Ocular morphology and corneal histology in four avian species with different ecological patterns

Abstract Vision plays a central role in the ecological success of birds, and morphological specialisations of the eye often reflect habitat use and behavioural strategies. As the principal refractive surface of the eye, the cornea is a key structure for visual performance, and its architecture may reflect ecological specialisation. In this study, we describe ocular morphology and corneal histology in four bird species with contrasting ecological profiles: the Eurasian sparrowhawk ( Accipiter nisus ), the booted eagle ( Hieraaetus pennatus ), the little bittern ( Ixobrychus minutus ), and the stone-curlew ( Burhinus oedicnemus ). Using light and transmission electron microscopy, we analysed morphometric parameters, tissue thickness, and cellular densities in both central and peripheral corneal regions. Interspecific differences were recorded in total corneal thickness, in the thickness of individual layers (epithelium, stroma, Bowman’s and Descemet’s membranes, and endothelium), and in keratocyte, epithelial, and endothelial cell densities. Accipiter nisus and Hieraaetus pennatus , two diurnal predators from open habitats, exhibited thicker corneas with greater stromal compactness and higher endothelial cell density. Ixobrychus minutus , a crepuscular species from dense wetlands, had a proportionally large cornea relative to eye size but reduced thickness, characterised by high epithelial cell density and frequent intracellular vacuolisation, possibly related to osmotic stress. Burhinus oedicnemus , adapted to arid and exposed habitats, presented a thickened Descemet’s membrane and more complex endothelial interdigitations. These descriptive findings document interspecific variation in ocular and corneal organisation among species occupying different ecological niches and establish a morphological reference for future comparative and functional studies.