At birds they comprise a very large and beautiful group of animals. They draw attention for beauty and singing. They are the only animals that have feathers.
The conquest of the flight allowed these animals to inhabit places that are difficult to reach and even impossible for other species. They present another major adaptation to terrestrial life, the homeothermia, which is the maintenance of body temperature, regulated by the metabolism itself. The study of birds is called Ornithology.
Birds evolved from reptiles and many modifications took place to conquer this whole way of life. Eggs began to develop outside the female's body, feathers appeared, the forelimbs gave rise to wings, nitrogenous excretion is uric acid, in a pasty compound to save water, bladder loss, endothermia, separation of venous circulation and arteries, air sacs that help to decrease density and dissipate heat, aerodynamic body and elaboration of voice and hearing.
The skin is thin, flexible and loosely attached to the underlying musculature. They do not have glands, except for the uropigian gland, which is near the base of the tail, where the bird passes the beak, collecting the secretion and passes the feathers to waterproof and also prevent the beak from becoming brittle.
The feathers are light and flexible. They grow from the follicles in the skin, form a thermal insulation and protect the skin, and are of enormous importance in flight.
There are several types of feathers such as: contour feathers, feathers, phylloplumas, bristles and powdery feathers.
During the bird's growth, pigments are deposited in the feathers, resulting in their coloration. The set of all feathers is called plumage. The process of exchanging feathers is called moulting.
Bird bones need to be light and delicate for flight and many have cavities for weight loss, they are called pneumatic bones.
In the sternum they have the keel or carena, where the pectoral muscles are inserted, these are responsible for the wing beats.
For greater agility of these animals, as in mammals, limb muscles are enlarged. The birds' pectoral muscles are responsible for wing movement during flight and are inserted into the keel.
Since the legs and paws are featherless, they have few muscles to prevent heat loss and ensure a more streamlined shape.
The tongue of the birds is small, pointed and has a horny coating. The shape of the beak is adapted to the diet of each species and has no teeth.
The digestive system is formed by the mouth, a short pharynx, tubular esophagus that dilates in the crop, where the food is stored and moistened. The stomach is divided into proventricle, which secretes enzymes, and ventricle or gizzard, where food is crushed by the movements of the muscles. The small intestine ends in the rectum, there are two caeca, the cloaca and the anus. The cloaca is the output of the reproductive and excretory apparatus.
The circulation is closed and the heart has 2 completely separated atria and ventricles, the right systemic aortic arch persisting.
There is no mixing between venous blood and arterial blood and this is very important in temperature regulation. The red blood cells are oval and nucleated.