Just to add to Rolands post: anything added to the water that is not liked by the pigeons will cause firmer droppings. adding anything to water to firm up the droppings is false economy. If your birds have watery droppings with just giving normal water you have a problem and you need to find it and cure it, adding stuff unless an antibiotic will only paper over the cracks. Just measure how much plain water your birds drink then add something to it and measure it then, its self explanatory.
John