I heard someone singing at an unusual time and thought it might be my BO pullet because she's getting close to laying age. Nope. My nutty BO roo was sitting outside his coop doing a beautiful imitation of his mate's egg song. lol
My roo (1 1/2 yr old SLW) does it all the time. Usually if someone lays an egg and starts singing they all start singing, including him. Sometimes I've even caught him doing it when nobody else is
I have found that this can also be a distress call. I had something scare one of my Brahma cockerels and he ran off belting out that song and of course all the others had to join in. I'm sure they use it for other comms as well.