The monarch butterfly feeds mostly on milkweed, which gives it a bitter taste and thus protects it from being eaten by birds. It has a Batesian mimic, the viceroy butterfly. The viceroy does not taste bitter, but because it looks very similar to the monarch butterfly, it is protected from being eaten by any bird which has tasted a monarch. The monarch is unusual in having a large annual migration between North America and central Mexico (those west of the Rockies migrate to California). Millions of butterflies migrate, and efforts are being made to protect their roosting sites.

The chrysalis of the monarch is very striking as it hangs from a twig. It is shiny bright green with a band of gold "beads" at the top.

The viceroy butterfly can be distinguished from the monarch by a transverse black line on each hind-wing. It is also smaller (3 inches) than the monarch (4 inches).