Flying in circles, a male butterfly will chase a female until she leads him to a branch. They join abdomen to abdomen and stay together, sometimes falling to the ground. The females store sperm from the male, and fly off north, laying eggs on milkweed plants on their way. There are chemicals in the body of a butterfly that come from the milkweed it ate when it was a caterpillar. These are called cardenolides.
Scientists can tell which species of milkweed they came from, and where that species grows.
So it is possible to tell where an adult butterfly lived when it was a caterpillar, where it was born. No one knows how the monarchs find their way. They can fly up to 80 miles in one day. They can fly as fast as 20 miles an hour, and as far as two miles high. If something is in their way, they fly up and over it instead of around.