I am not sure if this is a Monarch or a Viceroy butterfly as they are very similar in appearance. The Viceroy uses his close appearance to the Monarch as a way to protect himself from predators. Monarchs are poisonous to predators and those same predators leave the Viceroy alone because the two look so much alike. This butterfly is visiting a large patch of lantana. Lantana is a butterfly magnet; they love the tubular shape of the individual flowers. Planting masses of a color helps bring the butterflies to your garden. Plants with tubular flowers provide an inviting place for the butterlies to sip the nectar.
Addendum: One of the very sharp readers to this blog pointed out that this butterfly is a Gulf Fritillary. From the top of the wings these three butterflies look very much alike--especially to an amateur butterfly watcher like myself. However, the underside of the wings is very different. But since I love this photograph I will keep it.