As far as racing pigeons go I would say pigeons have to come into contact with bacteria even as ybs. Don't think u need to scrub and bleach drinkers every day as some do. Probably should be able to right ur name in the bottom of the bowl. If there's a little green algae in the bottom I wouldn't worry about it too much. The algae will be giving of healthy bacteria just like it does in the sea and rivers all over the world. Ybs are in inquisitive and will usually pick about in everything despite our best efforts to stop them. Pigeons have been seen eating some of their own dry droppings, and its said they are looking for something. Probably vitamins, probiotics etc its said to be as good as they can get and does them no real harm. wild birds also do this as do pet budges, cannaries etc.