BIRDS HAVE NEAR PERFECT VISION

Bald Eagle (CC BY-SA 2.0)

Bald Eagle (CC BY-SA 2.0)

Birds have near perfect vision and it is the most important sensory organ. They see far better than other animals. In most of the birds eyes are usually located on either side of the head and thus each has its own field of vision, but some like owls, however, have both eyes facing in front like humans. They can also move independently and thus each can be looking at different objects at one and the same time. The retina of their eyes has a greater density of sensory cells than the eyes of man, more than five times as many in the case of raptors, thus enabling them to see their prey even from a great height. Colour vision in birds is about the same as in man.

Birds, with the likely exception of diurnal primates (e.g., humans), are the vertebrates that may rely a great deal on vision to perform in their environment. The most apparent visually-dependent activity of birds is flight, but they also have remarkable range of visually-guided behaviours other than flight, for instance, choosing mate, foraging and keeping watch on predators.

Birds have larger eyes in relation to the size of their head & brain. This can be understood by the fact that eyes in humans make up about 1 per cent of the total mass of the head, while in European Starlings it is about 15 per cent. The advantages of larger eyes are larger and sharper images. While humans have two eyelids, birds have 3 – one upper and one lower plus a nictitating membrane. The third one, nictitating membrane is in between the two eyelids and the cornea with its own lubricating duct equivalent to our tear duct.

Eye Structure

Characteristic shape with front & back sections being spheres of unequal radii. Eye shapes vary among birds, ranging from globose or flattened (most diurnal birds) to tubular (owls)

Bird eye (Attribution - Jimfbleak at en.wikipedia & CC BY-SA 3.0)

Bird eye (Attribution – Jimfbleak at en.wikipedia & CC BY-SA 3.0)

Owls have tubular eyes with little extraocular musculature, to maximize the size of the eyes. To compensate, most owls can turn their heads nearly 270 degrees with such a speed that myths of complete 360° cranial revolution have sprung up. In owls eyes are frontally placed, providing as much binocular stereopsis as possible, as is typical for avian predators. The stereoscopic binocular visual field is approximately 60-70°, although the visual field is larger. The eyes together outweigh the brain as often occurs in raptorial species, especially the nocturnal owls.

Some nocturnal birds, like goatsuckers (Caprimulgiformes) posses a layer at the rear of the eye called the tapetum lucidum (meaning ‘bright carpet’) that acts as a mirror & reflects light back through the retina and, thus, makes it more likely that light will hit sensory cells in the retina. This is the reason that birds with a tapetum lucidum are able to see much better under low light conditions. It’s the presence of a tapetum lucidum that produces the ‘eyeshine’ of goatsuckers and some mammals.

Night-vision brain area in migratory birds

Twice each year, millions of birds migrate from colder regions to warmer areas and back. They also include night-migratory songbirds that cover thousands of kilometres while going from one place to another. To find their way in the dark of the night, they must process and put together spatiotemporal information from a range of signals, which also include Earth’s magnetic field and the night-time starry sky. Night-migrating songbirds possess a tight cluster of brain regions known as “cluster N”, which is highly active only during the time when night vision is needed by the birds. In night-migratory songbirds cluster N is seemingly involved in superior night vision, and could be incorporating vision-mediated magnetic and/or star compass information for night-time course-plotting.

Monocular vs. Binocular Vision

Most birds have their eyes on the side of their heads. This monocular vision enables them to see on each side at the same time. This kind of arrangement provides birds with a wide field of view. For example, Rock Doves (pigeons) are able to see objects within the range of 300 degrees without turning their

Avian Eye Field of view -pigeon (monocular) and owl (binocular)

Avian Eye Field of view -pigeon (monocular) and owl (binocular)

heads and American Woodcocks can see 360 degrees! With this kind of vision, birds may have a harder time making judgments about the distances and have poorer depth perception. Of course, these birds have a restricted field of binocular vision directly in front of them (woodcocks have a field of binocular vision behind them also). Some birds, like the owls, have their eyes in front giving them a much wider field of binocular vision. These birds may have a 180 degree field of overall vision with most of that binocular.

Airborne hunters don’t dive straight for a kill

Birds of prey do not dive straight to a prey; instead they spiral in towards their victim. This has been a mystery for a very-very long time, but now some scientists claimed to have solved it. According to them birds do it to make the most of their pin-sharp sideways vision. Tests have revealed that predatory birds see things in front of them most clearly when they turn their heads approximately 40 degrees to one side (adjusting the image to fall on the deep fovea). But doing this (turning of heads) in mid-flight might add to the birds’ aerodynamic drag and would slow them down. In an experiment it was found that at a wind speed of 42 km/hour, the drag on birds whose heads were turned 40 degrees was more than 50 per cent greater than on those looking straight ahead. To avoid this they keep their heads straight and follow a path called a logarithmic spiral. This helps them in keeping their one eye fixed on their prey while swooping down. While a spiral path is longer, the advantage of speed more than compensates it.

Avian retina

Unlike mammals, avian retina does not contain blood vessels (avascular), which averts shadows & light scattering. Avian retina has three types of photoreceptors that ‘translate’ light into nervous impulses:

  • cones – color vision
  • double cones – color vision
  • rods – black & white vision
  • Birds of the night, like owls, have retinas consisting entirely of rods. Diurnal birds’ retinas contain both rods and cones. Single cones, as well as one or both sections of double cones, contain oil droplets that consist of lipids in which carotenoid pigments are dissolved.
  • Can appear transparent, pale yellow, green, orange, or red.
  • Located at the distal end of the inner cone sections, covering the whole width of the receptor. Light passes through the droplet before entering the photosensitive outer segment.

Benefits of oil droplets:

  • they absorb light below their characteristic wavelength of transmission and, therefore, provide a protective shield against UV light.
  • they also probably act as lenses that focus light onto the photoreceptor & improve the reception of visual pigments

These oil droplets that are colourless or transparent found in many birds. They permit perception of very short spectral wavelengths (ultraviolet or near ultraviolet).

  • Such perception serves a signalling function in some species (bird plumage tends to have shorter wavelength reflectance than many other natural objects).
  • likely used as a cue in discriminating foods (e.g., plants, seeds, berries) or other natural objects.
Various birds use UV vision in different ways

Blue Tits (Parus caeruleus) use it to detect camouflaged caterpillars. Nestlings of some bird species make known their quality to parents via UV reflectance of their ‘flanges’ (rim around the edge of the mouth). Kestrels (Falco tinnunculus) use UV reflection of vole urine, left as scent-marks, to locate prey. Female of Blue-throats (Luscinia svecica) choose mates based on UV coloration of males.

Recent researches have shown that some species of birds of prey can see a wider colour spectrum than we do, together with ultraviolet light. The advantage of this can be seen in Kestrels (Falco tinnunculus) who are always on a lookout for small rodents that are fast, nimble and ranges over habitat that is often homogeneous and widespread. Searching for a prey in such habitat is extremely difficult for humans, but the Kestrel’s vision has made it easier. Rodents, while moving from one place to another, mark their tracks with urine and feces, which are noticeable in ultraviolet light. Bird’s ability to see UV light enables it to scan large areas in a relatively short time and to focus their hunting at ‘busy intersections’. Besides this, colour gradients in the sky due to UV or polarized light may also assist navigation when the sun is hidden by clouds.

Ultraviolet (UV) sparkle for mate selection

Females see something in their mates, which is not perceptible to human eye — the ultraviolet (UV) sparkle of the plumage. Birds, like many other creatures, can see ultraviolet light as well as the wavelengths invisible to humans; for example, insects use UV reflectance to reach flowers.

Foveae and Areas

Many birds have a horizontal band or “central area” across the retina with higher concentrations of sensory cells, usually with a fovea (area of highest concentration of sensory cells) at each end.

In some diurnal birds of prey, for example, American Kestrel, Photoreceptor (cone) densities in the foveae may be as high as 65,000 per square millimeter. While in humans, by comparison, the density of cones in the fovea is about 38,000 per square millimeter.

Visual acuity is defined as the ability to discriminate two points as being two points. Overall, the visual acuity of birds is comparable to humans, however, some birds, e.g., passerines, large raptors, & vultures, may have acuity 2.5 – 3 times that of humans. For example, the acuity of American Kestrels is such that they can recognize insects just 2 mm long from a distance of 18 meters.

Birds have overcome the problem of sleeping in risky situations by developing the ability to sleep with one eye open and one hemisphere of the brain awake. Such unihemispheric slow-wave sleep is in direct contrast to the typical situation in which sleep and wakefulness are mutually exclusive states of the whole brain. It has been found that birds can detect approaching predators during unihemispheric slow-wave sleep, and that they can increase their use of unihemispheric sleep as the risk of predation increases. It is believed this is the first evidence for an animal behaviourally controlling sleep and wakefulness simultaneously in different regions of the brain.

Archeopteryx_(public domain) (1880 photo of the Berlin Archaeopteryx specimen, showing leg feathers that were subsequently removed during preparation)Hearing

Hearing in Archaeopteryx vs. living birds

Archaeopteryx, 145 million-year-old creature and the earliest known bird, had a similar hearing range to that of Emus, suggesting that despite its reptilian teeth and long tail it was more birdlike than reptilian. Using computed tomography, or CT imaging the length of the inner ear of birds and reptiles could be used to predict accurately their hearing ability and even aspects of their behaviour. Scientists have proved that Archaeopteryx had an average hearing range of approximately 2000 Hz. This means it had similar hearing to modern Emus, which have some of the most limited hearing ranges of modern birds.

Hearing ranges of birdsMost birds hear best at frequencies between 1 – 5 kHz, but some are able to detect much higher frequencies (up to 10 – 12 kHz). Ability of birds to distinguish differences in the sound frequencies and to detect gaps between sounds is normally similar to humans. Their hearing is most sensitive at about 2 – 3 kHz. In general, the ears of owls (Strigiformes) are capable of detecting even the softer sounds compared to other birds over the entire range of frequencies. dBSPL is a measurement of sound pressure level in decibels, where O dBSPL is the reference to the threshold of hearing for a typical person.

Sound localization:- Substantial evidence are there to show that birds can and do localize the vocalizations (or other sounds) of  their own & other species. Many owls are specialized for pinpointing the source of sounds (and, therefore, prey). Some of them have facial discs that assist in sound localization. For example, Barn owls are well known for their noticeable ruff. The ruff serves as a sound collector and is very important for sound localization.

Little or no sense of taste

Birds have little sense of taste. This is because, firstly, their taste buds are situated quite deep inside the mouth on the soft upper palate and on the mucous membrane underneath the tongue. Secondly, unlike mammals, they straightaway swallow their food instead of chewing in absence of teeth. To make the digestion easier many of them swallow grit, which grinds the swallowed food inside the gizzard (bird’s second grinding stomach) and breaks it in small pieces to be digested. Later on this grit passes off with faeces. Besides gizzard birds also have a crop (a bag like structure) in their lower throat which serves for storing extra food when the gizzard is full. A bird can fill its crop with food and then fly somewhere safe to digest it.

Poorly developed sense of smell ?

Little work has been done on the avian sense of smell as it is believed that birds have very poorly developed sense of smell. Whatever has come out during the course of these studies, shows that the olfaction is important in the lives of only a few species like kiwis, albatrosses and their relatives (petrels and shearwaters) and some species of new world vultures. While in other birds it is probably much less important than vision and hearing. But recent observations and studies have shown that the aroma of cat scat wafting up from the base of a feeder is all it takes to keep certain songbirds away, while among domestic chicks, one bird will react excitedly to the smell of feces from a fearful peer, but only if both the smelled and smeller are eating a natural diet of insects and greenery.

Touch

Sense of touch in birds is developed in varying degrees. Generally they have such sensory organs inside the bill and on the tongue, but some also have them at the base of certain feathers and on the legs.

Bird Leg

Bird Leg

Feet

Since different species of birds lead different kind life their feet have varied shapes and sizes. The woodpeckers have short legs with zygodactylous toes, two pointing forwards and two backwards that are most suitable for climbing up the tree trunks. In case of birds of prey, those that feed by hunting have long, powerful, curved and sharp claws on all four toes, e.g. eagles, falcons, etc., whereas in the case of carrion feeders, like vultures, who also belong to the order of falcons and eagles, toes are blunt and straight.

In fact every bird has highly specially designed legs and feet even in the same environment. If we look at those birds that spend their time in water, we will find that every species has some sort of specialization according to the life it leads. For instance ducks and swans, etc., which spend most of their time swimming have webbed feet with three front toes connected with each other with a broad skin flap.

Most of the members belonging to the group of rails lead their life on water; among them some have fringed toes, while other species have fairly long legs with long toes suitable for running over the leaves of aquatic plants. Waders that are generally found on water’s edge, in mud flats, swamps etc. often have very long legs in proportion to their body. On other hand are the birds that belong to the order of storks that have very long legs enabling them to feed even in quite deep water.

Bird Beaks ( Creative Commons Attribution-Share Alike 2.5 Generic license) (Author - L. Shyamal)

Bird Beaks ( Creative Commons Attribution-Share Alike 2.5 Generic license) (Author – L. Shyamal)

Beaks

The individual bird group exhibits marked variations in the shape of the bill depending on the kind of food they eat. The avocet’s bill is long and thin with a slight upward curve, which it swings from side to side in the shallows to stir up the water for small crustaceans. Bills of ducks, geese and swans have a characteristic nail, or horny plate, at the tip of the bill, the upper and lower edges of which are serrated. Herons have a long, pointed bill that is often used to spear the prey.

As the name suggests bills in spoonbills are spoon-shaped, they are wide and flattened at the tip enabling the bird to dabble in mud or water rather like ducks. Waders normally have fairly long bills enabling them to forage in deep mud and water. In crossbills both the mandibles overlap each other, facilitating extraction of seeds from the cones of conifers. Bills of nightjars are smaller but very wide with stiff bristles at the gape, serving as sensory organ and help in catching insects in flight.

Raptors’ bills are strong, stout and down-curved. Its upper mandible is large, strong and sharp on the edge facilitating the bird to rip and tear the flesh and cut off softer pieces. On the other hand, seed-eating members of the pigeon family have short bills, whereas in wood-boring species like woodpeckers the bill is pointed and extra strong for chiselling wood and taking out hidden grubs.

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