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9.3.3. Birds

Class Birds

Birds are tetrapod vertebrate animals that emerged from reptiles in the Mesozoic, about 190 million years ago.

Their bodies are covered with feathers, which help keep their body temperature constant, at about 40º C, since they are homeothermic, warm- blooded animals. Some birds migrate to avoid times when temperatures are too high or too low.

The feathers have the following structure:

  • an axis.
    • Calamus, part that is inserted into the skin.
    • Rachis, from which the beards, the ramifications of the standard, come out.
  • Laminar or standard zone.

Estructura de una pluma

The body of birds can be distinguished:

  • the head The toothless horny beak stands out, which can have different shapes, adapted to different types of food. On the head are also the eyes and ears, although they do not have ears.
  • the neck It makes it easier for you to move your head.
  • The trunk.
  • The four extremities. They are tetrapods.
    • Two wings. The two forelimbs are anatomically adapted to be able to fly, although not all birds fly.
    • two legs They are covered with scales, like reptiles. The legs allow them to walk, jump or stand on them.

His body is adapted to flight. The skeleton of birds is very light, made up of hollow bones, and together with their streamlined shape, they are adaptations that facilitate flight.

Birds have very varied feeding methods. As they do not have teeth, they cannot chew the food, and have to swallow it whole, and accumulate it in the crop, where it is stored and softened. The food is crushed in the gizzard, and then it will pass to the stomach, the intestine, and finally, the cloaca.

The respiratory system of birds is also adapted to flight. The lungs have expansions called  air sacs that fill with air when you breathe and make the bird lighter. This reserve of air allows you not to have to use your respiratory muscles during flight.

The nervous system of birds is highly developed, especially the sense of sight, with eyes in a lateral position to increase the visual field.

Many species have a syrinx, an organ that allows them to sing and communicate.

Birds reproduce sexually, through internal fertilization. The individuals of the couple have to join their cloacas for fertilization to take place (cloacal kiss).

They are oviparous, laying eggs that they incubate until they hatch. The eggs are amniotic.

Types of birds

Birds can be classified into:

  • Superorder Paleognatas (Ratites)They are generally large birds. They do not have a keel or muscles for flight. His wings are stunted. It includes less evolved birds, runners and poor fliers, which hardly fly, such as kiwis, ostriches, rheas,...  


Ostrich in Cabárceno Park (Cantabia)

Humor: How to prepare a Kiwi.

  • Superorder Neognatas (carinates): They have a wide sternum, forming a structure called a keel that supports the muscles they need for flight. It includes the numerous orders of flying birds such as eagles, pigeons, ravens, owls, vultures, sparrows, gulls, pelicans, ducks, etc. They also include flightless birds such as penguins, which, although they do not fly, perform the same flight movements in the water.

Verderón (Chloris chloris) posado y posando en una ventanaGreenfinch (Chloris chloris) perched and posing in a window. (Zaragoza)