Why are Monarch butterflies toxic to birds?

Monarch butterflies are toxic to birds primarily because they consume milkweed during their caterpillar stage. Milkweed plants contain toxic compounds called cardenolides (or cardiac glycosides) that are unpalatable and can be harmful to many predators. When the caterpillars eat milkweed, they accumulate these toxins in their bodies, which they retain into their adult form as butterflies. This serves as a defense mechanism, making them distasteful and potentially harmful to birds and other predators. Consequently, the bright coloration of the adult butterflies serves as a warning signal to potential predators about their toxicity.

Other factors like diet and mimicry might play a role in the survival of some other species, but in the case of Monarchs, it is the consumption of milkweed that directly leads to their toxic nature.