Some background:
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The monarch butterfly's northward migration every spring looks like the sort of poorly planned bullshit you expect from mass animal migrations, with hundreds of millions spreading out across North America, presumably wherever they damn well please.

In August, they fly back south, and things get spooky. First of all, there's the fact that all hundred million fly directly to the same tiny patch of trees in Mexico 1/100th the size of Manhattan's Central Park. So what? It's not like you can have too many butterflies, right?

Wrong.
But the truly baffling part is that monarch butterflies live only for a few months. That means the migration spans generations. Every August, hundreds of millions of butterflies wake up from caterpillardom and know how to find the exact patch of trees that their great-great-grandfathers left six months earlier. This would be like being born knowing exactly how to get to the home your great-great-grandfather was born in, and your mother never told you and you don't even know that he exists or what a great-great grandfather even is because you're a butterfly.