Poison
Birds are sensitive to cleaning agents and other chemicals that can leach into the water. These might include the paint job on the bath itself or herbicides, insecticides, or pesticides used in the yard. If you paint a bird bath to make it more attractive, you should use a sealant that is waterproof and wildlife-friendly over the basin. Better yet, leave the interior of the basin unpainted to protect the birds. If chemicals have been sprayed near the bird bath, e.g., in a neighbour's yard, clean it thoroughly, rinsing and drying it well, as soon as possible afterward. Avoid using chemicals such as these around them if possible.Disease
In addition to chemicals, the water in a bird bath can also be contaminated by mould, feces, and debris that is allowed to rot after falling into the water. The contaminated water can become a breeding ground for disease-causing bacteria, and any bird that drinks or bathes in it could be exposed. You should avoid allowing feces to accumulate in the area around the bird bath by cleaning it often, including any nearby perches or ledges where the birds wait their turn. As for the bird bath itself, use a weak bleach solution to clean it on a regular basis. Before refilling it, make sure it is thoroughly dry.