Then she too flies off.
That is the last time we have seen Della!
So what has become of her? The most likely explanation is that she has been predated by, for example, a sparrowhawk which we have often seen hunting in the area. Della was not showing any signs of becoming bored with nesting - and she was continuing to be supported by her partner, Sam. It must be most unlikely that she has just gone off the idea of bringing up a brood of chicks.
This is the first time that a female that has started laying, has apparently disappeared into thin air. Let's hope we are luckier with Box1 and Box3!