My birds get YBS every season,I sit with them when feeding and catch them as soon as possible,anything not coming down for food is suspect,stop feeding them right away and withhold feed for up to 36 hrs,treat with whatever you suspect them to pick up as a secondary infection,light feed of diet or depurative several times a day with Spavins bitter stimulant powder on. Just about the worst season ever for most fanciers in my area some disposing of up to 17,I was lucky and disposed of 3 and only getting one race.Looks like the Welsh lads had a similar secondary and disposed of a lot of birds .