Abstract
Strong selection pressures are known to act on animal coloration. Although many animals vary in eye colour, virtually no research has investigated the functional significance of these colour traits. Passeriformes have a range of iris colours, making them an ideal system to investigate how and why iris colour has evolved. Using phylogenetic comparative methods, we tested the hypothesis that conspicuous iris colour in passerine birds evolved in response to (a) coordination of offspring care and (b) cavity nesting, two traits thought to be involved in intra-specific gaze sensitivity. We found that iris colour and cooperative offspring care by two or more individuals evolved independently, suggesting that bright eyes are not important for coordinating parental care through eye gaze. Furthermore, we found that evolution between iris colour and nesting behaviour did occur in a dependent manner, but contrary to predictions, transitions to coloured eyes were not more frequent in cavity nesters than non-cavity nesters. Instead, our results indicate that selection away from having bright eyes was much stronger in non-cavity nesters than cavity nesters, perhaps because conspicuous eye coloration in species not concealed within a cavity would be more visible to predators.
Highlights
Coloration is a prominent animal phenotype that is essential for several aspects of signalling including aposematism [1], species recognition [2] and sexual selection [3]
Ancestral reconstruction revealed that the ancestor of passerines was most likely a dark-eyed, cavity nesting bird
Rates of change away from bright eyes are higher in non-cavity nesting birds than cavity nesting birds
Summary
Coloration is a prominent animal phenotype that is essential for several aspects of signalling including aposematism [1], species recognition [2] and sexual selection [3]. Many birds [5], amphibians [6] and fish [7] have conspicuous eyes, but the function of iris colour and its role in communication outside the primate lineage is poorly understood. Passeriformes is the largest order of birds, and has a wide range of iris colour across species, making it an ideal system for studying the evolution and function of eye coloration. Conspicuous eyes may be important for communicating to competitors—the white iris in jackdaws (Corvus monedula) may signal to conspecifics to keep away from occupied cavities [8].
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