It looks like you have attracted a Red Admiral (Vanessa atalanta). You can tell by the distinctive reddish orange forewing band. Red Admirals are named after the red chevrons on a naval uniform.
Mature Red Admirals tend to feed on fermenting fruits, sap from trees and flower nectar. If you want to make your birdbath into a butterfly feeder you can soak a sponge in hummingbird nectar and place it in the bath.
Red Admirals are considered to be people-friendly butterflies that will approach and perch on human beings. But I think it would be alright to shoo the butterfly to another perch and fill the bird bath with water.