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Understanding Avian Vision

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Understanding Avian Vision 1 t has been suggested that the popular- synchronized. In this way, birds per- entire image is in focus, not blurring 1 2 ity of birding, at least in part, stems ceive a three-dimensional image (in the towards the periphery. Not only does a 2 3 Ifrom the similar ways in which man same way as we do). Whilst excellent bird’s eye have a more high-tech screen 3 4 and birds perceive their environment: it for hunting, this does limit their field than ours, but birds also have far great- 4 5 is much easier for us to relate to a bird’s of view – owls, for example, have to er focusing ability. They can change 5 6 view of the world than, for example, to rotate their heads in order to see behind their focal distance very rapidly, and can 6 7 a mole’s view. A bird’s view of the world, them. also achieve focusing ‘tricks’ that we 7 8 however, has long been recog- could only manage with 8 9 nized as being considerably extremely strong glasses. 9 10 more acute than ours. Consider, for example, birds 10 11 Expressions such as ‘eyes like a which hunt for their food in 11 12 hawk’ are widely used, but anal- water, such as cormorants and 12 13 ogies are rarely, if ever, drawn some of the kingfishers. As you IN THE 13 14 between human eyesight and move from air to water, refrac- 14 15 that of any other mammal. tion increases by some 20 15 16 It is fairly easy to understand diopters. We would normally EYES SKY 16 17 why acute vision is of such ben- wear glasses to correct for a 17 18 efit to birds – theirs is an deficiency of one diopter, and 18 19 intensely visual world. Finding the glasses you would need to 19 20 food and mates, avoiding preda- correct for 20 diopters would 20 21 tors and flying through struc- be so thick you would probably 21 22 turally complex habitats such as need a neck brace to wear 22 23 forest, all rely on excellent eye- them! The underlying secret to 23 24 sight. Simple evidence for the this ability in birds is that the Understanding 24 25 importance of vision in birds is lenses in their eyes are very 25 26 provided by the size of their much softer than those of 26 27 eyes: proportionally, they are mammals and therefore their avian vision 27 28 much larger than those of shape can be changed more 28 29 mammals. In absolute terms, easily. When light enters the 29 Text by Phil Hockey 30 our eyes are about the same size eye, it passes first through the 30 31 of those of a large owl or raptor. cornea (the clear outer wall of 31 32 The Ostrich claims a unique the front part of the eye) and 32 33 optical record – its eyes are larg- then through the lens. Both 33 34 er than those of any other ter- the cornea and the lens change 34 35 restrial vertebrate! But having the direction of the light rays, 35 36 large eyes is not in itself ade- but it is primarily changes in 36 37 quate to explain why birds see the shape of the lens that effect 37 NIGEL J. DENNIS 38 so well; to do this, we have to focusing. Both bird and mam- 38 39 delve deeper into the position- The Ostrich claims a unique optical record – its eyes are larger mal eyes have muscles that 39 40 ing, shape and structure of the than those of any other terrestrial vertebrate. press up against the lens, chang- 40 41 eye itself and examine some of ing its shape and bringing about 41 42 those features that make it such focusing. In mammals this is 42 43 a masterpiece of evolutionary design. Birds with laterally positioned eyes the only real option available for focus- 43 44 have a very large field of view – more ing, but birds have others. They can also 44 The hunters and the 45 than 300° in pigeons – but gain this change the shape of the cornea by using 45 hunted 46 advantage at the cost of having a much muscles which drag the edges of the cor- 46 47 The positioning of the eyes in the head reduced field of binocular vision. A wide nea backwards, thus increasing the cur- 47 48 differs between different bird species. In field of view is, however, of great value vature in the middle. 48 49 some, such as owls and birds of prey, the in detecting approaching danger. We are unable to do this, although the 49 50 eyes are positioned towards the front of principle of changing the cornea’s shape 50 51 the head. In others, including many Projection screens and underlies the radial keratotomy operation 51 52 ground-feeding birds such as doves, the focusing mechanisms used in correcting short-sightedness. The 52 53 eyes are positioned on the side of the The shape of the mammalian eye, includ- third focusing technique used by birds is 53 54 head. These two different designs serve ing our own, is one of evolution’s less employed under extreme conditions, 54 55 different functions. Birds which hunt successful designs. Because it is essentially such as underwater, and brings the iris 55 56 fast-moving prey must track their prey spherical in shape, only a small part of into play. The iris is a muscular ring 56 57 during the chase. To be successful, they the image falling on the retina is in focus. that opens and closes like the aperture 57 58 need accurate perceptions of speed and The problem is akin to trying to project of a camera to control the amount of 58 59 distance. These are achieved through an image from a slide projector on to a light passing into the eye through the 59 60 binocular (or stereoscopic) vision, that strongly concave screen. A bird’s eye, by lens. By holding the iris rigid, and using 60 61 is, both eyes are locked on to the object contrast, is flattened at the back, forming the muscles adjacent to the lens, the 61 62 simultaneously and their movement is a much more even ‘screen’ on which the soft lens can be forced to bulge GOLIATH HERON 62 PHOTOGRAPH: NIGEL J. DENNIS 63 50 AVIAN VISION AFRICA – BIRDS & BIRDING 1 1 2 2 3 3 4 4 5 Over evolutionary 5 6 6 7 time, the avian eye has 7 8 8 9 developed a plethora 9 10 10 11 of special and unique 11 12 12 13 features which make it 13 14 14 15 one of Nature’s finest 15 16 16 17 designs. 17 18 18 19 19 20 20 21 21 22 22 PETER STEYN NIGEL J. DENNIS 23 23 24 through the opening of the iris, achiev- Above The large, frontally positioned and cones. These differ in the circuitry pecten. So as not to disrupt picture Above Birds which hunt their prey sky. Red droplets would remove much of 24 25 ing extreme curvature. The more rigid eyes of owls provide them with excellent through which they pass messages to reception, the pecten is anchored to a under water, such as this Pied Kingfisher, the green, which would help birds search- 25 26 lens of a mammal precludes this focus- stereoscopic vision. Because these birds the brain. In the case of rods, several small area of the retina called the optic must be able to cope with 20 diopters ing for insects in a forest. 26 27 ing option. hunt primarily in low light conditions, their converge on a single nerve chain to the disc: this is where the nerves from the of refraction – much more than the human Unlike us, birds are sensitive to ultra- 27 28 retinas are rich in rods. The Spotted Eagle brain. This design increases the strength photo-sensitive cells come together and eye can handle. violet light. They also seem to be able to 28 29 Receiving the signal Owl shown here is a typical example. of the signal through a multiplier effect. leave the eye en route to the brain. There Below A Lesser Double-collared detect polarized light, and this ability 29 30 When light enters the eye and falls on Below Eyes positioned on the side of the Cones, by contrast, each have their own are no rods or cones at the point where Sunbird feeds at an erica. Birds can may be particularly important in naviga- 30 31 the retina, an image is perceived and head, as in this Namaqua Dove, do dedicated line to the brain. Because the this nerve cable leaves the eye, so vision probably sense ultraviolet light and may tion. The pattern of light polarization in 31 32 decoded by the brain. The retina thus not work well for stereoscopic vision but strength of incoming light determines is not impaired. In both birds and mam- use this ability in the same way as the sky changes during the day as the 32 33 acts as an interface between the outside can detect the approach of danger the strength of signals sent to the brain mals, there is a small area in the middle nectar-feeding insects to detect ultraviolet sun’s position moves. This may well 33 34 world and the biological computer, the from almost any quarter. (which is why your eyes hurt in bright of the retina that is thicker than the rest.
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