The dose makes the poison, maybe OTC zinc supplements and cold remedies contain les than acid rain leaches off pennies.
The RDA is 8-11 mg per day for adult humans.
The Tolerable Upper Intake Level is 40 mg per day.
http://ods.od.nih.gov/factshee...
I found one site that mentioned cold-remedy-levels like "daily doses of zinc acetate higher than 75 milligrams", which they said was seven times more than is generally recommended. Indeed, 75 mg is around twice what the NIH thinks is the max dose that's unlikely to cause adverse health effects. That's probably why the fine print advises not taking zinc cold remedies for more than five days.
http://en.wikipedia.org/wiki/C...
"Experts recommend that zinc should not be taken for more than five days.
Long-term use of zinc -- for more than six weeks -- can lead to copper deficiency."
Or maybe birds are more vulnerable to it than humans are.
