Birds are animals with aerodynamic bodies covered with feathers, anterior limbs transformed into wings, pneumatic bones and horny (corneous) beaks.
Birds are terrestrial animals but the majority of them also explore the aerial environment by flying.
The features of birds that allow them to fly are: wings attached to a well-developed pectoral musculature, pneumatic bones, less accumulation of feces in the bowels due to the absence of the colon, the absence of a bladder (no urine storage), an aerodynamic body and lungs with specialized air sacs.
Birds have lightweight bones with internal spaces filled with air. These bones are called pneumatic bones. This feature reduces the density of body of the animal, facilitating flight.
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Like reptiles and mammals, they carry out gas exchange through lungs.
Birds, like every vertebrate, have a closed circulatory system. Their heart is similar to the heart of mammals, as it has four chambers (two atria and two ventricles) and no mixture of venous and arterial blood. (In mammals, however, the aorta curves down to the left and in birds it curves down to the right).
Birds are uricotelic, meaning that, like reptiles, they excrete uric acid. This substance needs less water to be eliminated and it helps to reduce body weight, making flight easier.
Regarding external coverage, birds are similar to reptiles in that they present impermeable keratinized outside. Concerning reproduction, fertilization is internal in both and the embryo develops within a shelled egg. Regarding excretion, both excrete uric acid.
Birds, like every vertebrate, use sexual reproduction. Their embryos develop within shelled eggs containing extraembryonic membranes. Therefore, their embryonic development occurs outside the mother’s body.
Birds copulate. Fertilization is internal and occurs only before the female gamete is wrapped in the calcareous eggshell.
Embryonic development in birds is direct. There is no larval stage.
The eggshell is basically made of calcium carbonate. The white, or albumen, is composed by albumin, a protein. The yolk is predominantly made up of lipids, but also contains proteins and vitamins.
Reptiles are heterothermic, meaning that they do not control their body temperature. Birds are the first homeothermic animals, as they are able to maintain a constant body temperature.
Many birds live in regions of intense cold. Penguins are an example of birds that live in polar region.
Zoonoses are human diseases transmitted by animals. Psittacosis, a bacterial disease, and histoplasmosis and cryptococcosis, which are fungal diseases, are examples of zoonoses transmitted by birds.
Examples of representative species: chickens, sparrows, parrots, ostriches, penguins. Basic morphology: aerodynamic body, feathers, pneumatic bones, horny beaks. Skin: impermeable keratinized, feathers, uropygial gland. Respiration: pulmonary. Circulation: closed and complete, heart with four chambers. Nitrogen waste: uric acid. Thermal control: homeothermic. Types of reproduction: sexual, internal fertilization, shelled eggs with extraembryonic membranes.
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