Why do eastern North American monarch butterflies fly south in autumn?

Answer: They seek overwintering conditions they cannot find in the cold north

They need a hot climate to keep breeding all winter — The migrants do not go south to breed continuously in heat. They delay reproduction and spend winter in relatively cool forest conditions. They become reproductively active again as the overwintering period ends.

They follow flowering milkweed as the adult's only food — Milkweed is essential food for monarch caterpillars, but adults drink nectar from many kinds of flowers. The autumn journey ends at overwintering forests rather than simply tracking milkweed blooms.

They seek overwintering conditions they cannot find in the cold north ✓ — The southward flight takes these monarchs to overwintering sites in central Mexico, away from harsh northern winters. Cool, sheltered forest conditions help them conserve energy while reproduction is delayed. In spring, survivors start north again; later generations continue the journey to northern breeding grounds.