FEATURE
Main Picture: A Military Macaw taking a proffered nut, again able to keep an ‘eye’ on the treat as well as potential predators. Bright red brow coloured with manufactured pigment, shades of green resulting from a combination of yellow pigments and blue refracted light.
Below: A Pyrrhura conure, showing its nictitating membrane (third eyelid).
carotenoids, which are largely obtained from dietary components, whereas parrots manufacture yellow and red psittacofulvins. Having dealt with colour, Mark returned
to visual acuity by explaining that humans have muscles in the eye that change the shape of the lens, allowing focus on objects just 8” from the eye. Parrots however can not only change the shape of their lens, but also have muscles that influence the cornea, meaning that they can focus on something as close as just ½” from the eye, right up to infinity. Watch your parrot holding something in its foot while it eats,
and observe the eye flicking from a downward view of the food up to watch its surroundings. More facts and figures followed swiftly: human retinas have some 200,000 cones per mm2 but parrots have twice as many at 400,000 cones per mm2. Moreover, humans have just one nerve fibre (axon) serving 7 cones, to take impulses to the brain. Parrots have one axon for each cone, thus having seven times more efficient nerve fibre transmission. The central part of the retina – the fovea – is the area of sharpest focus in the eye, and this is far larger in
BIRD SCENE 31
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