Birds are a class of vertebrate tetrapods characterized by bipedalism, wings and a toothless beak and feathers.
Cosmopolitan, birds inhabit all environments, from the ice of Antarctica to equatorial forests and deserts. This distribution was made possible thanks to varied anatomical, physiological and behavioral adaptations (in particular the migratory phenomenon)..
There are, in 2020-2021, around 10,700 to 11,150 recorded species of birds (including 9,700 wild species). Very different both in their ecology and their behaviors, each of them has a common set of obvious characteristics allowing them to be grouped together, in particular a beak, a short bony tail, forelimbs transformed into wings and hindlimbs that serve alone. normally to progress on land or in water. In addition, they are all oviparous, that is to say, they lay eggs surrounded by a thin hard shell.
The anatomy and physiology of the majority of birds tend to favor their flight as much as possible: weight reduction (vertebral column characterized by the high number of vertebrae fused into a single and very solid piece of bone, the synsacrum, lightening of the skeleton by pneumatic bones, reduction of ventral and dorsal musculature).The aerodynamic body and a strong pectoral musculature are also favorable factors for flight. Finally, a very active metabolism favored by energy food and an efficient respiratory system by the presence of air sacs allow them to produce the high and lasting power required for flight. The speciation of terrestrial avifauna has however given rise to some flightless birds (ostriches, penguins, etc.).