A reader wrote to ask:

"Why do monarchs leave Mexico in the spring and migrate north? Is it because it gets too hot in Mexico during the summer? Does milkweed not grow in Mexico? Do they have to head where the milkweed supply is coming back in the spring? I have yet to find an article that explains why monarchs leave Mexico in the spring and I have googled this endlessly."

Scientists think monarchs evolved in the tropics and move north each spring to take advantage of all the milkweed that grows in the summertime.

Right now, at the overwintering sites in Mexico, the butterflies are essentially running out of resources. Water is scarce, their lipid reserves are depleted, and there aren't enough flowers to feed them. Soon they will have tremendous demands for milkweed. Imagine how much milkweed 300 million butterflies would require for egg-laying and the hungry caterpillars that follow.

Meanwhile, the growing season is beginning in the north and milkweed will soon be plentiful. By migrating north they can expand into over 1 billion acres of breeding habitat from the 15 acres of habitat they occupied this winter in Mexico.

Of course, we can never really know what causes migration. The answer is buried in evolutionary history.  But this is the hypothesis and reasoning that supports it.