Partial honesty in a hummingbird polymorphism provides evidence for a hybrid equilibrium

Abstract

Animal signals, while informative, are unlikely to be entirely reliable. Models of such partially honest communication have traditionally taken the form of ‘honest-enough’ signalling, in which a subset of signallers can signal at lower cost and therefore exaggerate their perceived ability or condition. Although support for these models has been demonstrated, alternatives are rarely tested in nature. Recent theory has highlighted an alternative model that also results in partial reliability, yet functions through a different mechanism. In so-called hybrid equilibria, all signallers pay the same costs given their condition, yet low-quality signallers sometimes spoof the high-quality signal, which receivers sometimes heed and sometimes ignore. Although theoretically well established, documentation of hybrid equilibria in nature is rare. Here, using previously collected behavioural data from the field and literature, we detail a game-theoretic model based on the natural history of hummingbirds. We demonstrate that an unusual female plumage polymorphism found in these birds is best explained as a hybrid equilibrium. In addition to explaining the persistence of polymorphism, the model also offers testable parameters that may predict the wide range of sex variation in plumage found across hummingbirds and other taxa, including bright and dull monomorphism and sexual dimorphism. Ultimately, our findings show that intersexual mimicry can be modelled as a hybrid equilibrium, that hybrid signals likely exist in nature, and that there is the need for a greater diversity of models to explain stable communication.