I haven't visited this thread in a while, so forgive me if this has already been posted.
Here is a quote from this link:
http://ezinearticles.com/?Parr...
"Copper is an essential trace element required for hemoglobin formation and is a component of several enzyme systems. Copper is soft metal usually used for plating (not only US Mint cent coins - copper 2.5% plated zinc 97.5%, but also hardware and decorative elements on your cage), kitchen utensils, copper wire, welding. Copper can be potentially poisonous to birds - usually oxidized copper causes troubles - storing acidic food in copper containers or slightly acidic water from copper piping may contribute to elevated levels of this metal in your bird's diet and it's easy to avoid. Copper is a popular algaecide and fungicide (used in commercial and residential pools as well as to spray vegetables and grains to prevent fungus and algae growth)."
Article Source:
http://EzineArticles.com/30870...
The article was written for owners of caged parrots, not for wild birds, but essentially it is the oxidized copper which would be a problem to birds, and probably more so to the captive birds than to the wild ones - a piece of pure copper in a bird bath will not oxidize if it's under the water. Copper does not react with water (that's one of the reasons it was used for plumbing) but required oxygen in the atmosphere to oxidize.
I gotta go clean the bird bath now...
