Kingdom Animalia; Phylum: Chordata (Chordates), Subphylum: Vertebrata (Vertebrates)
Class: Birds (Aves)

The vast majority of these vertebrates move by flying. Birds have a unique cover – feathers. These consist of a special substance called keratin which gives birds extraordinary skills. Birds are extremely old animals. First birds lived 140 million years ago! These days there are approximately 8 600 different birds’ types which live and fly all around the world.

Anatomy: Their skeleton is completely bony and aerodynamically suitable for flying. Long bones are tubular and therefore extremely light. The skull is fixed to the backbone only with one vertebra just like in the Reptilia Class. A beak is created by prolonging the upper and the lower jaw. A neck is quite long and very movable. A chest bone is wide with a number of muscles, especially those responsible for flying, are fixed to it. Even the bone in the wings is attached to the chest bone. The birds’ body is covered with feathers. It is extremely light and on the other hand it is also a thermal isolation. Feather lies in two layers.

Physiology: The birds breathe with lungs however these cannot be extended. The lung capacity is extended by five pairs of air sacks which store the air. These sacks are located next to some inner organs and tubular bones. Some birds have got even a vocal organ. The heart is divided into two atriums and two ventricles. Deoxygenated blood flows through the right heart whereas the oxygenated blood flows through the left heart. So there are two separated blood circulations. The blood circulation starts in the right atrium and then the blood flows into the right ventricle. From the right ventricle the blood flows into the lungs where it is oxygenated. Subsequently it flows into the left atrium and then into the left ventricle. From the left ventricle the oxygenated blood flows into the entire body to all organs. There it gives inevitable oxygen to tissues and last but not least the deoxygenated blood comes through veins into the right atrium. In the right atrium it starts all over again. Birds belong to endothermic animals, so their body temperature is stable and is not influenced by the environment.
The average body temperature is about 42 Celsius degrees. In the brain we can observe a huge development especially in the frontal part. Birds use their instinct very much, besides they take care of their offspring. Their orientation skills are remarkable too.

Life span: All birds lay eggs. There are three main layers in the egg. Amnion is filled with amniotic fluid (reptiles have got amnion as well), then chorion and alantois. Alantois is used as a respiratory and a secretory organ. The female birds have got only one ovary. There are often differences in sexes. This is called sexual dimorphism.
The Bird Class can be divided into two infraclasses: Saururae and Ornithurae.
Infraclass Saururae includes already extinct birds. They had been a link between reptiles and birds. They were able to climb trees but unable to fly. Their jaws were pretty sharp.
The second Infraclass includes few extinct birds and also a number of birds which can be seen all around the world. This infraclass can be divided into several categories such as: Ratitae, Natantes and Volantes. Ratitae: These bords cannot fly. Their wings are not developed enough. They can walk and run therefore their extremities are very strong. Nantes: cannot fly as well. They live mostly in water in the Antarctica. They use their extremities mainly for swimming (for instance: penguins (Phenisciformes)). Volantes: They are the biggest category of all living birds these days. They can fly. This group can be also divided into several subcategories such as: Columbiformes (e.g.: pigeons (Columba)), Anseriformes (swans (Cyngus)), Ralliformes, Pelecaniformes (pelicans (Pelecasnus)), Ciconiiformes, Cuculiformes, Falconiformes (eagle (Aquilla)), Strigiformes (Owels (Stix)), Coraciaformes, Piciformes and Passeriformes (sparrow (Passer)).

Bird Class includes a number of animals which differ in size, food habits, locomotion, vocalization, colour etc. Some birds take care of their small young others do not. All in all birds have always been a great fascination for humans.