Copper is a well known ingredient in algaecides but I do not know whether it is effective with mold or mildew.
Regardless, it would need to be in a form that is water soluble - which a penny is not.
For this same reason, I doubt it would have any ill effect on a bird's health.
Perhaps a chemist can weigh in on this.
In any event, you are most likely already replacing your hummer food often for other reasons - at least often enough to avoid algae.
So I see neither a benefit nor a danger of using a penny.
On Aug 3, 2017, at 6:31 AM, Stu Gibeau <sgibeau...> wrote:
A friend told me he was putting a penny in his hummingbird feeders to keep the mildew down. Is this bad for the birds? I would think it's not good for them. Although there's not much copper in pennies anymore