The lungs of birds are compact and very efficient. They are linked to very important structures called air bags, which work to decrease bird density during flight.
At the base of the trachea is a structure called syrinx, with vocal muscles, responsible for singing.
These sounds made by birds enable communication between individuals of the same species - which is important for the defense of the animal, for marking the territory, and for approaching males and females in the reproductive period. There are birds, such as the ostrich and vulture, that do not have the syrinx.
Birds have no bladder. The kidneys communicate directly with the cloaca through two channels. The urine is whitish, half pasty and eliminated along with the feces.
A bird's brain is proportionally larger than a reptile's brain and has 12 pairs of cranial nerves.
The birds are dioecious, with internal fertilization, oviparous and with direct development. Fertilization usually occurs in the upper region of the oviduct, the posterior glands secrete the shell membranes when the egg is ready for laying.
The female has only one ovary, which produces large eggs. The egg, also called a yolk, when fertilized by the male sperm, forms the zygote, embryo of the new living being. Passing through a long canal, the egg exits through the cloaca.
The eggs are hatched by the female, male or both, usually in a nest. The body of the adult bird on the eggs guarantees them the warmth needed to develop the embryo. The incubation period lasts from 20 to 30 days.
In eggs, there are substances (the calf) that nourish the young in formation. The shell is porous and has tiny holes that allow gas to be exchanged, but not water to escape from inside the egg, which would leave the embryo dehydrated and lead to death.