How are monarchs toxic to predators?

Monarchs become toxic to predators by sequestering or storing toxins from the milkweed plants that they eat. Milkweed contains toxins called cardenolides, or cardiac glycosides, which are toxic to predators. This makes monarchs very distasteful or unpleasant to predators. Some predators have evolved ways to avoid or tolerate these toxins, such as the bird predators found in the Mexican overwintering colonies.