Tuesday, 30 April 2013


Bird Vision (2) - ocular motility

Nay if thou be that princely eagle's bird
Show thy descent by gazing 'gainst the sun.
Henry VI, Part III, 2.1

Q: Why does Monty keep looking round everywhere when he is on his perch?

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A: He has to. Monty's eyes are very large in proportion to his skull – by volume, they take up about a third of his entire head. Because of this, Monty's eyes cannot move in their sockets the way ours do. In order to watch what is going on around him, he has to make constant head movements, both laterally and in elevation.

Monty can use his eyes in two different modes: he can use both eyes together, looking forwards (“stereopsis”) which helps him to estimate the distance and size of objects at short to medium ranges. He also has a second “fovea” (the dense layer of photoreceptor cells at the back of his eye) which comes into operation when he looks SIDEWAYS, out of one eye. This mode works best for his longer range high-acuity vision, to distinguish small or distant objects.

It is difficult for us humans to imagine what a picture of two different scenes with eyes pointed in different directions would look like to a bird, but there is some evidence* (from functional MRI scanning) that their brains combine the images to produce a coherent overall view of the landscape.

[*- Tame parrots appear to like MRI scanners and will happily sit inside one while it works. It may be that the humming noise emitted by the machine interests them.]

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