Monarchs have always been assumed to taste terribly bitter, because of the acrid milkweed on which the caterpillars feed. You always run into monarchs and viceroys when you read about mimicry: viceroys look enough like monarchs that keen-eyed birds who have tasted monarchs once will avoid the viceroys as well. New studies indicate that milkweed-fed monarchs are not so much evil-tasting as literally nauseating, since milkweed contains “heart poisons similar to digitalis” that make the bird ill.
