I guess I was wrong: in NB-1 and NB-4 brodding hast not begung yet.
The misleading factor: both females spend a significant amount of time in the nest. More than I had witnessed last year. I wonder if the territorial disputes play a role in this, what appears to me, over-proportionate "nesting" (i.e. just sitting around in the nest).
I have not recently witnessed any more disputes between the Sparrows and the new Great Tit pair. This however does not mean that the tension is not high.
What I am still wondering: are there any eggs at all in NB-1 and NB-4. When I look closely I see none in NB-4 (and of course I cannot see so deep inside the nest in NB-1).
I wonder if:
a) The Great Tit female is guarding an empty nest from (i) Sparrows or (ii) other Great Tits, or if
b) The Great Tit female is guarding the nest and lays eggs and hides them so well that it appears as though none are there
Interestingly enough, the male brings her food quite frequently. I wonder if the male is more aware than I am of what is going on or if he simply feeds the female because she is in the nest and will continue to do so until the hatchlings come. I cannot stop marvelling at the complexity of all of this wonderful process.
If I recall last year, nest building started on the 1st of May and the first egg was laid on the 4th. So if I compare, then theoretically it would not be too late if neither the Sparrow nor Great Tit have not laid eggs yet. However 2 things must not be forgotten:
1) Last year's winter seemed never to end and the temperatures were very low until end of April
2) The Blue Tit female in NB-3 is currently brooding 9 eggs...
I have started a statistics page/section as it seems as though it could be interesting to follow developments over the year and perhaps identify a trend if any.