Eye Morphology, Foveal Structure and Photoreceptor Composition in Both Foveae of Common Kestrel (Falco tinnunculus Linnaeus, 1758)

Vision is considered the most important of the five primary senses in birds, particularly in raptors, and the relatively large size of the avian eye reflects its importance. This study provides a morphological and histomorphometric characterisation of the eye and retina of the Common Kestrel (Falco tinnunculus), a diurnal raptor with bifoveate retinal organisation. Two adult Common Kestrels, obtained through a wildlife rehabilitation programme, were examined. Eye morphology was characterised using the ratio between corneal diameter and transverse eye diameter, while retinal morphology and layer thickness were evaluated using conventional histological procedures, and opsin expression was examined in both foveae. The eyes showed a globose morphology with a strongly protruding cornea and anterior segment, within the range reported for diurnal birds of prey. Marked regional variation in retinal morphology was observed, with the central retina displaying the greatest overall thickness (254.4 ± 27.04 µm), compared with 108.6 ± 15.58 µm in the peripheral retina. Two distinct foveae were identified: a deep convexiclivate fovea within the area centralis and a temporal fovea with a deep pit and steep walls. Both foveae showed displacement of the inner retinal layers and reduced thickness at the foveal pit. The central and temporal foveae exhibited depths of 217.66 µm and 106.38 µm, respectively. S- and L/M-opsin immunoreactivity was detected in both foveae, and the absence of rhodopsin immunoreactivity in the central foveal pit suggests that high-acuity vision in both foveae is predominantly mediated by cones.