[quote="Resin"]I'd not try it - copper content in the water high enough to kill the algae could well be toxic for birds too.
That was my initial concern, too, but I've given it more thought. The algae are in constant contact with the copper, while the birds who visit are in, at best, infrequent, intermittent contact. Since my birdbath is not the sole source of water for even the few birds who do visit it, I would think the effect of the copper would be minimal, maybe even infantesimal.
I doubt that it would work very well. I don't think you would get enough Copper to oxidize for it too work. Copper sulfate is a common algaecide and used to kill aquatic weeds. It is less toxic to birds than it is to other animals, pigeons have an LD50 of 1000mg/kg of body weight, while rats have an LD50 of 30 mg/kg of body weight. An LD50 is the dose needed to kill 50%. There are also some long term effects to chronic exposure. Copper sulfate is also one of the oldest fungicides known.
I just wash my bath with a dilute bleach solution every couple of weeks to keep the algae in check.
Hack, I bought some barley straw concentrate. It cost an arm and a leg and at first I didn't think it worked as there didn't seem any loosening of the algae, but when I squirt it with the hose, even weeks later, all the algae just washes off. Algae still seems to grow, although maybe not as vigorously. I love it!! The bonus is, it's non-toxic and hosing the birdbaths into the garden isn't going to cause any problems to surrounding plants or good stuff in the soil.