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Vision of birds


bubo bubo Birds have highly developed eyes, even more so than humans in many aspects. The eyes make up a much larger percentage of the weight of the head in birds than in man, i.e. 15% for a common Starling but only 1% for man.

The eyes of most birds are on the side of their heads. Some birds have a 360 degree overall field of view. Thanks to this placement they can detect a potential danger immediately. This type of vision is called monocular vision. However, with monocular vision, birds have a harder time judging distances and have worse depth perception. This may be one reason why birds collide so often with glass panes. In contrast, their resolution in time is much better: While we can distinguish less than 20 pictures per second, birds manage to make out up to 180!

The vision that occurs when the field of vision from each eye overlaps is called binocular vision. Birds like raptors have their eyes placed far to the front giving them binocular vision as people have. These birds may have a 180 degree field of overall vision with much of that binocular. They have much sharper vision to the front than their monocular cousins.

The structure of a bird's eye is similar to humans. However, there are some very important differences. Most birds' have eyes that are flatter than human eyes. This flatness allows birds to have a larger area in focus at one time while we focus one smaller area at a time.

Cones allow the brain to perceive colours. A human may have 10,000 cones per square millimeter while some birds may have up to 120,000 per square millimeter! Birds who forage and feed in the daylight see colours very clearly from far off. In particular, they can distinguish greenish colours much better than we can. In addition, most birds have a UV sensivity with a maximum around 370 nm. It's function is not yet completely understood but this ability allows them e.g. a better detection of fruits or rodents (scent marks) and a better differentiation of plumage patterns.

For more detailed information, see e.g. http://users.mis.net/~pthrush/lighting/cvb.html

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