A sticky drop of nectar clinging to the tip of a hummingbird’s beak drips into the next flower the bird visits. With that subtle change, the microbes within that drop are now in a new environment, teeming with other microbes. This small example of species forced to coexist in the real world has helped the Fukami Lab at Stanford University unravel the relative importance of two theories scientists have had about how species can live together.