Intro to Bird Vision
All animals on Earth, including humans, including birds, must navigate a complex and physical environment in order to survive and possibly leave a genetic legacy of a few ungrateful offspring.
Animals need to perceive features of the physical world. Due to the fact that numerous elements such as sharp rocks, cliffs, quicksand, and predators can result in harm or even death Others, like food and water, are essentials for animals to survive.
Due to the lengthy and harsh process of natural selection during evolution, animals have senses that are vital for their survival. Our ancestors relied on their senses to learn about their surroundings, stay safe, and find essential resources. And senses continue to serve us very well today.
Animals find electromagnetic radiation, whether it comes from the Sun or another source, to be especially helpful in gathering information about the environment. This energy is also thought of as photons, and it is present everywhere as visible light, ultraviolet light, microwaves, gamma rays, and other waves.
These waves/photons bounce off of objects or move through them. Animals’ sense of sight enables them to recognize some of these waves, which in turn enables them to recognize the objects themselves.
So sight is pretty dang helpful.
Even though they don’t have the best sense of humor, birds have very good vision. One could argue that birds have the best vision of all animals.
When most species go about their daily business as birds, their primary sense is sight.
To the best of our knowledge, birds have spent their entire evolutionary history mostly living during the day, and vision has been crucial to them throughout.
I bring this up because mammals spent a considerable amount of time being nocturnal. Mammals were tiny, nocturnal creatures with keen senses of smell but poor vision during the dinosaur era. It was once more advantageous for certain mammals, such as our primate ancestors, to be able to see color when they resumed their diurnal, daytime existence. So they re-evolved color vision and regained some visual acuity.
In the meantime, birds have been flying around, honing their vision the entire way. We mammals have yet to catch up to them.
Anatomy of the eye edit Anatomy of the avian eye
Bird eyes share many of the same main structures as other vertebrates. The transparent cornea at the front of the eye and two layers of sclera, or tough white collagen fiber layer that surrounds and supports the entire eye, make up the outer layer of the eye. The anterior and posterior segments are the two primary internal divisions of the eye caused by the lens. The aqueous humour, a watery fluid, fills the anterior segment, while the vitreous humour, a transparent jelly-like substance, is found in the posterior segment.
The lens is a transparent body that is convex or lens-shaped, with an inner layer that is softer and an outer layer that is harder. It focuses the light on the retina. The ciliary muscles, which are directly connected to the lens capsule through the zonular fibers, have the ability to change the shape of the lens. Apart from these muscles, certain birds possess an additional set called the Cramptons muscles, which have the ability to alter the cornea’s shape. This allows birds to have a wider range of adaptation than mammals. Certain diving water birds, like mergansers, can quickly adapt to their surroundings. In front of the lens, the iris is a colored, muscular diaphragm that regulates how much light enters the eye. The pupil, the movable circular opening through which light enters the eye, is located in the center of the iris. [4][19] Hummingbirds are amongst the many birds with two.
A smooth, curved structure with multiple layers, the retina is home to photosensitive rod and cone cells along with related blood vessels and neurons. The highest possible level of visual acuity is largely dependent on the density of the photoreceptors. The common buzzard has one million receptors per millimeter, compared to 400,000 in the house sparrow and 200,000 in humans. Resolution is influenced by the ratio of nerve ganglia to receptors because not all photoreceptors are individually connected to the optic nerve. The white wagtail has 100,000 ganglion cells to 120,000 photoreceptors, which is extremely high for a bird. [4].
Cones, which are less sensitive to light, are able to see color, while rods are more sensitive to light but do not provide any color information. 80% of the receptors in diurnal birds (or 90% in some swifts) may be cones, whereas nearly all of the receptors in nocturnal owls are rods. Like in other vertebrates, with the exception of placental mammals, some cones might have two of them. These can account for up to 100% of all cones in certain species. [20].
The fovea, also known as the less specialized area centralis, is located towards the centre of the retina and is the region with the highest forward visual acuity. It has a higher receptor density. e. sharpest, clearest detection of objects. In 25.44% of birds, such as kingfishers, hummingbirds, swallows, and birds of prey, there is a secondary focal point for improved sideways viewing. A collection of nerve fibers called the optic nerve transmits information from the eye to the appropriate areas of the brain. Similar to mammals, birds have a tiny area at the optic discwhere the optic nerve and blood vessels meet the eyethat is blind and devoid of photoreceptors. [4].
The poorly understood pecten is a body that protrudes from the retina and is made of folded tissue. It appears to maintain the retina’s nutritional supply and is richly supplied with blood vessels. It may also protect the retina from harsh light or help detect moving objects. [4] Melanin granules are widely distributed throughout Pecten oculi, and it has been suggested that they absorb stray light that enters the bird eye to lessen background glare. The metabolic rate of pecten has been suggested to be increased by a slight warming of the pecten oculi caused by melanin granules absorbing light. This may contribute to increased nutrient secretion into the vitreous body, which the birds’ avascular retina will eventually absorb for better nutrition. [21] It has been suggested that the exceptionally high enzymatic activity of alkaline phosphatase in pecten oculi supports the high secretory activity of pecten, which supplements nutrition for the retina. [22].
The layer behind the retina called the choroid is home to a large number of tiny veins and arteries. These drain venous blood and supply arterial blood to the retina. Melanin, the pigment that gives the inner eye its dark color, is found in the choroid and helps to block distracting reflections.
Movement edit A
Compared to humans, birds are better at interpreting fast movements; to them, flickering at a rate faster than 50 light pulse cycles per second appears to be continuous movement. Because of this, humans are unable to discern between individual flashes of a fluorescent light bulb oscillating at 60 cycles per second; however, budgerigars and chickens have thresholds for flicker or cycles per second of more than 100. [citation needed] A Coopers hawk can hunt swift prey through forests, dodging branches and other obstacles while pursuing them at a high rate of speed; to humans, this would appear as a blur. [11].
Birds can also detect slow moving objects. Birds are able to detect how the sun and constellations move across the sky, but humans are unable to perceive this movement. Migrating birds can correctly orient themselves thanks to their ability to detect these movements. [11].
Birds use compensatory reflexes to hold their heads as steady as possible while they are flying or perched on swaying branches. Maintaining a steady is especially relevant for birds of prey. [11] Most falcons dive using a spiral path to approach their prey after they have locked on to a target individual because they can only focus on the deep fovea of one eye at a time. Spiraling does not greatly lower speeds, but turning the head to get a better view slows down the dive by increasing drag. [57][58].
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