Most of the perching birds are song birds. They use songs and calls to communicate with each other. Birds sing by pushing air out through their open mouths. Birds song has a number of different functions. Male birds may sing to defend their territories. Their song warns other males not to approach, but it may also serve to attract females. Females in their turn will sing a greeting song so that the male will know not to attack. Young birds sing to let their parents know they are hungry, and their parents will sing to warn them of danger, or to gather them together if they stray. They are found in black, yellow, grey and red colors. They are differentiated from others according to their beak colours.