I'm at about the same stage with my new girls - getting close to 17 weeks and I've noticed a change - maturity - beginning to develop and they are making more sounds than they use to - what exactly is the "egg song"? I assume they start making some type of throaty grunting cackle - which is what mine are doing. I need to get their next boxes in the coop. I don't want to start finding eggs in all types of add ball places!
I put my hens on fermented finisher feed plus oyster shell in a separate feeder at 16 weeks. I won't give them layer feed until I have eggs from all of them. This morning 2 of my very mature looking hens (20 weeks) started running around clucking like crazy for at least 30 minutes. Another mature looking hen (20 weeks) did the same thing today, but stayed inside; every time the 2 outside clucked hysterically, she clucked back. They then settled down and were their usual quiet selves the rest of the day. I opened the laying boxes also at 16 weeks. They have been peeking and investigating them for the last week or so. Yesterday, I put a fake egg in one of them. I think they're getting close to laying. On of these hens has also been squatting more often. The hens involved are a buff orp, ee, lace Wyandotte. Also note post 120 from Sumi.