Who's a Pretty Birdie

Hi all, I currently have 7 budgies in a large outdoor aviary, having recently lost 4! Two of these were my big breeding males On checking the 4 that died, I discovered small bugs with wings scurrying in and out of their feathers. I dont know what they are or how to treat them. I checked the remaining budgies yesterday and they dont seem to have them. Their cage is clean, seed fresh etc. Can someone please help as I dont want to lose any more. Thanks,