A couple of weeks ago I was walking in the local woods when I heard a lovely gentle warbling sound. It turned out that it was coming from a blackbird in a nearby tree, who was singing very quietly without opening its beak. It was definitely this bird making the sound, not another one further away. Do birds rehearse their songs before committing themselves to singing out loud? This song was surely not intended to be heard by other birds at any distance.

I've read about some studies with electrodes in the brains of sleeping zebra-finches which have suggested that they 'dream' of singing, but is this kind of 'closed-beak singing' a known behaviour?