There’s a break in the canopy above them, so there are dozens of birds congregated to soak up the sunlight. The scientist is eagerly taking notes and muttering to himself, “28…29…30… there’s 31 distinct species all in this one clearing! It’s amazing!”

The birdwatcher tells him, “No, they’re all the same species, some of them just look a little funny.”

“That’s preposterous! Every one of those birds has a distinct coloration—some are blue or purple or green, for God’s sake—you’re telling me those are ALL varieties of sparrow or something?”

“No, not sparrows, they’re robins. I guarantee it.”

They argued back and forth like this for several minutes until the scientist decides to carefully move closer to the birds to get a better look and prove the birdwatcher wrong. He moves slowly to keep from scaring the birds away, and is shocked when he gets right to the edge of the sunlit clearing. Despite their differences, each bird has a red belly and the head and beak shape of a common robin. He returns to the birdwatcher, dumbfounded, and asks, “How could you have possibly known that?”

“It’s easy,” he says. “Basking robins come in 31 flavors.”