The two main signs are green sticky droppings & food retention. Look for pigeons that are reluctant to fly and take no notice of you when trying to call them in. Its not the Y/B/s that kills them its a secondary infection taking advantage whilst the pigeons immune systems lowered by fighting the original infection. Nine times out of ten the secondary infection is e-coli. There's no cure for Y/B/S but you can put them on a course of treatment to nullify any secondary infection. De Weerd do a 4 in 1 treatment thats as good as anything else.