This image shows the size difference of a Monarch compared to a Viceroy. The tagged Monarch is considerably larger. (approximately 1″) The Viceroy is distinguished by its arching black band on its hind wing. Monarch caterpillars feed  exclusively on milkweeds and Viceroy caterpillars feed on willow and poplar. One birder commented that seeing a Monarch and a Viceroy together is like seeing a Downy Woodpecker and a Hairy Woodpecker. They are somewhat similar, but when viewed together you can really notice the differences in size and in other features.