External eye appearance in diverse avian taxa has been proposed to be driven by social and ecological functions in different studies. Recent research using quantitative measurements and phylogenetically informed analyses in primates suggest that instead photoprotective functions are important drivers of external eye appearance. Using similar methods, we examine the variation in external eye appearance of 132 parrot species (Psittaciformes) in relation to their ecology and sociality. Breeding systems, flock size and sexual dimorphism, as well as species’ latitude and maximum flying altitude were used to explore the relative contribution of social and ecological factors in driving external eye appearance. We measured the hue and brightness of visible parts of the eye and the difference in measurements of brightness between adjacent parts of the eye. While we found no link between social variables and our measurements, we found a significant negative association between the brightness of the inner part of the iris and latitude and altitude. Darker inner parts of the irises were more prevalent farther away from the equator and with higher flying altitudes. Because the amount of UV radiation varies according to these factors, our results suggest that irises are at least partially adapted for photoprotective functions. The results inform our growing understanding of the drivers of external eye appearance across land vertebrates, highlighting the importance of photopic factors across such distantly related taxa as anthropoid primates and psittaciformes.
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