The babies will begin to flex their wings and stand up on the edge of the nest when they get close. They will also venture out on the beam, the nest is built on, if they do as the previous clutch did.
Here's a thought I had today: I wonder if the parents know how many eggs will be laid and that determines the size of the nest. The four babies are still a few days off from leaving the nest and the nest is so full I can't imagine a fifth baby would fit!
We saved the nest from the previous clutch of 5. It will be interesting to measure both and see if the nest, for a clutch of 4, is any smaller than a nest for a clutch of 5.