An ostrich's eye is larger than its entire brain.

The ostrich (Struthio camelus) holds the record for the largest eyes of any land vertebrate. Measuring approximately 50 millimeters in diameter, their eyes are significantly larger than those of an elephant or a horse. This evolutionary trade-off prioritizes visual processing over complex cognitive functions, as their brain weighs only about 40 grams compared to the combined weight of their eyes.The size of the eye is directly related to the quality of the image produced on the retina. Larger eyes allow for a larger retinal image and more light-gathering power, which is crucial for a bird that lives in wide-open habitats. This biological design gives them a field of vision spanning 210 degrees, ensuring they can detect movement across the horizon with incredible precision.Research published in journals like 'Vision Research' highlights that the ostrich eye has a unique tubular shape. This shape allows the lens and cornea to focus light onto a massive number of photoreceptor cells. These adaptations enable the bird to identify potential threats, such as lions or cheetahs, from distances exceeding 3 kilometers in daylight.To protect these massive organs, ostriches possess a nictitating membrane, often called a third eyelid. This clear membrane sweeps horizontally across the eye to moisten it and remove debris without blocking vision. Additionally, their prominent eyelashes are actually modified feathers that act as a sunshade and a barrier against the abrasive sands of their natural environment.