What is the meaning of a bird on a coat of arms?

Different birds can mean different things and animals in general can be symbols for different virtues or vices, for example:

A Martlet or Swallow means 'one who has been dispossessed of land', or the sign of a fourth son.

An Eagle means 'a protector, a person of action, noble, power and strength'.

A Peacock signifies beauty and pride.

A Swan is a sign of a musical person, of poetry and harmony.

A Dove naturally means love and peace.

The Eagle has the presumed ability to stare at the sun without becoming blind and this became a source of metaphors.

The pelican was supposed to be able to open its breast with its beak to feed it's young in times of great distress, so it became a symbol of generosity, parental devotion and redemption. Its generally represented in the act of feeding it's young.