Biskit, there is more in the article on that subject:

Q: Do you think, as has sometimes been suggested, that birds sing for the pure pleasure of it?

A: here is a possibility. Konrad Lorenz had a jackdaw, and he said that when the bird had had a nice meal and was relaxed and had no stress it would sit on a tree and sing. It would use all the calls in its vocabulary and he would do these movements like a Shakespearean actor doing a soliloquy. Lorenz suggested it was a form of play.