Monarch butterflies lay their eggs on milkweed plants. These plants are toxic to many creatures that attempt to eat them, but not to Monarch caterpillars, which feed on milkweed vegetation until they pupate. Adult Monarchs, thanks to the milkweed toxins, are distasteful to most birds, and it takes just one taste to convince these birds to avoid orange-and-black butterflies with white spots. A similarly colored butterfly, the Queen, is a close relative of the Monarch that also tastes bad because of its milkweed-eating caterpillar.
The Viceroy, on the other hand, is a mimic. It too is an orange-and-black species, but it does not feed on milkweeds and it is probably pretty tasty (to a bird). But because it resembles the Monarch in some parts of its range, and the darker Queen in other parts, birds avoid it, too. This type of adaptation, with an innocuous species gaining protection by mimicking a harmful or distasteful species, is known as Batesian mimicry.
The Monarch is not the only species that has imitators. The Pipevine Swallowtail is a beautiful butterfly that, in its caterpillar stage, feeds on the leaves of pipevines (Aristolochia species), which impart an unpleasant taste to the adult. Among the Pipevine Swallowtail's mimics are female Black, Tiger, and Spicebush Swallowtails, the female Diana Fritillary, and the Red-spotted Purple.
Because butterflies are subject to many kinds of predators and parasites at all stages of their lives, from egg to adult, they have evolved an impressive array of protective adaptations. Eggs, caterpillars, chrysalises, and adult butterflies may have camouflage coloration that allows them to blend superbly with their environment. They may resemble inedible objects such as bird droppings or sport eyespots or other markings meant to frighten off predators. All of these adaptations are fascinating, and the case of the Monarch and the Viceroy adds an additional twist: The Viceroy represents an adaptation (mimicry) to an adaptation (feeding on toxic vegetation)!
Learn more about the Monarch or the Viceroy.