When I set up the bird bath, I wasn't thinking of the bees. There is water in my neighborhood - a nearby creek and feeder streams all over - so plenty of water available. But the first warm day this winter, I noticed bees in my bird bath, struggling to maintain their footing and a couple of dead bees in the water. I immediately added wine corks. Only a couple. I want the birds to use the water AND I wanted to accommodate the bees.
I noticed a very interesting feature of the corks when I was watching the bees in the bird bath yesterday. You'll see too when you look closely at this photo: