The "milk" of milkweed is poisonous to most animals. Monarch catepillars are one of the few organisms that can handle the toxins in their bodies.
Thus, monarch caterpillars eat the leaves of milkweed, building up the toxin in their body. When a bird eats a monarch butterfly the butterfly tastes bitter and the bird will avoid eating other monarch butterflies.
The Viceroy butterfly does not eat milkweed and therefore would taste very good to birds. However, the viceroy has similar color markings as the monarch. Therefore, birds also avoid eating viceroy butterflies since the birds relate the butterfly coloring to a bitter taste. This survival technique is known as mimicry.
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