Birds, also known as birds. Definition: feathered oviparous vertebrates, dictionary explanation: a class of vertebrates, which lay eggs with warm blood, breathe with lungs, almost all have feathers, hind legs can walk, forelimbs become wings, and most of them can fly. In zoology, the main feature of birds is that their bodies are streamlined (spindle-shaped or spindle-shaped), and most of them live by flying. The body surface is covered with feathers, and the forelimbs generally become wings (some kinds of wings degenerate); The pectoral muscles are developed; Short rectum, large appetite and fast digestion, that is, the digestive system is developed, which helps to lose weight; The heart has two atria and two ventricles, and its heartbeat is fast. The body temperature is constant. In addition to the lungs, the respiratory organs also have air sacs protruding from the lung wall to help the lungs perform double breathing.
Feathers are divided into three types: normal feathers, down feathers and hairiness. Feather branches are densely distributed on both sides of the feather branches, and there are hooks or grooves on the feather branches, and adjacent feather branches are hooked with each other to form a flat and elastic feather sheet. The positive feathers on the body surface form a windbreak shell, which makes the bird's body streamlined. Positive feathers on wings and tails play a decisive role in flight and balance. The structural characteristics of down feather are that the shaft of down feather is long and thin, and the hooked protrusions of down feather branchlets are underdeveloped, which can not form solid feathers and has the function of heat preservation. Duck down is duck down. Hairiness is very thin, like hair, mixed with normal feathers and down, which can only be seen after the normal feathers and down are removed.
The skin of birds has no sweat glands, and the only sebaceous gland is the tail sebaceous gland. The oil secreted by it is smeared on the feathers by the beak, which makes the feathers moist but not wet. The secretion of caudate finger gland also contains ergosterol, which can be converted into vitamin D under ultraviolet irradiation. When birds rub their feathers with their beaks, vitamin D can be absorbed by the skin, which is beneficial to the growth of bones.
Birds adapt to life in Yu Feixiang. Their bones are light and strong, with fine bone fragments, long hollow bones and penetrating air sacs. Many bone fragments are connected together to increase their firmness. The spine can be divided into five parts: cervical vertebra, thoracic vertebra, lumbar vertebra, sacral vertebra and coccygeal vertebra. The large number of cervical vertebrae and saddle-shaped vertebral bodies make the neck extremely flexible (the owl's head can move up to 270).
The last few thoracic vertebrae, all lumbar vertebrae, sacral vertebrae and some coccygeal vertebrae are completely healed together, which is called comprehensive sacrum and is a strong pillar of the waist. There are hooked protrusions on the ribs, which hook each other to make the chest stronger.
The forelimbs become wings, the bones are arranged in a straight line, there are movable joints between the bones, and the carpal bones, metacarpal bones and phalanges at the end heal and deform, so that the wings become a whole when they flap. The shoulder strap consists of scapula, coracoid and clavicle. The slender and elastic clavicle is V-shaped, which can prevent the closure of the left and right coracoid bones when flapping their wings, and also enhance the elasticity of the shoulder strap.
The whole weight of the bird falls on the hind limbs, and the bones of the hind limbs are strong. Compared with the bones of the hind limbs of other terrestrial vertebrates, the tarsal bones of birds are stretched and play a role in increasing elasticity. Birds usually have four toes. In adult birds, the ilium, ischium, pubic bone and comprehensive recommended bone of the belt are integrated, which increases the firmness of the belt.
The pectoral muscles related to flight in birds are particularly developed, accounting for about 1/5 of their weight. They can send out powerful forces and flap their wings. And the back muscles degenerate, contrary to fish.
The pectoral muscles of birds can be divided into pectoralis major and pectoralis minor. The former starts from the keel process and ends at the ventral surface of humerus. When contracting, the wing descends. The latter starts from the keel process, and the long tendon passes through three bone holes composed of clavicle, coracoid and scapula, and ends behind the proximal humerus, so that the wings can be raised when contracted.
The muscles of the hind limbs are concentrated in the upper thighs, and each has a long tendon connected to the toes. In this way, the muscles that control the movement of forelimbs and hind limbs are concentrated in the central part of the body, which is of great significance to keep the center of gravity stable during flight.
Modern birds lack teeth, and the chewing function is replaced by sandbags. During the breeding period of female pigeons, the crop wall can secrete "pigeon milk" to feed young pigeons. The digestive glands (liver and pancreas) of birds are well developed. They secrete bile and pancreatic juice, which are injected into the duodenum and participate in the digestion in the small intestine. Pigeons have no gallbladder, while most birds, such as chickens and ducks, have gallbladder. Birds have a strong digestive ability and eat a lot when they are not hungry, which is related to the high energy consumption of birds when flying.
Birds have developed brains, cerebellums and midbrains. The cerebral hemisphere is larger, which is mainly due to the increase of striatum at the bottom of the brain. In birds, the striatum is an advanced part of sports management, and it is also related to some complicated living habits. Experiments show that the normal excitement and inhibition of domestic pigeons are destroyed, their eyesight is affected, and the habit of courtship and nesting is lost after removing part of their striatum. Birds have underdeveloped cerebral cortex and developed cerebellum, which is related to the coordination and balance of bird flight movements. The midbrain forms a pair of developed optic lobes on the back.
Among the sense organs of birds, the most developed is the visual organ that plays an important role in air flight, while the olfactory organ is underdeveloped. Bird's eyes can quickly adjust their vision by relying on developed ciliary muscles, from hyperopia to myopia. Therefore, when birds fly in trees, they never touch branches; Or dive to the ground for food from high altitude, or adjust from "hyperopia" to "myopia" in an instant. The bird's eye blink membrane is developed, which covers the eyeball during flight and plays a protective role.
Birds' kidneys are very large, accounting for more than 2% of their body weight, even surpassing mammals in proportion. The development of kidney is related to the metabolism of birds. Birds have no bladder, and the urine has little water and is white and thick paste, which is discharged with feces and does not urinate alone.
Pigeons, like most birds, have no diplomatic connectors (ostriches, ducks and geese have connectors). When the male and female pigeons mate, the cloaca holes contact each other, and the semen enters the female pigeon for fertilization. After the egg matures, it breaks through the ovarian wall and is sucked into the corner of the fallopian tube. If there is sperm, it is fertilized here. Whether fertilized or not, when the egg descends along the fallopian tube, it is first wrapped by protein, then the eggshell membrane is added, and finally the calcareous eggshell is added to the uterus. The incubation period of pigeon fertilized eggs is about 16 d, chicken is about 2 1 d, and duck is about 28 d.