Monarch butterfly larvae feed on milkweed and a few closely related plants, whereas adults forage for nectar. Although both larvae and adults are toxic and bad-tasting due to the presence of stored cardiac glycosides in their bodies, they are still preyed upon by birds, mice and praying mantises.
Birds that eat monarch butterflies include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays, pinon jays, black-headed grosbeaks and orioles. Some of these birds avoid eating the parts of the monarch with the highest levels of glycosides, which is a strategy also used by the Chinese mantis. Others have a high tolerance for the toxins, which is a characteristic shared by some mice. Asian lady beetles eat monarch eggs and newly hatched caterpillars.