The Monarch butterfly is poisonous to birds. The Viceroy butterfly is not (or so the popular theory goes), but it mimics the Monarch in coloration and patterns. There is a subtle difference in the markings on the lower wings, but birds don't take the time to figure it out, they just tend to leave them both alone. Others say it isn't so, and they both taste bad. I don't eat either one.
This one is a Monarch, and we don't see very many of them around here.