Monarch caterpillars feed only on Milkweed of which there are 14 known varieties of this plant in Canada.

Milkweed contains poisons known as cardiac glycosides. The poisons are stored in the caterpillars body and passed along to the adult butterfly. Birds in general do not eat Monarch butterflies as the stored poison in the Monarch's body causes very adverse reactions, such as vomiting, in the bird when eaten.

However, a few birds have developed a resistance to the Monarch's toxic properties such as the Black-headed Grosbeak and the Black-backed Oriole at the Monarch's wintering sites in Mexico. - Info from Parks Canada.