The chicks have some temporary distinguishing features: One has a large tuft of white feathers in the middle of her back, and the other has tufts on the sides.
The middle-tufted one was on the nest box edge for quite awhile, pulling feathers and wingercizing. I know they need to build strength and agility, but ... yikes! I hope they stay off when the wind gusts are strong today. Oh good grief, now the other one is up there and not looking too steady. And they are both up!
(Wasn't it just yesterday when they were tiny and we were worried about the rain and the cold?)