Functional morphology of the pecten oculi in the nocturnal spotted eagle owl (Bubo bubo africanus), and the diurnal black kite (Milvus migrans) and domestic fowl (Gallus gallus var. domesticus): a comparative study

The pecten oculi is a highly vascularized and pigmented organ that overlies the optic disc and projects into the vitreous body in the avian eye. First reported over 300 years ago, its function(s) remains a puzzle to ornitholo gists, ophthalmologists and anatomists. Morphometric study of this unique o rgan was undertaken in birds exhibiting apparently different visual acuitie s, namely two species of diurnal birds (the ground-dwelling domestic fowl G allus gallus var. domesticus and a highly active predator bird, the black k ite (Milvus migrans) and a nocturnal bird (the spotted-eagle owl Bubo bubo africanus). The volume of the owl's eye was 4.8 and 2.2 times larger than t hat of the fowl and the kite, respectively. However, the pecten of the fowl consisted of more pleats (16-18) compared to the kite (12-13) and the owl (5-6). The volume of the pecten of the kite was 1.4 and 2.7 times larger th an that of the fowl and the owl, respectively (P < 0.05). Similarly, the su rface area of the pecten of the kite was 2.6 and 4 times larger than that o f the fowl and owl, respectively (P < 0.05). The volume density of blood ve ssels (lumen and wall) in the pecten of the kite, fowl and owl comprised 67 .7%, 66.9% and 62.6%, respectively, the pigmented tissue constituting the r est. Both the volume density and the volume of the blood in the pecten were higher in the diurnal birds (kite, fowl) than the owl (P < 0.05). The surf ace area of the capillary luminal surface was 1.7 and 5.3 times higher in t he kite than in the fowl and the owl, respectively (P < 0.05). These result s suggest that the functional morphology of the pecten correlates with the life-style of the bird and with functional need, and lends further support to the nutritive role of the pecten.