Why Do Birds Sing

Why do birds sing? Has their music a meaning, or is it all a matter of blind impulse? Some bright morning in March, as you go out-of-doors, you are greeted by the notes of the first robin. Perched in a leafless tree, there he sits, facing the sun like a genuine fire-worshiper, and singing as though he would pour out his very soul. What is he thinking about? What spirit possesses him?... Birds sing when they are happy, and cry out when they are frightened, just as children do.
Only they have songs and cries of their own. You can always tell when the little song-birds are happy, for each one trills out his joyous notes as he sits on a branch of a tree, or the top of a hedge...