Largest eyes for a bird

Largest eyes for a bird
Who
ostrich Struthio camelus
What
50 millimetre(s)
Where
South Africa
When
28 February 2018

The largest eyes of any land animal are those of the ostrich Struthio camelus, native to Africa (in particular South Africa and eastern Africa) and formerly the Middle East too. The diameter of an ostrich eye measured from the centre of the cornea at the front of the eye to the centre of the retina at the back can be up to 50 mm, which is larger than its brain (no bigger than a walnut), and about twice the diameter of a human eye. In comparison, the largest eyes of any land mammal, those of the horse Equus caballus and the moose Alces alces, measure a "mere" 34 mm (some sources claim up to 40 mm for the moose but without supplying details of confirmed cases).

Possessing such huge eyes, it is not surprising to learn that ostriches have very keen eyesight, and can see for up to 3.5 km, thereby enabling potential predators such as lions to be noticed while still at a very safe distance away. In fact, researchers have calculated that the high number of photoreceptors in each ostrich eye's retina combined with the size of the image produced by the eye's lens means that its eye size is actually the maximum possible for efficient eyesight – any larger and the image captured by it would be distorted.