Comparison of anatomical visual features of the eyeball, lens, and retina the diurnal common kestrel (Falco tinnunculus rupicilaeformis) and the nocturnal little owl (Athene noctua glaux)

Our study aimed to compare the anatomical features of the eyeball, lens, and retina between the two raptor birds of different visual active clock hours: the diurnal common kestrel (Falco tinnunculus rupicilaeformis) and the nocturnal little owl (Athene noctua glaux) using gross, morphometric analysis, histological, and scanning and transmission electron microscopy techniques. The semi-spherical eyeball of the kestrel had less convexity on the anterior surface than on the posterior surface; meanwhile, it was relatively larger in the owl. There is a relationship between the corneal diameter (CD) and the eye axial length (AL). There were significant differences in the retinal layer thickness between the two raptors, in which the diurnal kestrel had a thinner pigmented epithelium and photoreceptor layers compared to the nocturnal owl. Moreover, the inner nuclear and outer nuclear, inner plexiform, and outer plexiform layers in the diurnal kestrel were larger than those in the nocturnal owl. The differences in the pigmented epithelium layer lead to the higher visual acuity and better color vision of the diurnal kestrel compared to the nocturnal owl. The photoreceptor layer in diurnal kestrel was composed of single large and double cones, which are composed of chief cones and accessory cones; meanwhile, the photoreceptor layer in nocturnal owl had only single elongated rods. We concluded that the lens and retina of the two raptors revealed great variations in reflecting the adaptation of each bird to different modes of life. The statistical analysis found a strong positive correlation between the axial length of the eye and the corneal diameter in both birds, as indicated by the Pearson correlation coefficient.