if its ybs keep all the birds in the loft for a week,does not matter a jot about isolation this is a viral condition,depurativeor a diet feed is required,glucose in the water,and as a precaution give them amoxycillin for 2-4 days to combat any chance of secondary infections (ie -e-coli),this will soon pass and the birds will be top notch within a week.crop retention is always related to ybs,but it may just be a greedy b*****d. all the best