well, good and bad news...
As one pair had all 5 eggs infertile and the other pair had all 7 eggs fertile, I decided to swap 2 eggs, so the first pair could take care of two chicks and the second pair would be left with just 5 of them and would have less mouths to feed..
Everything was fine. All 7 eggs hatched, but the 2 chicks that hatched in the first pair's nest I found tossed and dead
That pair has never ever tossed a chick, so I'm wondering whether it has been any of the already fledged siblings that still go into the nest every now and then (parents don't seem to care at all when they do)
Does it make any sense to you? This pair has been excellent both brooding and feeding their previous clutch...