Birds have feathers and a beak. Living species are toothless. Most birds fly using their modified fore-limbs as wings and use their hind limbs for terrestrial locomotion. A few groups are flightless. The eggs are fertilized internally and laid on land, usually in a nest, enclosed in a hard, but permeable, shell.
These typical bird features are illustrated by a stuffed pigeon and a pigeon skeleton. The model of the chick embryo (right) shows embryonic gill pouches and metameric segmentation. The pigeon brain (front) has a large cerebrum but the cerebellum, which coordinates motor activity and equilibrium, is also very large. These features relate to the fact that birds fly and need good balance and coordination in three dimensions.