Moll I'd say its most likely mite, do as skycat and Bunny's says and burn all the perches and nests, if you still have young in the nests or eggs in them take them out,get a ball of moss, put it in the nest pan and fashion a new nest with a light bulb then replace the chicks or eggs in the new nest.
You need to spray all your cages with an antimite treatment, if you soak a beer mat in Jezes fluid for a few hours hook it behind the nestpan and any remaining mite wont get near the new nests...