Update 5/12: The birds are getting bigger, and are nearly ready to move on. One left with the parents last night, while the other stayed in the nest. I expect that they'll leave entirely within a couple of days.
The parents are getting even more protective of the babies than when they first hatched. When the mother returns to feed them, the father stands by.
They have started walking around and occasionally flying off.
When they fly, the first stop is the roof at the other end of the balcony:
Well, I'll be.
Here we go again!