parrots than it is in man. There is also a major difference in image persistence – the ability to detect flicker. Humans can see images changing no faster than 50 or 60 cycles per second (hertz), and films on television will therefore appear to us as a fluid motion. Parrots can detect up to 140- 160 hertz, so such a film would appear as a series of rapidly changing individual pictures. All these facts, coupled with the superior


processing power of the brain mentioned earlier, ensure that parrots are successful at avoiding danger and flying at speed past obstacles. Still with eyes, Mark told us next that


parrots have voluntary control over the muscles that dilate and constrict the pupils of their eyes. This ‘flashing’ or ‘pinning’ is used to communicate moods such as anger, fear or excitement.


In common with most prey species,


parrots’ eyes are on the sides of their heads, giving a 330° angle of vision. Not only that, but their brains can process and track two separate images simultaneously on either side of the head. Against that however is that the angle of forward binocular vision is very small, so judging distance for parrots is more difficult. They compensate for this by ‘sampling’ – moving the head back and forth quickly and comparing the different angles of view of an object to calculate distance away. Finally, there is the ‘third eyelid’, or nictitating membrane – a semi-transparent structure that can protect the eye against dust or wind when the bird is flying. Dr Stafford moved on to other senses, describing how the delicate ‘hair cells’ in a parrot’s ear are capable of regenerating if


A Panama Amazon parrot resting on one leg. Its other leg is drawn up into its body feathers. Colour again mostly green, being a combination of yellow pigment and blue refraction. Some pure yellow and red pigments showing also.


32 BIRD SCENE


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