I've seen them fly well around the loft everyday, droppings good, no food retention or vomiting and birds feathering tight but still have a touch of YBS...If they fail to trap with the speed as they usually do and look at you with a slight glare in their eye, Generally the birds aren't switched on, well this is YBS in its mild form and can be missed by most fanciers.How to eradicate it,and never get this. Well , that's the million dollar question..