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.
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