As I said above, I just don't see how anyone could conduct a controlled study on this. Pet birds are kept in a closed environment. Wild birds range so far and wide, and they also migrate. They sample many different water sources, including natural and polluted storm water. Without the capability of collecting dead bird bodies on a fairly large scale, how could you determine exactly what killed them? You'd need to collect them within a short time after they died to do toxicity testing as well. Impossible!
My personal opinion is that it isn't worth worrying about. There are so many other much bigger hazards that we create for wildlife, like garden chemicals and household cleaners etc. Worry about the bigger picture, use your copper bird bath, and just remember to flush the water regularly. You're providing cleaner, safer water than what's in the ditch in front of your house, for sure!
