In nature, and especially in the animal kingdom, survival is the priority for the individuals of most species. Their survival will largely depend on the mechanisms they have to perform their vital functions effectively, as well as on how they adapt to the environments and ecosystems they inhabit. They also need to be able to avoid potential dangers and predators.
Not all animal groups have the same requirements, so their adaptations vary considerably. For birds, it’s essential to have excellent vision in order to survive. They need to be able to locate their prey in any situation, even at night or in aquatic environments. It’s safe to say that sight is their most developed sense.
Each bird species has different needs, so the structure and characteristics of their eyes will change from one to another. In this article, we’re going to explain the main morphological and structural differences in the eyes of different bird groups.
Diurnal birds of prey: experts at hunting from afar
Unlike most birds, raptors have forward-facing eyes, which gives them binocular vision. In other words, both eyes analyse the same image. This type of vision helps them detect their prey and gives them a fairly wide field of vision (35° to 50°).
Additionally, their eyes are considerably large, accounting for 10-15% of their head weight (in humans, for example, they represent around 2%), and have a high concentration of receiving cells in the retina. The high density of these cells (cones and rod cells) allows them to spot small prey while flying at great heights, as they can project their image with more precision.
Raptors can calculate with accuracy the exact distance of their prey, which is a huge advantage when hunting and attacking. This is possible thanks to the existence of a structure called ‘fovea centralis‘, an area of the retina with a high concentration of cones in which the image projected is the sharpest of the visual field. Many birds of prey also have a second fovea centralis, which allows them to further increase their visual acuity.
Granivorous species and monocular vision
Granivorous bird species have a remarkably wide visual field, as, unlike raptors, their vision is mainly monocular: their eyes are located on both sides of their head, thereby increasing the field of vision. However, having their eyes on the sides of their head can also be a disadvantage; as both eyes are not on the same plane, each of them will perceive a different image, reducing their perception abilities.
According to various research and studies, it has been confirmed that this bird group perceives colours in great detail and sees a much wider range of colours than humans, for example. They also perceive environmental details better, which allows them to control their surroundings more efficiently. This is possible thanks to the high density of cones in birds’ eyes, which are the receiving cells of the retina sensitive to colours. Another interesting adaptation is that they can perceive certain colours of the ultraviolet range.
Nocturnal birds of prey and their incredible adaptation to the dark
Nocturnal birds of prey, such as owls, are one of the most interesting groups, as their activity takes place during twilight and at night, when lighting conditions are far from ideal. These birds are extremely sensitive to light, as their eyes have an abundance of receiving cells that give them extraordinary night vision. Additionally, their eyes are tubular and large, which allows them to better capture light.
As with diurnal raptors, the eyes of night raptors face forward, giving them outstanding binocular vision. This bird group also has a very flexible neck, which allows them to turn their heads almost 300° and look in almost any direction. This, combined with binocular vision, gives them a very wide visual field.
How diving birds protect their eyes under water
Aquatic bird species that spend part of their time swimming or diving, such as penguins or some anatidae, require their eyes to be protected. For this purpose, diving birds (and other bird groups) have a double membrane, a double eyelid that acts as defence and a third membrane (the ‘nictitating‘ membrane) that cleanses and protects the surface of the eye. These membranes also increase the refractive power and improve visual perception under water. They also have a powerful lens that increases accommodation power during dives.
Thanks to their eye anatomy, these species have clear vision both inside and outside water, as well as a wide visual field.
The unique vision of the Eurasian woodcock
The Eurasian woodcock is characterised by its large globular eyes, which are set far back on its head to give it one of the widest visual fields found in birds (up to 300°). It can see in almost any direction, even behind and above its head. This ability is a great advantage, as, despite its weak binocular vision, it can detect predators in advance, regardless of the direction they’re coming from.
Translated by Carlos Heras