Br J Ophthalmol 2004;88:613 613 Br J Ophthalmol: first published as 10.1136/bjo.2004.045492 on 16 April 2004. Downloaded from COVER ILLUSTRATION ...... Halcyon days

n classical Greek mythology, the drops towards its prey, acuity Halcyone, daughter of Aeolus, was becomes less critical but the ability to Imarried to Ceyx, King of Thessaly. On see prey movement especially in the a voyage at sea, Ceyx fell overboard and three dimensional world of water drowned. Halcyone turned into a king- becomes paramount. The infula or fisher when she saw his corpse. Her visual streak which connects the two lamentations awakened the gods who foveae allows the image to swing then turned Ceyx into a , and temporally as the bird drops onto the Ceyx and Halcyone mated. Their nest of prey, and if the prey moves, the tem- bones and shells floated on the open sea poral fovea senses the difference in as Halcyone brooded over her young. position and the kingfisher can adjust preponderance of red oil droplets in Aeolus, her father and the god of wind, accordingly. their retinas. Many avian species that quieted the sea for seven days on either When hovering 12–15 metres above must look through water for their prey, side of the winter solstice to protect this their prey, aquatic are pre- such as kingfishers, have a preponder- union. Milton immortalised these sented with an unusual problem. The ance of red oil droplets. The reason red ‘‘Halcyon’’ days in the Hymn of the simultaneous movement of the and Nativity when he wrote ‘‘While of droplets predominate is not understood, but the droplets may help with the glare of the bird as it hovers would create a calm sit brooding on the charmed neurological nightmare. The solution wave.’’ or the dispersion of light from particu- late matter in the water. A woodland produces a dramatic real life aerial In reality, only one species of king- dance for those who have witnessed it. fisher is pelagic and none nests on the kingfisher, the (right half of cover) has five different types of oil The pied kingfisher is the only species sea, but they have evolved other inter- that is pelagic, and during its hover esting traits, most notably ocular adap- droplets in each of the four types of single cones and one type of double before the plunge-dive it keeps its head tations that are deserving of recognition. rock steady relative to the movements of Evolution has found specialised adap- cone. Centrally coincident with the nasal fovea, the cones become thinner wing and body. Curiously, the depth of tations to fit specialised niches. the prey will vary the form of the Coloured oil droplets, located in the and the oil droplets lose much of their colour (fig, left) compared to the thicker plunge-dive. The deeper the prey, the inner segments of cones, can be found steeper the dive angle (with achieved in lungfish, diurnal reptiles, turtles, and cones at the periphery (fig, right). This distribution pattern is also seen in speed as high as 4.5 m/s), perhaps to birds. Amphibians, , crocodiles, reduce the effects of refraction on the and placental mammals have lost these another woodland species, the ( sanctus) but its apparent position of the prey. structures, presumably because of an http://bjo.bmj.com/ The position of the monocular fovea extended period of nocturnality during function is not completely understood tends to be fairly constant in all bifove- evolution. All species with coloured oil (Hart NS, J Comp Physiol A ate birds, but the temporal fovea is droplets also have different types of 2001;187:685–97). different. The two foveae in kingfishers visual pigment; many avian species have As with many avian predators, king- have very large angles of separation four such pigments, so the function of fishers have two foveae in each eye. As which measure up to 52 compared to coloured droplets cannot be to add previously discussed in the February ˚ 15 in eagles and kites. This anatomical colour to a black and white world. 2001 BJO cover essay, bifoveate birds ˚

separation means that the binocular on October 2, 2021 by guest. Protected copyright. However, coloured oil droplets function have a monocular fovea and a binocular foveae are sighted along the beak much as long pass filters and will absorb fovea, but this species has a real twist. photons below a specific wavelength. The temporal fovea is at the ora and like laser sighting a gun. Coloured oil droplets decrease overall shares the binocular field with its Even the imagination of Greek photon capture, shift the peak sensitiv- compatriot in the contralateral eye even mythology cannot match the reality of ity of the cones to wavelengths longer if the neural integration is not under- this bird’s evolutionary creativity, than the peak absorbance of the visual stood. This temporal fovea is shallow although birds of calm may not be the pigments they contain, and narrow their and surprisingly inferior anatomically. best name for them. spectral bandwidth. Current evidence The ganglion cell density around the suggests that oil droplets not only monocular nasal fovea is much higher I R Schwab enhance colour vision (by increasing than the cell density around the bino- University of California, Davis, Sacramento, both discrimination and the number of cular temporal fovea, suggesting that CA, USA; [email protected] discriminable colours in the ’s the acuity is significantly higher at the N S Hart ‘‘colour space’’), but also reduce chro- nasal fovea (Moroney MK, Pettigrew Vision Hearing and Touch Center, University of matic aberration and glare (Vorobyev M, JD, J Comp Physiol A 1987;160:137–49). Queensland, Australia Proc R Soc Lond B 2003;270:1255). This would not be a surprise. Pre- Cover photographs by the author. Retinal The aquatic kingfishers, as repre- sumably, a kingfisher will sight its prey photographs by Nathan Hart, PhD, of the sented by the green and rufous king- with the higher acuity nasal monocular Vision Hearing and Touch Center, University fisher (left half of cover), have a fovea and, if aquatic, hover above it. As of Queensland, Australia.

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