One can often hear the question: are birds animals or not? Having studied all the features of the structure and life of the representatives of this class, it will be possible to answer it with confidence.
General characteristics
The Bird class includes 9000 species, united in the following orders: ratites, or running (ostriches, kiwi), penguins, or swimming (emperor penguin, spectacled, Magellan, Galapagos, crested and others), keel, or flying (chicken, pigeon, passerine , goose and others).
The birds are similar in structure to reptiles and represent a progressive branch that was able to adapt to flight. Their forelimbs transformed into wings during evolution. Birds are characterized by a constant body temperature characteristic of higher vertebrates, therefore, birds are warm-blooded animals. This is the first answer to the question "Is a bird an animal or not?"
Birds owe their origin to the oldest reptiles pseudosuchia with a similar structure of the hind limbs.
Body and skin
The body of the birds is streamlined with a small head and a long movable neck. The trunk ends with the tail.
The skin is thin, dry, almost devoid of glands. Only some birds (waterfowl) have a coccygeal gland that produces a fat-like secret with water-repellent properties. Horn formations (derivatives of the epidermis of the skin) cover the beak, claws, scales of the fingers and foregut (lower part of the leg). Feathers are also derivatives of the skin. They are divided into two groups: contour and down. Contouring, in turn, are steering (flight control), fly-by (keeping the bird in the air), as well as coverts (located on top of the body). Feathers are under the outline. They help maintain body heat. In the process of molting, older feathers completely fall out, and new ones grow in their place.
Skeleton and muscle system
In birds, the skeleton is particularly durable and light due to cavities in bones filled with air. It consists of the following departments: cervical and thoracic, lumbar and sacral, as well as the caudal. The cervical region is extremely mobile due to the many vertebrae. In the thoracic region, the vertebrae are tightly fused and bear ribs that are movably connected to the sternum and form the rib cage. To attach the muscles that cause the wings to move, there is a protrusion on the sternum - the keel. As a result of fusion of the lumbar and sacral, and also partially of the caudal vertebrae between themselves and with the pelvic bones, a sacrum forms, which serves as a support for the hind limbs.
The muscular system in birds is well developed. Depending on the ability to fly, certain departments achieve particular development. In birds that fly well, the muscles that move the wing are well developed, and those who have lost this ability have muscles in their hind limbs and neck.
Digestive and excretory systems
The digestive system is characterized by a lack of teeth. To grab and hold food, a beak with horn covers on the jaws is used. Through the mouth, food enters the throat, and after it into the long esophagus, which has a pocket-like expansion (goiter) to soften it. The posterior end of the esophagus opens into the stomach, which is divided into two sections, glandular and muscular (here the food undergoes mechanical grinding). The intestine consists of the duodenum, where the ducts of the liver and the pancreas open, as well as the small and short rectum ending in the cloaca. This structure contributes to the rapid removal of undigested residues to the outside.
The excretory organs of birds include paired kidneys and ureters, which open into the cloaca. From it, urine, along with feces, is excreted.
Respiratory system
Respiratory organs in birds are maximally adapted to flight. Through the nasal cavity, air enters the pharynx and trachea, which is divided into two bronchi in the chest. Here is the voice unit. Once in the lungs, the bronchi are highly branched. The lungs themselves have a complex structure and consist of numerous through tubes. Some of them expand, forming air sacs, they are located between the internal organs, muscles and tubular bones. Birds are characterized by double breathing. This happens due to the fact that during flight the air passes through the lungs twice: when it is sucked in by a wing-stroke and expelled when it is lowered due to compression of the bags.
Nervous system
The organization of the nervous system in birds is quite complex and similar to that in higher vertebrates. This again gives an affirmative answer to the question "Is a bird an animal or not?" The system consists of two departments: the brain and spinal cord. The cerebellum, which is responsible for the coordination of movements, as well as the front hemispheres and the midbrain, which are responsible for complex forms of behavior, are well developed in the head department. The spinal cord is most developed in the brachial, lumbar and sacral regions, which provides good motor functions. These features also provide a clear affirmative answer to the question "Is the bird an animal or not?"
The behavior of birds is based on unconditioned (innate) reflexes: feeding, breeding, nesting, laying eggs, mating, singing. In contrast to the class of reptiles, conditioned (acquired in the process of life) reflexes can form and be fixed in them, which indicates their highest stage of evolution. One example of conditioned reflexes may be the fact of their successful taming by humans. It is believed that birds are domestic animals that easily rearrange their behavior and lifestyle from the wild (natural) type to the cultural (domestic) one.
Circulatory system
The organs of the circulatory system of birds, as in most higher vertebrates, are represented by a four-chamber heart consisting of atria (2) and ventricles (2), as well as vessels. Their blood is completely divided into venous and arterial. It passes two circles of blood circulation (small, large).
Breeding
Birds are dioecious animals with a complex and highly developed system of mating behavior, breeding offspring with eggs and caring for them.
All of the above characteristics of the class give an unambiguous answer to the question "Is a bird an animal or not?" Of course, birds belong to animals.