Eyes in the sky: understanding avian vision

It has been suggested that the popularityof birding, at least in part, stemsfrom the similar ways in which manand birds perceive their environment: itis much easier for us to relate to a bird’sview of the world than, for example, toa mole’s view. A bird’s view of the world,however, has long been recognizedas being considerablymore acute than ours.Expressions such as ‘eyes like ahawk’ are widely used, but analogiesare rarely, if ever, drawnbetween human eyesight andthat of any other mammal.It is fairly easy to understandwhy acute vision is of such benefitto birds – theirs is anintensely visual world. Findingfood and mates, avoiding predatorsand flying through structurallycomplex habitats such asforest, all rely on excellent eyesight.Simple evidence for theimportance of vision in birds isprovided by the size of theireyes: proportionally, they aremuch larger than those ofmammals. In absolute terms,our eyes are about the same sizeof those of a large owl or raptor.The Ostrich claims a uniqueoptical record – its eyes are largerthan those of any other terrestrialvertebrate! But havinglarge eyes is not in itself adequateto explain why birds seeso well; to do this, we have todelve deeper into the positioning,shape and structure of theeye itself and examine some ofthose features that make it sucha masterpiece of evolutionary design.The hunters and thehuntedThe positioning of the eyes in the headdiffers between different bird species. Insome, such as owls and birds of prey, theeyes are positioned towards the front ofthe head. In others, including manyground-feeding birds such as doves, theeyes are positioned on the side of thehead. These two different designs servedifferent functions. Birds which huntfast-moving prey must track their preyduring the chase. To be successful, theyneed accurate perceptions of speed anddistance. These are achieved throughbinocular (or stereoscopic) vision, thatis, both eyes are locked on to the objectsimultaneously and their movement issynchronized. In this way, birds perceivea three-dimensional image (in thesame way as we do). Whilst excellentfor hunting, this does limit their fieldof view – owls, for example, have torotate their heads in order to see behindthem.The Ostrich claims a unique optical record – its eyes are largerthan those of any other terrestrial vertebrate.Birds with laterally positioned eyeshave a very large field of view – morethan 300 ° in pigeons – but gain thisadvantage at the cost of having a muchreduced field of binocular vision. A widefield of view is, however, of great valuein detecting approaching danger.Projection screens andfocusing mechanismsThe shape of the mammalian eye, includingour own, is one of evolution’s lesssuccessful designs. Because it is essentiallyspherical in shape, only a small part ofthe image falling on the retina is in focus.The problem is akin to trying to projectan image from a slide projector on to astrongly concave screen. A bird’s eye, bycontrast, is flattened at the back, forminga much more even ‘screen’ on which theentire image is in focus, not blurringtowards the periphery. Not only does abird’s eye have a more high-tech screenthan ours, but birds also have far greaterfocusing ability. They can changetheir focal distance very rapidly, and canalso achieve focusing ‘tricks’ that wecould only manage withextremely strong glasses.Consider, for example, birdswhich hunt for their food inwater, such as cormorants andsome of the kingfishers. As youmove from air to water, refractionincreases by some 20diopters. We would normallywear glasses to correct for adeficiency of one diopter, andthe glasses you would need tocorrect for 20 diopters wouldbe so thick you would probablyneed a neck brace to wearthem! The underlying secret tothis ability in birds is that thelenses in their eyes are verymuch softer than those ofmammals and therefore theirshape can be changed moreeasily. When light enters theeye, it passes first through thecornea (the clear outer wall ofthe front part of the eye) andthen through the lens. Boththe cornea and the lens changethe direction of the light rays,but it is primarily changes inthe shape of the lens that effectfocusing. Both bird and mammaleyes have muscles thatpress up against the lens, changingits shape and bringing aboutfocusing. In mammals this isthe only real option available for focusing,but birds have others. They can alsochange the shape of the cornea by usingmuscles which drag the edges of the corneabackwards, thus increasing the curvaturein the middle.We are unable to do this, although theprinciple of changing the cornea’s shapeunderlies the radial keratotomy operationused in correcting short-sightedness. Thethird focusing technique used by birds isemployed under extreme conditions,such as underwater, and brings the irisinto play. The iris is a muscular ringthat opens and closes like the apertureof a camera to control the amount oflight passing into the eye through thelens. By holding the iris rigid, and usingthe muscles adjacent to the lens, thesoft lens can be forced to bulge NIGEL J. DENNIS50 AVIAN VISIONafrica – birds & birdingEYESGOLIATH HERONPHOTOGRAPH: NIGEL J. DENNISSKYIN THESKYUnderstandingavian visionText by Phil Hockey

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