If you've ever watched Black Terns, you know that males and females are nearly impossible to tell apart. But our quantitative analyses showed a subtle, but significant difference: males are darker than females. 2/5

Even though the difference is minor, avian visual models suggest it is perceptible to the signal receiver. Especially since Black Terns occur in wide open habitat conducive to visual signaling, we're pretty confident they can use plumage to distinguish the sexes. 3/5