The eyes of most birds are almost fixed in their sockets, so that the bird must turn its head to alter its field of view. Their eyeballs are usually flattened at the front, to give good wide-angle vision, and are so large that they almost fill the skull.

A hawk flying at a height of 300 m can spot a field mouse on the ground. Its eyes have to change focus very rapidly when it swoops in for the capture.

The detector cells in the retina of a birds’ eyes are very small, about a million in one square millimeter. This means that they can detect very small detail. Birds of prey, such as the hawk, have sight that is about 8 times keener than ours. Some birds also have coloured filters to sharpen contrast and help long-distance vision in hazy conditions.

In birds, sight has reached a level of perfection found in no other animal. Their vision is sharp and accurate and most have good colour vision.

Because of the position of their eyes, many birds focus close objects better with one eye than with two – this is obvious when you watch a hen feeding.