4 weeks old
Everything continues to proceed normally in the nest. The chicks are spending a lot of time exploring the ledge and starting to practice flapping. Here's a video clip of one of the females (presumably because the chick is so large) out for a "flap-about." Watch how she reacts when a fluffy down feather floats up in front of her!
Earlier, Apollo (presumably because this nestling is so small) was begging one of the adults for food inside the nestbox.
Here's a good shot of a couple of the chicks to document their size at the age of 4 weeks: