Scientists had previously observed sleeping birds making movements that resembled lip-syncing. In earlier work, Mindlin and his colleagues implanted electrodes in two Zebra Finches; for a recent study in Chaos, they did the same for two Great Kiskadees. This let them record and compare neuron and muscle activity in the sleeping birds.

When awake, Zebra Finches sing a well-regulated line of staccato notes. But their sleeping song movements are fragmented, disjointed and sporadic—“rather like a dream,” Mindlin says. A dozing finch seems to silently practice a few “notes” and then add another, producing a pattern of muscle activity that reminds Mindlin “of learning a musical instrument.”