Light conditions and the evolution of the visual system in birds.

The article investigates how exposure to different light conditions has influenced the evolutionary divergence of the avian visual system, focusing on eye architecture and visual brain centers across 760 bird species. It finds that nocturnal birds have evolved larger corneal diameters and, to a lesser extent, longer eye axial lengths compared to diurnal species, enhancing light sensitivity but potentially compromising visual acuity. Contrary to expectations, nocturnal species tend to have smaller or unchanged sizes of the main visual brain centers—the optic tectum and the wulst—indicating a mosaic pattern of evolution where sensory and processing components do not always coevolve. These results suggest that adaptation to low-light environments in birds involves optimizing eye optics for photon capture while reducing demands on neural processing capacity.