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

We also compared plumage color between mated individuals and found an unexpected pattern of disassortative mating: more black and saturated birds pair with more gray and unsaturated birds. Plumage may be a signal allowing individuals to choose a genetically dissimilar mate. 4/5