Biologists over the years have found that birds sing to stake out and defend territories, to identify themselves, to attract the opposite sex, to stimulate nest- building and mating, and to encourage the female to incubate her eggs. Some studies suggest that sometimes, male birds sing simply because they're happy or enjoy singing. There is strong evidence that baby birds hear their species' song while still inside their shell. The song is imprinted on their brain circuitry at this time. They are able to produce a rudimentary song, but are unable to produce the species' adult song.
Birds have accents.
While it's easy enough to identify a cardinal song in Key West or in Indiana, there is marked variation in the details of those songs. As in human speech, however, the farther apart the birds, the more marked the dialect or accent. Nobody knows exactly why this is true, but it may have something to do with the process that eventually creates sub-species of the same bird as with the western thrashers or kingfishers around the world. Songs and appearance. In some species, there is a direct relationship between the species' song and appearance.