Translated by Nick R
A lot has been said about the extraordinary abilities birds have to fly and see from the skies with great accuracy. But do you know how their vision works? Read on to take a closer look.
Table of Contents
What is the vision?
In general, the vision or eyesight sense can be defined as the sensory ability to interpret the environment by means of specialized receptor organs such as the eyes, which are sensitive to electromagnetic rays: light. These light rays are measured as a wavelength in nanometers (nm), the shorter the wave, the bluer it tends to be (close to X-rays) and the longer it is, the redder it tends to be (close to microwave rays).
You must keep in mind that the electromagnetic spectrum (all light waves) has a wide size and the eye is able to reach certain wavelengths, not all of them. Some animals are able to see certain wavelengths such as infrared (above 700 nm) and others only visible light (380 nm – 750 nm).
How does the sense of sight work in animals?
In general, all animals, depending on their species, have a different way of seeing: some animals can see things that others cannot, some see colors, and others black and white. But what is behind the evolution of eyes?
Well, what lies behind the eyes are biological adaptations of living beings to their environment. Animals, including birds, have a viewing system connected to their brain that receives, processes, and responds to visual stimuli produced by the ocular receptors (eyes). This all happens thanks to a series of cells with specific functions.
Well, if the eyes are the main organs of vision, how do the eyes work?
The eye: the lens of nature
When we talk about the eye, it’s inevitable to think of a camera, I know, because it’s a very straightforward way to summarize the way it works. The eye is actually a receptor organ, it is the lens of the camera and not the camera as such. This is because the visual system is more complex and the brain is the real image-creating organ.
The eye consists of a fabulous structure based on these seven functional parts:
The cornea
This is one of the most important parts of the eye as it protects the eyeball’s inner area and allows light to pass through to the retina, helping to focus.
The retina
The retina transforms the light into electrical stimuli that are later converted into images in the brain. In this area are the photoreceptor cells that translate light.
There are two photoreceptor cells in the eye:
- Cones, cells that sense light better and function in full daylight to identify colors.
- Rods capture light at night, identifying black-and-white images.
The lens
The crystalline lens is a transparent membrane that adjusts the focus of the lens, adjusting to the distance.
The iris
It is like a diaphragm with the ability to contract to regulate the passage of light to the retina, acting as a door.
The pecten (birds)
It is the most peculiar part of all. It functions as a special, non-sensory system that nourishes the retina without obstructing the eye with blood vessels.
The fovea (birds)
The fovea is an area of the eye where the light rays focus (more cones), allowing for better resolution. Most birds have one in each eye, raptors have two.
Nictitating membrane (birds)
A special transparent membrane that protects and moisturizes the eyeball.
How does the eyesight sense work in birds?
Why do birds have such a sophisticated vision?
Birds indisputably have a far superior sense of sight relative to many animals, including humans. These animals have such highly developed vision that they depend on it for the most important tasks in their life cycle, showing incredible mastery in doing so. Believe me when I tell you that no animal can see what birds can see.
In general, animals have a sight sense that differs among species, based on their evolution. Birds, in particular, have a super powerful vision, given that they have been able to conquer diverse habitats, adapting to live in the air as well as in terrestrial and aquatic environments. Vision is the most important sense of birds, their life depends largely on their ability to see and translate their environment into images.
What is bird vision like?
The vision of birds is very similar to the vision of reptiles, with which they have a close evolutionary lineage. The bird’s eye is a highly developed organ consisting of a sophisticated design that evolved to give birds the ability to see from anywhere at any time of day or night. Birds have acquired several genius adaptations during their evolution, such as a larger number of light-receiving cells and a thicker optic nerve.
The sense of sight of birds can be explained from different perspectives, depending on the environment in which they live and their ecological niche or their function in the environment. In that regard, to better understand how the “owners of the sky” see, we must determine whether they are predators or not, as well as whether they are nocturnal or diurnal birds.
What types of vision exist in birds?
Diurnal raptors
Birds of prey have binocular vision since they are net hunters. Diurnal raptors (Falconiformes) depend on light and require better color rendition. Therefore, this type of bird’s vision developed more cones, for a better definition of images. Imagine comparing an HD TV and a 4K TV, which one has a better definition?
Nocturnal raptors
Contrary to daytime raptors, nocturnal raptors (Strigiformes) require a developed vision system in the absence of light. These birds see in black and white, so they developed a greater number of rods and a structure called tapetum lucidum to focus their vision better at night and capture the low light in their environment.
Granivores
Granivores are all birds that like seeds, fruits, and vegetables; many of which you have at home as pets. In relation to their vision, they are called monoculars since their eyes are flat and are located on each side of the face. This is because many of these species are prey to other animals and must be vigilant.
Divers
As for aquatic birds, adapted to water rather than to air, they have a very complex eye that allows them to see underwater. Ducks, for example, protect their eye with three membranes and two eyelids.
Special case
An example of originality is the woodcock (Scolopax rusticola), which has two large eyes in the shape of a sphere on its head very close to each other. This species is a relic, a unique animal that can be said to have a 360° scope of vision.
Differences between birds’ and humans’ vision
Birds and humans are vertebrates (have skeletons) and share much more than a friendship. They share certain parallels when it comes to intelligence, language, and sociability. However, these two animals have many differences, so here are the five main parameters that distinguish the sight of one from the other:
- Eye anatomy (physically).
- The physiology of the vision system (how it works).
- Field of vision.
- Perceived electromagnetic spectrum.
- Ecological niche or life habits.
Characteristics of bird vision
Well, you’ve probably understood by now why and how birds see. But what features make bird vision unique? Excellent question! To really understand how birds’ eyes work, you must understand what are the main adaptations that have provided birds with their great vision power. Birds can see more because they…
They have different types of eyes
The eyes of birds have different forms, depending on their ecological niche and habits, classified into different types.
According to their shape, they can be…
- Tubular for better focus.
- Globular for the highest resolution.
- Flat for a better panoramic focus.
According to their field of view
- Monocular, when each eye has an independent field of view of up to 340°, usually with low image definition.
- Binocular, when they have a field of vision with a plane shared by both eyes (focused on a single image) between 50° and 70°. Although, some raptors can reach a field of view of up to 240°.
Birds’ eyes perceive UV rays
The most amazing feature of birds is their ability to perceive a wider electromagnetic spectrum, achieving to perceive ultraviolet light and the three-dimensional structure of foliage (leaves). For example, a pigeon could easily distinguish four colors and several of their shades.
That is, birds are tetrachromatic (four types of cones), so they can see both visible lights in different shades such as red, orange, green, yellow, blue and violet; as well as capture ultraviolet light (commonly represented as an intense violet), which is located in a spectrum of light between 300 and 380 nanometers, a wavelength not visible to humans.
See beyond your nose…
One of the most prevalent characteristics of birds is the great distance from which they can see, thanks in large extent to the order of Falconiformes. If you were a mouse, a hawk could see you from a distance of 1 km, so no matter how small you were, you could not escape the clutches of a bird of prey equipped with such magnifying eyes. Some birds of prey have such acute vision, they can see objects from a mile away – amazing!
They have the all-seeing eye
As you could already see, the lens helps focus. Well, birds are able to rapidly change the crystalline lens: “the avian eye”. That is, in a hypothetical case, if you were an owl and you had large tubular eyes, you could focus your vision as if it were a super binoculars or a natural telescope embedded in your eye sockets, adjusting the lens according to the distance in just milliseconds.
How many frames per second?
Another spectacular characteristic of birds is the fluidity with which they perceive moving images. Birds can see at 100 FPS (frames per second), visualizing the flapping of other birds and the movement of stars such as the sun. In fact, scientists have reported that migratory birds are able to see magnetic fields, for use as navigational maps in their travels.
In addition…
Do birds have eyelids?
Over the course of evolution, birds have developed peculiar ocular anatomy very similar to reptiles, with obvious differences; among them, the lack of eyelids and the presence of membranes that protect the eye in different environments without blinking for even a second. The nictitating membrane is the most important web of the eye, covering, moisturizing, and protecting it at all times.
They see in any direction
Birds have different fields of vision, usually between 50° and 240°. However, there are some species of birds that break this rule and have a much wider visualization of their environment, achieving in some cases a vision of up to 360° horizontally. In other words, they can see in all directions.
More eye than face
The eyes of birds are so important that they account for a large part of the skull tissue (much in comparison to other vertebrates). For example, in an ostrich, it can reach a ratio of 12% with respect to its head.
They have a rotating head
Birds of prey have great neck mobility, so they can turn their heads considerably to detect their prey (almost 300°), as is the case with owls.
Would you like to look like a bird?
Bird’s eyes activated!
After several investigations, scientists at Lund University in Sweden invented a camera that simulates the vision of birds, allowing us humans, with a more limited vision, to see like birds… Amazing, don’t you think?
Get bird eyes in just one click, see here.
Characteristics of human vision
Humans have good vision. But, “Humans see what we want to see”. In other words, our vision is limited because our photosensitive organs can only see certain wavelengths, which allows us to interpret our environment in a certain way in relation to the information we receive and the way we analyze it.
Humans have visual limitations
No matter how 20/20 your vision is, our nature has limitations, including the distance at which we can see. If I were to compare you with an eagle, it is very likely that it can see a prey from a distance of 1 km with total clarity, while you, with perfect eyes and on the ground, can’t see the bus.
Now, add to our limitations our capacity to see moving images. Humans can only see at 25 FPS or 25 images per second, that is, a quarter of what birds can see.
Humans only see three colors
The human eye only perceives the visible light of the entire electromagnetic spectrum, which has wavelengths ranging from 380 to 750 nanometers. This characteristic makes us animals with trichromatic vision (three types of cones). That is, humans can only interpret images in terms of visible light (blue, green, and blue).
Our field of view is limited
Humans possess only one arrangement of the eyes that provides a binocular field of vision. That is, we humans have our eyes in front of our face: frontal vision, which limits the angle of vision to a maximum of 170° horizontally.
Curious fact
Chickens are able to see much farther without using their eyes. Yes, just as you read it. It sounds a bit far-fetched, but have you ever wondered why chickens wake up at a certain time of the day with such accuracy? Why do we use the crowing of the rooster as an alarm clock?
Well, our barnyard friends can see with their eyes, receiving light in the retina but also perceive light through their pineal gland and hypothalamus. What? Yes, this is because the pineal gland and hypothalamus are mainly responsible for the circadian cycle (wake and sleep); that is, when to wake up and when to sleep. So, by receiving certain wavelengths in these organs, chickens activate their special vision.
Now you know a lot about the eyes. Go, watch and learn! Many worlds await you to discover. What are you waiting for?
