Thank you for the answers!!
The 12 birds are now in a cage that is 120 cm long, 60 cm wide and 80 cm high.
It's getting too small for them, so tomorrow they'll move to a cage that is twice bigger.
The ceiling of the cage is flexible, because it's attached very loosely... I've seen them flight till the ceiling a couple of times, but more like "playing" together, and after that they have continued a bit jumping etc. If they have been frightened by something while I haven't seen them, I think the ceiling should be safe enough anyway... I bought the quails when they were only 5 days old, so they haven't been in other cages.
I used to put hiding places to them, but they didn't use them. The previous time I had quails (bought them when they were 3 weeks old, were not as tame), they loved to have a good hiding place.The next cage will contain hiding places, because like you said, it's natural for quails to hide in bushes. They will move outdoors, so hiding places will certainly be a good thing! They also need a place where they can lay eggs alone. But I don't think getting frightened and hurting the head is the cause of the problem...
I have tried yo be quiet when they sleep, I even clean the house after they have had their naps!! One day they were tired because my 6-year old daughter started her summer holiday with me, and there was more noice. I don't have other pets, I live alone - with the quails. At my livingroom!
There is 4 quails acting this way, 2 of them are very big-sized - and they all eat with a good apetite!! I haven't noticed any difference. I really don't understand this behaviour... They don't do it very often, actually the last days only one bird has been acting like this. It started as the last doing this, prehaps it gets better??? The vets are on holiday, or they have never heard of this behaviour.
I think I will separete the 4 birds anyway..? I can keep them in the cage they are now, and the others can move to the new cage. Don't know yet, what should I do with the quails!