Through the eyes of a bird

In following the life of a bird through the cycle of a year, we have seen how visual patterns and stimuli tend to guide and direct its actions. A bird is evidently above most other creatures eye-minded," and a thorough knowledge of how its eyes function and how they differ from our own, is of great use in interpreting many aspects of its life.

The first thing that strikes one on examining a bird's eye, is the relatively great size of the eyeball, and this fits in at once with what we have said about the " eye-mindedness " of birds as a group. Some birds' eyes are so large, and protrude into the interior of the skull so far, that the backs of the eyeballs roll on one another when the eyes are moved. There is thus little room for a large brain in the head of a bird. Only the smallest finches and warblers have eyes {6 to 8 millimeters in diameter) as small as the average reptiles, the group of animals to which birds are most closely related. Hawks and owls have eyes as big as humans, or even larger, which is quite remarkable when one considers the relative sizes of the heads. Birds' eyes can be grouped roughly into three classes, (i) Flat, (2) Globose, (3) Tubular. In Fig. 14 are shown as typical examples of these, in life-size cross-section, the eyes of a swan, an eagle, and an owl. As in the case of the human eye, the eye of a bird is divided into two main compartments bounded by the outer surface or cornea, the crystalline lens, and the rear inner surface known as the retina. The general functions of these main features of the eye are comparable with those of a camera ; the lens of the eye merely projects an image on to a sensitive recording screen. In the photographic process this is normally a plate or film coated with a chemical which is sensitive to light: in the vertebrate eye we find instead a membrane,