With a diameter of about 5 cm, the ostrich’s eye is five times larger than a human eye and even larger than its own brain.
The Ostrich (Struthioniformes) is quite unusual, but they also hold many records on Earth. Not only are they the largest birds on the planet, reaching heights of up to 2.8 meters, but they are also the fastest running flightless birds. Another impressive record is that they have the largest eyes among terrestrial animals. In fact, an ostrich’s eye is even larger than its brain.
Ostrich with unusually large eyes. (Photo: Evgenia Bolyukh).
The diameter of an ostrich’s eye is approximately 5 cm, which is about five times larger than a human eye. In ostriches, the lens and cornea contribute equally to the total optical power.
Analysis shows that the ostrich’s eye is almost a larger-scale version of the eyes of other bird species, such as the European Starling (Sturnus vulgaris) and the Tawny Owl (Strix aluco). This is quite impressive as the eye shapes of these bird groups appear different. Experts hypothesize that the differences may stem from their varying weights.
For instance, the overall shape of an owl’s eye may result from the evolutionary process of having a large eye—both in terms of pupil size and axial length (measured from front to back of the eye)—that needs to fit into a small skull and remain light enough for efficient flight. However, since ostriches do not fly, they are not restricted by weight-related issues. Consequently, their enormous eyes have evolved into the distinctive shape we see today. This may provide numerous benefits, especially regarding their visual field size.
Ostriches have a monocular field of vision—the view from each eye—of about 155 degrees, allowing them to scan the horizon for potential dangers. However, they have significant blind spots above and behind their heads due to the arrangement of their eyes in the skull to avoid damage from the tropical sun. This natural shielding may be necessary to prevent retinal damage, but it does give ostriches a rather peculiar appearance.
Ostriches also possess binocular vision (both eyes looking in the same direction to obtain a three-dimensional image of their surroundings) in front of their beaks. This ability is very useful for foraging. Their visual field extends vertically to 80 degrees and horizontally to 20 degrees.
Such a range is relatively small but still helps ostriches locate food on the ground. Since they primarily feed on grass, they need to focus on small objects like leaves, seeds, and flowers. Occasionally, they also eat insects and small vertebrates, so this visual field helps them target moving prey.
Contrary to their gigantic eyes, ostrich brains are modest in size. Their brains are about 5.9 cm long and 4.2 cm wide. However, ostriches have evolved in a way that does not require a large brain. If hiding is ineffective, they can often run away very quickly.
