The strange dark-adapted eyes of owls and the nightjar Caprimulgidae are difficult to explain. The feathered areas are well insulated and there might be infrared engineering within the feathers to stop the loss of infrared heat. There is little they can do about the eyes which should be bright but they seem darker than the eyes of mammals. The outside ring of the eye is bright but the centre is cool. The cool area presumably corresponds to emission from the vitreous humor.
In Britain the nightjar Caprimulgidae is a summer visitor to woodland edges with low vegetation. The author has filmed them regularly. They are crepuscular and nocturnal and can be seen hawking silently for food at dusk and dawn. There presence is first indicated by the male’s hypnotic churring song rising and falling as the head is turned. They are nearly impossible to see in the daytime as they lay along the bough of a tree with wonderfully camouflaged plumage. With a thermal imaging camera they can easily be located nesting on the ground. This video shows several nightjars filmed at Silverwoods in Romsey and thanks to the owners Brian and Jean Sutton. One of the problems filming birds in the infrared is setting the exposure. Patterns in the plumage need high gain but this saturates the eye area. As this video progresses the gain is turned down so in the last sequence the gain is very low showing details in the eyes. The iris is presumably wide open so the cold retina is a surprise as you would expect this to be close to the brain.
The pecten oculi is an unusual and poorly understood organ in the eyeball of most birds. The only bird to have a much diminished organ is the New Zealand kiwi which has abandoned vision in favour of touch.
The pecten oculi is a comb-like structure of blood vessels belonging to the choroid. (Dayan, 2013, and Kiama, 1994). It is a non-sensory, pigmented structure that projects into the vitreous body from the point where the optic nerve enters the eyeball. It therefore minimizes obstruction of the birds vision The pectin is present in the eyes of all birds and some reptiles. Most flying birds have a pleated topology with an almost rectangular profile consisting of about 12 pleats, held together apically by a heavily pigmented ‘bridge‘. Ascending and descending blood vessels of varying calibre are joined by a profuse network of capillaries reported to be about 13-20 μm in diameter. There are reports of a distinct distribution of spherical melanosomes of 1.6 to 2.2 microns diameter, densest on the bridge and upper end of the structure and strongly associated with the capillary network. This is assumed to be for protection from UV because unlike mammals birds do not have UV filtering in the front lens. It has been proposed as an organ to nourish the vitreous humor. However the organ has very high levels of carbonic anhydrase and other enzymes compatible with the catalysis of blood reactions. The working assumption is that birds use this organ to control blood chemistry.
Carbonic anhydrase catalyses the bicarbonate reaction which is highly endothermic so the blood should cool. If the pecten is an organ for cooling the vitreous humor it might explain the thermal images of a dark adapted eye. It introduces a new question – why do birds need to cool their eyes (or regulate the temperature of them).
M.O. Dayan and T. Ozaydin, “A Comparative Morphometrical Study of the Pecten Oculi in Different Avian Species”, The Scientific World Journal, Volume 2013 (2013), Article ID 968652, http://dx.doi.org/10.1155/2013/968652
S.G. Kiama, J. Bhattacharjee, J.N. Maina and K.D. Weyrauch, “A scanning electron microscope study of the pecten oculi of the black kite (Milvus migrans)”, J, Anat, 1994 Dec; 185(Pt 3): 637–642.