Male birds sing to attract female birds. Birds sing to defend their territory. Birds sing to convey their motivational message. Birds get information about position of other birds through singing. One song can contain many types of information.
Some species learn their song during social interaction.
HVc is important in moment to moment control of singing. Lesion of HVc, birds can't sing, but can still call. HVc of males is much larger than HVc of females.