4. how do birds fly safely in darkness
Answers
Answer:
The ability of a bird, or for that matter a cat or any other creature, to see in the dark has to with structure of the eyes. The receptor cells in eyes are called rods and cones. All animal eyes have these photo-receptors shaped like cones and rods. Retinal cones function best in bright light and are responsible for colour vision. Rods are much more sensitive and function best in dim lights. Owls have almost a million rods per square mm so they see well at night but can hardly distinguish any colour. Humans have only a fifth of the number of rods, but they have many more cones making vision in vibrant colour during the day a delight, but our night vision is not nearly so good. .
Not all birds can see well at night, those that can–owls, nightjars, oilbirds etc–all have more rod receptor cells than cones.
What makes Owls very interesting is the fact that they have no eyeballs at all. Instead, their eyes are shaped like tubes, held rigidly in place by bones called sclerotic rings.
If you are observant you will have noticed that owls twist their whole head to look around; the reason for this behavior is that they cannot roll their eyes around the way other creatures do– they have to move their entire head to look in a different direction. They frequently twist their head and “bob and weave” to expand their field of view.
The eye of an owl in relation to body weight is 10 000 times as big as a human eye! On the same scale a human eye would be about 2.25 kgs in 75kg male [more than the liver and the heart combined].
Owls also have what is called “eyeshine.” Eyeshine is a result of an animal’s having a layer of tissue behind the retina that reflects visible light. This reflection dramatically increases the light available to the animal’s photo receptors and gives it superior night vision.
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