This happens every year between April and September.
On coming downstairs in the morning no flies are visible in the open plan living area. I open a window, or the external door, and within 15 minutes about two dozen flies can be observed flying around erratically close to the ceiling. They occupy a small area at the centre of the living space, say 4’ by 4’. The rest of the room is unaffected, as are all the other rooms in the house. There is nothing notable about the territory colonised by these flies. The room is beamed at the ceiling and there is no light fitting installed there. My standards of household hygiene are, of course, impeccable.
I’m not so much requesting hints on controlling these flies as I am curious to know why they behave in this way. If anyone has a clue then please advise accordingly.
If they do this every year, I’m guessing there’s something there. Can you get up near the ceiling and smell it, touch it and check for dampness, maybe get up in the attic and check to see if there’s something going on above it.
I forgot to mention that the flies only congregate in sunny weather. On dull, overcast days with the door and windows open they are in absentia. Furthermore the windy conditions as described by RedSwinglineOne do not apply in this case as today is calm and the flies are in attendance as per usual.
There is no attic in the house as such but this may be irrelevant since the Fly Zone described in the OP is beneath the (upstairs) master bedroom. Now, there are items under my bed which I do not wish to discuss on a public message board, but I’m reasonably sure that an animal is not among them, especially a dead one.
In the absence of a more persuasive reason for the presence of these flies, and I’m still trying to digest the link of Squink, I believe it’s possible that the cause can be put down to religious fervour. This area of the living room is probably a shrine of some kind which the flies come to worship. The fact that they only come when the weather is fine is clearly an example of insects imitating human behaviour.
As an interim solution I decided to hang an effigy of a spider in the affected area in order to test the flies’ faith. Regrettably I don’t have such an effigy handy right now so I’ve nailed a toy frog to the beam instead. The frog moves its limbs and opens its mouth when you pull a cord at the bottom.
Someone in the past may have sprayed something that fluoresces in the UV, an air freshener or touch up paint, on that part of the ceiling. That’d make that bit of ceiling appear brighter than the rest, on sunny days.