I've seen (or rather heard) Common Blackbirds and Black Redstarts quietly singing during what seemed their resting time. The birds were just sitting on their perches, not too high from the ground. I encountered such songs outside of their breeding season too, so it seems to be something other than a common "loud" song used to attract a mate. What can it possibly mean? i've heard that birds can sing "for themselves" when their mood is good. Is that true, or there is another explanation to this?