Some days hens here are quite vociferous and some days silent.
It is important to understand here what the "egg song" is and why it happens.
Many of the characteristics of the chickens' ancestors, red jungle fowl, still exhibit in our birds' behaviors.
When flocks of wild chickens would forage, a hen would have to lay an egg and would go back to a nest to lay while the rest of the flock would continue to forage.
After she was done, she would call out "Hey, where are you guys" (egg song) and the rest of the flock would call out, "Here we are". The call is primarily an effort to reconnect with friends because I've heard an isolated, separated rooster sing the song.
They can't help themselves sometimes. Even when they are all in the same pen, they continue to sing to each other.