We're going to put it out there: ostriches are weird, but they are also very cool. Not only are they the largest bird on the planet, growing up to 2.8 meters (9.2 feet) tall but they are also the fastest flightless birds out there. But did you also know that they also have the largest eyes of any land animal? In fact, their eyes are so large that they dwarf their own brains by comparison.

Ostrich eyes are really big. They are about 2 inches (5 cm) across, which makes them about five times larger than a human eye. The optical design of their eyes is set up in a way that the lens and cornea contribute equally to the total optical power.

Interestingly, analysis of ostrich eyes has revealed them to effectively be scaled-up versions of the eyes of other birds, like the common starling and even the tawny owl. This similarity may be quite striking at first, considering that the eyes of these two bird species appear to have different shapes. This, it has been hypothesized, may be a result of the birds' differing weights.

For instance, the overall shape of an owl eye may result from the evolution of a large eye – both in terms of its aperture and axial length (the measure from the front to the back) – which needs to fit into a small skull and still be light enough for powerful flight.

However, because they are flightless birds, ostriches are not constrained by the same weight issues, so they have developed the characteristic global shape to their massive peepers. This may offer a range of advantages, especially in relation to the size of the visual field it would offer the bird.

Ostriches have a monocular field – the vision in each eye individually – that extends to about 155°, which helps them scan the horizon for potential dangers. However, ostriches have a substantial blind spot above and behind their heads which is caused by the way their eyes sit in their skulls to protect them from the tropic sun.

This type of natural shielding may be needed to prevent retinal damage, but it does make them look like angry sock puppets.

Ostriches also have binocular vision in front of their bills which is invaluable for searching for food. This visual field extends vertically through 80° and up to a width of around 20°.

It’s quite a small range, but it helps them to forage for food on the ground. As they are principally herbivores, this means ostriches need to be able to focus on small objects like leaves, seeds, and flowers. They also sometimes eat small insects and vertebrates when the feeling takes them (we’ve all been there) so this visual field also helps them target potentially moving prey.

Yet while their eyes are colossal and bulging, they are ridiculous compared to the bird’s modestly sized brain. Ostriches have brains that are about 59.26 mm (2.3 inches) long and about 42.30 mm (1.6 inches) wide. Visually, this would look like a golf ball wired up to two billiard balls.

Still, the birds have clearly evolved in a way that does not need them to have a large brain. If hiding doesn’t work, the bird can usually just outrun any problems that come its way.