Unfortunately the visibility in nesting box #1 has decreased dramatically over the last day. The Eurasian Tree Sparrow has been indeed extremely diligent and has (as perhaps expected) added twigs to the upper section of the nest.
Although I am extremely happy that the birds have picked the box as nesting location, I am bit worried as to future visibility conditions.
With some luck, the second nesting box will also have guests...of another species!
I will post a video of the nest progress later today or in the coming days.