I bought a heated dog bowl and wanted to make sure that the birds wouldn't either end up in the water or poo in the water. I've seen a few ideas online using bent wire, but I didn't fancy drilling holes in the bowl housing. Instead I added a vac formed dome to cover it. It looks a little like a space helmet, but with the holes in the side, the birds don't seem to mind. I'm lucky that I have a vac former setup to do this (I design and fabricate consumer products for a living). So far, no wet birds and no poop.
The original bowl:
The vac formed bowl still in the form:
The finished assembly with holes cut (note the protective film hasn't been removed in this pic, hence the cloudiness of the material):