No one knows if birds really sing for joy, but we do know birds have other good reasons for singing.
Bird song is a form of advertisement, sending two messages to other birds of the same species. One message is a form of courtship. The singing male tells females he is available. The other message warns other males to stay out of his nesting territory. Male birds fly about and sing from different perches in their territory to announce its boundaries. This keeps the area from being invaded by competitors and protects the food for his family.
Singing peaks during spring when birds pair off and claim territories. There is great variety among bird songs, from the red-winged blackbird’s trill to the robin’s melodious warbling. Even the noisy hammering of a woodpecker functions like a song. A common forest bird, the red-eyed vireo, may be the champion songster. A dedicated bird-watcher (who apparently had time on his hands) observed a red-eyed vireo singing 22,297 songs within a day.
So despite its beauty, bird song is serious business. Without song, most of our common birds could not reproduce.