Subtle, but perceptible, sexual dichromatism and disassortative mating based on plumage reflectance in Black Terns (Chlidonias niger)

In birds, sexual selection on plumage often leads to sexual dichromatism and male ornamentation. However, even in drab species with no obvious sexual dichromatism, both sexes may still use plumage for mate choice. A previous study found weak sexual size dimorphism in North American Black Terns (Chlidonias niger surinamensis), but no assortative mating based on morphology. However, the relevance of plumage variation to mate choice is yet untested. Here, using reflectance spectrometry and avian visual modeling revealed that Black Tern males and females exhibited a subtle but significant difference in brightness (males darker). Importantly, the achromatic contrast between the sexes should be perceptible during mate choice. Moreover, there was evidence of disassortative mating for plumage color, but not brightness: more black and saturated birds paired with more gray and unsaturated birds. There was no relationship between plumage color or brightness and body mass. This finding suggests that visual signals unrelated to body condition may be relevant to Black Tern mate choice. The pattern of disassortative mating was unexpected, and potential benefits of choosing a mate dissimilar from oneself are discussed. This study highlights the importance of considering the avian visual system when studying plumage variation elusive to human observers.