adapted to life on land, so birds can be explained as reptile-like adapted to flight. Flying involves energy and freedom of movement. Despite flight there are 2 other things which of equivalent significance to birds in the adaptations to environmental changes are the steady temperature and large brain. The bird's skill to fly allow it to keep its internal steadiness by flying to appropriate surroundings and food, and it demands use of the lot of energy. Those that stay as birds on land, are structurally like the reptile, running on 2 legs, with head erect, and front limbs set free for other uses. It is supposed that they developed from walking reptiles; that their wing that should have developed before skill to fly was required by need to keep the constant body temperature.
The feathers are of 3 basic kinds:
a) Down feathers:
These are plain in structure. They like feathers are made of keratin that is made by skin and are restricted in patterns on skin like scales of reptiles are. Several young birds and few adults have this body cover. They contain short shaft and barbs which don't join.
b) The Quill Feathers:
These composed of central rod (shaft) that bears the flat expanded part known as vane. Lower part of central rod is hollow, and forms quill. At the base of shaft is the hole at base of which tuft of papilla or tissue fits into the pit-like follicle. Vane is composed of several processes known as barbs arranged on either side of rachis like teeth of a comb. Every barb has 2 rows of small branches known as barbules that are also prearranged obliquely so that adjacent barb overlaps. Barbules hold barbs together. There are small hooks on the barbules on lower surface of feather, while upper barbules contain ridges. These can only be seen under microscope. Barbule hooks of on lower barb fits into ridge of upper one. This binds whole vane together to give the surface capable of beating air.
c) The Covert Feathers:
They are alike structurally to quill feathers except that they are minor. They cover body. There are filoplumes all over body between covert feathers. They comprise of the slender thread-like shaft with the tuft of barbs at end.
Forelimbs of birds serve as wings. They function like wings and propeller at same time. There are 3 parts that match with upper arm, forearm and hand of mammal. When not in flight whole arm is folded in the Z shape. This way they are folded neatly to body so that they don't impede with other movements like walking or hopping. 3 parts of arm are kept by 2 fold of skin, one from upper arm to wrist and other across armpit. Wings are enclosed with feathers in most parts. This gives large surface area for bird to bit wind. Whole structure is light as nature of feathers.
They are composed of keratin. Muscles and bones of forearm give base to which feathers are joined. There are only 3 digits, thumb that remains dividing, whereas second and third are tightly fixed together by the web of skin.
Uses of Feather:
Feathers are given with muscles at base. When these contract or relax, they bring about the control of flight, body temperature, or shows feather in connection to observe and so on. They are supplied with nerve fibres so that they are sensitive to touch. They provide shape to the body.
Birds are able to fly as they have developed feathers. It though can also run and walk on 2 legs, and maintain itself balanced. They are capable to fly also as feathers have been formed in wings, there are hollow bones, they are warm blooded and have astonishing effective respiratory system and strong breast muscles. All these make for high power and little weight. Bird skeleton is light and strong as bones are hollow and thin and some parts have been fused together. Lower vertebra are made tip of hip, sacrum are combined into the tube like structure. Hollow finger bones are combined together. Tail is decreased to the small plate at end of vertebral column. Ribs are flat and thin and with their overlapping joints form the strong flexible basket. Skeleton of bird helps it to fly and walk on two legs when essential. It is comprised of breast bone for attachment of huge muscles which move wings. It is the adaptation of sternum that has become the big keel that has elongated for below ribs. Wing is lifted or lowered by another muscle that is also joined to sternum but passed over shoulder to other side of humerus. They rotate humerus. Other muscles within wing serve to fold and extend it. Much of bulk of body has been pushed back. The edge of every wing bends upward in down stroke due to its flexibility. So every stroke causes bird to move upwards and forwards. When bird is to land or take off, wing beats causes quill feathers at tip to spread out to form the wing slot that stops bird from stalling. Legs are folded in under body in flight but extended for landing.
Oxygen Supply and Energy:
With such apparatus for flying, there is requirement for the effective machinery for energy supply. This is plain in totally divided heart. Right part pumps blood solely to lungs whilst left pumps to oxygenated blood from lungs to body. Big arteries provide muscles. Oxygen is delivered to tissues at high artenal pressure and red blood cells carry extremely large amount of oxygen that they can provide suddenly at fairly high oxygen tension. Body is high 420C - 450C (bird maintain the high body metabolism). To maintain this, birds require the efficient respiratory system. The air sacs assist make bird light. They extent into all parts of bird, comprising hollow bones and toe bones. The air sacs are filled when bird inspires with oxygen to rinse out utilized air from tissues and this rising respiratory efficiency. Air also assists to cool overheat body of bird. For flight the great amount of food which can be rapidly digested is needed. For this cause, birds eat high calorie food such as, seeds, insects, fish, worms, rodents and fruits. Daily food ingestion can reach 30% of body weight, compare to 10% of that of mammals. Beak is structure with which bird takes in food. It is the horny structure covering small toothless jaws. It is utilized for getting all kinds of food. Its special shape and adaptations rely on kind of food particular species requires.
Birds go through the period of courtship that is generally long and complex. Colors of male bird become very bright and he show himself to female. At times displays are performed to fright other males from recognized mate. After mating, nest is made. Well grown eggs are fertilized in ovary of bird. It is only after fertilization that shell glands secretion is obtained as egg moves down oviduct. The average of egg is laid a day. It is only when eggs are laid that is clutch is finish that bird starts to sit on them and utilize body heat to hatch them. Eggs are comparable in structure with reptilian egg. Dissimilar to reptiles birds hatch their eggs. At times both male and female share incubation in other cases it is only hen.
The good number of birds migrates yearly along routes which turn out to be extremely customary for them. In migration, they can cover extremely huge lands across continents like Swallow departs Europe for Africa in September just to return to Europe in April the next year.
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