Correlated evolution between nest architecture and the visual system of Passerine birds

Nest architecture is central to avian life history evolution, and the degree to which visual acuity and sensitivity mediate nest construction, nest defense from invaders (including predators and parasites), and interactions with eggs and nestlings under variable lighting conditions remains poorly understood. Here, we use a comparative approach to reveal correlated evolution between eye size and nest architecture for 1,662 species of terrestrial birds (order Passeriformes). After controlling for body mass allometry and known ecological predictors of relative eye size variation, such as habitat, foraging behavior, and diet, we assessed several hypotheses regarding the relationship between eye size and nesting behavior. Open cup and cavity nesters had the largest and smallest residual eye sizes, respectively, especially for metrics related to contrast sensitivity in dark environments. Specifically, forest specialists constructing open cups had the largest eyes, and habitat generalists constructing cavities and domes had the smallest eyes. Birds appeared to compensate for having smaller eyes that collect less light in dark nest conditions by producing relatively brighter eggshells and nestling gape coloration. Eye size was also positively correlated with intricate nest material stitching behaviors for species within the weaver songbird family (Ploceidae), revealing a previously undocumented relationship between nest construction and visual systems. In general, we provide evidence that avian eyes have evolved to enhance visual acuity and contrast sensitivity across a wide range of nest lighting conditions. Our results point toward the evolution of a pervasive set of interactions between the visual system and avian reproductive dynamics as mediated by nest architecture.