Eyes in the sky: understanding avian vision

Over evolutionarytime, the avian eye hasdeveloped a plethoraof special and uniquefeatures which make itone of Nature’s finestdesigns.PETER STEYNNIGEL J. DENNISthrough the opening of the iris, achievingextreme curvature. The more rigidlens of a mammal precludes this focusingoption.Receiving the signalWhen light enters the eye and falls onthe retina, an image is perceived anddecoded by the brain. The retina thusacts as an interface between the outsideworld and the biological computer, thebrain. Light falls on to, and triggers,nerve cells in the retina. The signal ispassed from the retina along a nervechain. There are two types of light- orphoto-sensitive cells in the retina – rodsAbove The large, frontally positionedeyes of owls provide them with excellentstereoscopic vision. Because these birdshunt primarily in low light conditions, theirretinas are rich in rods. The Spotted EagleOwl shown here is a typical example.Below Eyes positioned on the side of thehead, as in this Namaqua Dove, donot work well for stereoscopic vision butcan detect the approach of dangerfrom almost any quarter.PETER STEYNand cones. These differ in the circuitrythrough which they pass messages tothe brain. In the case of rods, severalconverge on a single nerve chain to thebrain. This design increases the strengthof the signal through a multiplier effect.Cones, by contrast, each have their owndedicated line to the brain. Because thestrength of incoming light determinesthe strength of signals sent to the brain(which is why your eyes hurt in brightlight), cones are most effective undergood light conditions – sending very clearimages to the brain – and rods work bestunder low light conditions – amplifyingthe signal but losing some of the definition.Nocturnal birds have retinas richin rods, and diurnal birds have retinasrich in cones.High-quality reception...and magnifying glassesMammals also have rods and cones inthe retina, so these alone are not a reasonfor birds’ excellent eyesight. Theirgreater efficiency in birds can only beunderstood in conjunction with the wayin which the eye receives nutrition(without which it could not function).The mammalian retina is richly suppliedwith blood vessels. These take up spaceand therefore compromise the numberof rods and cones that can be accommodatedin the retina. There is no bloodsupply to the avian retina, so that it canbe dedicated entirely to receiving andpassing on light signals. Nutrition isprovided to the bird’s eye by way of asmall, blood-rich structure called the pecten. So as not to disrupt picturereception, the pecten is anchored to asmall area of the retina called the opticdisc: this is where the nerves from thephoto-sensitive cells come together andleave the eye en route to the brain. Thereare no rods or cones at the point wherethis nerve cable leaves the eye, so visionis not impaired. In both birds and mammals,there is a small area in the middleof the retina that is thicker than the rest.This is a region particularly rich in conesand is unimaginatively termed the ‘centralarea’.In birds, the central area may have adepression within it (called a fovea).This acts as a small magnifying glass,and projects a slightly enlarged imageon to the retina. Some birds which pursueprey at high speed have two foveasin each eye – these are thought to helpin stereoscopic vision.Colour vision and beyondMany birds are brightly coloured and frequentlyuse these colours in social andother displays. This in itself is good evidencethat they have colour vision. Thecolour image is provided by visual pigmentsin the cones. In addition to these,however, birds also have brightly colouredoil droplets incorporated in their cones. Itis thought that these work not in producinga colour image per se, but by acting asfilters to enhance contrast in the sameway as photographic filters do. Thus, yellowdroplets would remove much of theblue from the background, enhancingcontrast between an object and a blueAbove Birds which hunt their preyunder water, such as this Pied Kingfisher,must be able to cope with 20 dioptersof refraction – much more than the humaneye can handle.Below A Lesser Double-collaredSunbird feeds at an erica. Birds canprobably sense ultraviolet light and mayuse this ability in the same way asnectar-feeding insects to detect ultraviolet‘nectar-guides’ on flowers.PETER STEYNsky. Red droplets would remove much ofthe green, which would help birds searchingfor insects in a forest.Unlike us, birds are sensitive to ultravioletlight. They also seem to be able todetect polarized light, and this abilitymay be particularly important in navigation.The pattern of light polarization inthe sky changes during the day as thesun’s position moves. This may wellhelp migrating birds which use the sun’sposition as a compass – especially onovercast days.At the beginning of this article, I statedthat birds were highly visual animals.Over evolutionary time, the avian eyehas developed a plethora of special andunique features which make it one ofNature’s finest designs. Perhaps we arecorrect in drawing the analogy betweenthe way in which man and birds perceivethe environment, the differencebeing that birds see it with a clarity wecould only dream of!Many birds are brightlycoloured and frequentlyuse these colours insocial and otherdisplays. This in itself isgood evidence that theyhave colour vision.52 AVIAN VISIONafrica – birds & birding1997 – volume 2, number 6AVIAN VISION53