Although there are differences among species, in general, a bird's eye weighs more than its brain. For example, the combined weight of an owl's eyes is around a quarter of an ounce (7 g), or about 4% of its total body weight. Human eyes comprise well under 0.1% of our body weight. The average bird's eyes take up approximately 15% of its head, while human eyes comprise just 1% of the whole head. Because of the keen vision necessary for spotting food sources from high in the air, birds' eyes are of vital importance for their survival. In fact, birds' eyes take up a greater proportion of their total body mass than the eyes of any other vertebrate.
More about birds and their eyes:
- Birds have a third eyelid that is located between the upper and lower lids to provide extra protection and to improve cleanliness.
- The location of a bird's eyes on its head indicates whether it is the hunter or the hunted. Predators' eyes are located in the front, while prey species have eyes on the sides of their heads.
- Birds' eyes are nearly immobile; for a bird to see from side to side, its eyes trigger a reflex that causes the entire head to move.