Scientists have reason to believe birds have music on the brain when they're sleeping. Researchers think that birds dream about their songs.
The key is a structure in birds' brains that controls the nerves that make singing possible. Not only does this structure control the bird's singing, but it also responds to sounds.
What's interesting is that this structure has been shown to be significantly more responsive to sound, especially to recordings of the bird's own voice, when the bird is asleep. Even when scientists aren't playing recordings of the bird's own songs to its sleeping brain, this structure shows bursts of activity throughout the duration of the creature's sleep cycle. It seems the bird is hearing music in its head.
The theory is that dreaming about its songs helps the bird learn new tunes and possibly improve them. As with most activities in life, birds learn to sing by studying and practicing. To be good singers they need to listen to songs and reproduce them. Scientists believe that when the bird sleeps it might be re-playing songs from the day, possibly memorizing the songs and trying out variations on the tunes.