Jennifer Ackerman, The Genius of Birds (New York: Penguin Books, 2016), p. 157:
Sleep seems to play a role in song learning for young birds, just as it does in human learning. A growing body of research suggests that the human brain continues to process the learning of a new motor skill after active training has stopped and during the sleep that follows it. This may be true for birds, too. Zebra finches practice their songs by day and sleep by night.