Bird vision - AbsoluteAstronomy.com

Vision is the most important sense for bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, since good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other vertebrate

Vertebrate

Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

groups; a pigeon has been described as "two eyes with wings". The avian eye resembles that of a reptile

Reptile

Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

, with ciliary muscles that can change the shape of the lens

Lens (anatomy)

The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

rapidly and to a greater extent than in the mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s. Birds have the largest eyes relative to their size within the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi

Pecten oculi

The pecten or pecten oculi is a comb-like structure of blood vessels belonging to the choroid in the eye of a bird. It is non-sensory and is a pigmented structure that projects into the vitreous body from the point where the optic nerve enters the eyeball. The pecten is believed to both nourish the...

, unique to birds.

Birds, like fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, amphibian

Amphibian

Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s and reptiles, have four types of colour receptors in the eye. Most mammals have two types of receptors, although primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s have three. This gives birds the ability to perceive not just the visible range but also the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

part of the spectrum

Spectrum

A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

, and other adaptations allow for the detection of polarised light or magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

s. Birds have proportionally more light receptors in the retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

than mammals, and more nerve connections between the photoreceptors and the brain.

Some bird groups have specific modifications to their visual system linked to their way of life. Birds of prey

Bird of prey

Birds of prey are birds that hunt for food primarily on the wing, using their keen senses, especially vision. They are defined as birds that primarily hunt vertebrates, including other birds. Their talons and beaks tend to be relatively large, powerful and adapted for tearing and/or piercing flesh....

have a very high density of receptors and other adaptations that maximise visual acuity. The placement of their eyes gives them good binocular vision enabling accurate judgement of distances. Nocturnal species have tubular eyes, low numbers of colour detectors, but a high density of rod cells which function well in poor light. Tern

Tern

Terns are seabirds in the family Sternidae, previously considered a subfamily of the gull family Laridae . They form a lineage with the gulls and skimmers which in turn is related to skuas and auks...

s, gull

Gull

Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...

and albatross

Albatross

Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...

es are amongst the seabird

Seabird

Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...

s which have red or yellow oil drops in the colour receptors to improve distance vision especially in hazy conditions.

Extraocular anatomy

The eye

Eye

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

of a bird most closely resembles that of the reptiles. Unlike the mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

ian eye, it is not spherical, and the flatter shape enables more of its visual field to be in focus. A circle of bony plates, the sclerotic ring

Sclerotic ring

Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

, surrounds the eye and holds it rigid, but an improvement over the reptilian eye, also found in mammals, is that the lens is pushed further forward, increasing the size of the image on the retina.

Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant

Great Cormorant

The Great Cormorant , known as the Great Black Cormorant across the Northern Hemisphere, the Black Cormorant in Australia and the Black Shag further south in New Zealand, is a widespread member of the cormorant family of seabirds...

. Birds with eyes on the sides of their heads have a wide visual field

Visual field

The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...

, useful for detecting predators, while those with eyes on the front of their heads, such as owls, have binocular vision

Binocular vision

Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...

and can estimate distances when hunting. The American Woodcock

American Woodcock

The American Woodcock , sometimes colloquially referred to as the Timberdoodle, is a small chunky shorebird species found primarily in the eastern half of North America...

probably has the largest visual field of any bird, 360^o in the horizontal plane, and 180^o in the vertical plane.
The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane

Nictitating membrane

The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

, a third concealed eyelid that sweeps horizontally across the eye like a windscreen wiper. The nictitating membrane also covers the eye and acts as a contact lens

Contact lens

A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic or therapeutic reasons. In 2004, it was estimated that 125 million people use contact lenses worldwide, including 28 to 38 million in the United...

in many aquatic birds when they are under water. When sleeping, the lower eyelid rises to cover the eye in most birds, with the exception of the horned owl

Horned owl

The American horned owls and the Old World eagle-owls make up the genus Bubo, at least as traditionally described. This genus, depending on definition, contains about one or two dozen species of typical owls and is found in many parts of the world. Some of the largest living Strigiformes are in...

s where the upper eyelid is mobile.

The eye is also cleaned by tear secretions from the lachrymal gland and protected by an oily substance from the Harderian gland

Harderian gland

Harderian gland is a gland found within the eye's orbit which occurs in vertebrates that possess a nictitating membrane....

s which coats the cornea and prevents dryness. The eye of a bird is larger compared to the size of the animal than for any other group of animals, although much of it is concealed in its skull. The Ostrich

Ostrich

The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...

has the largest eye of any land vertebrate, with an axial length of 50 mm (2 in), twice that of the human eye.

Bird eye size is broadly related to body mass. A study of five orders (parrots, pigeons, petrels, raptors and owls) showed that eye mass is proportional to body mass, but as expected from their habits and visual ecology, raptors and owls have relatively large eyes for their body mass.

Behavioural studies show that many avian species focus on distant objects preferentially with their lateral and monocular ﬁeld of vision, and birds will orient themselves sideways to maximise visual resolution. For a pigeon, resolution is twice as good with sideways monocular vision than forward binocular vision, whereas for humans the converse is true.

The performance of the eye in low light levels depends on the distance between the lens and the retina, and small birds are effectively forced to be diurnal because their eyes are not large enough to give adequate night vision. Although many species migrate

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

at night, they often collide with even brightly lit objects like lighthouses or oil platforms. Birds of prey are diurnal because, although their eyes are large, they are optimised to give maximum spatial resolution rather than light gathering, so they also do not function well in poor light. Many birds have an asymmetry in the eye's structure which enables them to keep the horizon and a significant part of the ground in focus simultaneously. The cost of this adaptation is that they have myopia

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

in the lower part of their field of view.

Birds with relatively large eyes compared to their body mass, such as Common Redstart

Common Redstart

The Common Redstart , or often simply Redstart, is a small passerine bird in the redstart genus Phoenicurus...

s and European Robin

European Robin

The European Robin , most commonly known in Anglophone Europe simply as the Robin, is a small insectivorous passerine bird that was formerly classed as a member of the thrush family , but is now considered to be an Old World flycatcher...

s sing earlier at dawn than birds of the same size and smaller body mass. However, if birds have the same eye size but different body masses, the larger species sings later than the smaller. This may be because the smaller bird has to start the day earlier because of weight loss overnight.

Nocturnal birds have eyes optimised for visual sensitivity, with large corneas relative to the eye’s length, whereas diurnal birds have longer eyes relative to the corneal diameter to give greater visual acuity. Information about the activities of extinct species can be deduced from measurements of the sclerotic ring and orbit depth. For the latter measurement to be made, the fossil must have retained its three-dimensional shape, so activity pattern cannot be determined with confidence from flattened specimens like Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...

, which has a complete sclerotic ring but no orbit depth measurement.

Anatomy of the eye

The main structures of the bird eye are similar to those of other vertebrate

Vertebrate

s. The outer layer of the eye consists of the transparent cornea

Cornea

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

at the front, and two layers of sclera

Sclera

The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

a tough white collagen fibre layer which surrounds the rest of the eye and supports and protects the eye as a whole. The eye is divided internally by the lens

Lens (anatomy)

into two main segments: the anterior segment

Anterior segment

The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.Within the anterior segment are two fluid-filled spaces:...

and the posterior segment

Posterior segment

The posterior segment is the back two-thirds of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor, retina, choroid, and optic nerve....

. The anterior chamber is filled with a watery fluid called the aqueous humour, and the posterior chamber contains the vitreous humour, a clear jelly-like substance.

The lens is a transparent convex or 'lens' shaped body with a harder outer layer and a softer inner layer. It focuses the light on the retina. The shape of the lens can be altered by ciliary muscles which are directly attached to lens capsule by means of the zonular fibres. In addition to these muscles, some birds also have a second set, Crampton’s muscles, that can change the shape of the cornea, thus giving birds a greater range of accommodation

Accommodation (eye)

Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

than is possible for mammals. This accommodation can be rapid in some diving water birds such as in the mergansers. The iris

Iris (anatomy)

The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

is a coloured muscularly operated diaphragm in front of the lens which controls the amount of light entering the eye. At the centre of the iris is the pupil, the variable circular area through which the light passes into the eye.

The retina

Retina

is a relatively smooth curved multi-layered structure containing the photosensitive rod

Rod cell

Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

and cone cell

Cone cell

Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

s with the associated neurons and blood vessels. The density of the photoreceptors is critical in determining the maximum attainable visual acuity. Humans have about 200,000 receptors per mm², but the House Sparrow

House Sparrow

The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...

has 400,000 and the Common Buzzard

Common Buzzard

The Common Buzzard is a medium to large bird of prey, whose range covers most of Europe and extends into Asia. It is usually resident all year, except in the coldest parts of its range, and in the case of one subspecies.-Description:...

1,000,000. The photoreceptors are not all individually connected to the optic nerve, and the ratio of nerve ganglia

Ganglion

In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....

to receptors is important in determining resolution. This is very high for birds; the White Wagtail

White Wagtail

"Pied Wagtail" redirects here. For the related African bird, see African Pied Wagtail.The White Wagtail is a small passerine bird in the wagtail family Motacillidae, which also includes the pipits and longclaws. This species breeds in much of Europe and Asia and parts of north Africa...

has 100,000 ganglion cells to 120,000 photoreceptors.

Rods are more sensitive to light, but give no colour information, whereas the less sensitive cones enable colour vision. In diurnal birds, 80% of the receptors may be cones (90% in some swift

Swift

The swifts are a family, Apodidae, of highly aerial birds. They are superficially similar to swallows, but are actually not closely related to passerine species at all; swifts are in the separate order Apodiformes, which they share with hummingbirds...

s) whereas nocturnal owls have almost all rods. As with other vertebrates except placental mammals

Eutheria

Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...

, some of the cones may be double structures. These can amount to 50% of all cones in some species.

Towards the centre of the retina is the fovea

Fovea

The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....

which has a greater density of receptors and is the area of greatest forward visual acuity, i.e. sharpest, clearest detection of objects. In 54% of birds, including birds of prey

Bird of prey

, kingfisher

Kingfisher

Kingfishers are a group of small to medium sized brightly coloured birds in the order Coraciiformes. They have a cosmopolitan distribution, with most species being found in the Old World and Australia...

s, hummingbird

Hummingbird

Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings...

s and swallow

Swallow

The swallows and martins are a group of passerine birds in the family Hirundinidae which are characterised by their adaptation to aerial feeding...

s, there is second fovea for enhanced sideways viewing. The optic nerve

Optic nerve

The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

is a bundle of nerve fibres which carry messages from the eye to the relevant parts of the brain and vice-versa. Like mammals, birds have a small blind spot

Blind spot (vision)

A blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...

without photoreceptors at the optic disc, under which the optic nerve and blood vessels join the eye.

The pecten is a poorly understood body consisting of folded tissue which projects from the retina. It is well supplied with blood vessels and appears to keep the retina supplied with nutrients, and may also shade the retina from dazzling light or aid in detecting moving objects.

The choroid

Choroid

The choroid, also known as the choroidea or choroid coat, is the vascular layer of the eye, containing connective tissue, and lying between the retina and the sclera. The human choroid is thickest at the far extreme rear of the eye , while in the outlying areas it narrows to 0.1 mm...

is a layer situated behind the retina which contains many small arteries and vein

Vein

In the circulatory system, veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart...

s. These provide arterial blood to the retina and drain venous blood. The choroid contains melanin

Melanin

Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

, a pigment which gives the inner eye its dark colour, helping to prevent disruptive reflections.

Light perception

There are two sorts of light receptors in a bird’s eye, rods

Rod cell

and cones

Cone cell

. Rods, which contain the visual pigment rhodopsin

Rhodopsin

Rhodopsin, also known as visual purple, is a biological pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light,...

are better for night vision because they are sensitive to small quantities of light. Cones detect specific colours (or wavelengths) of light, so they are more important to colour-oriented animals such as birds. Most birds are tetrachromatic, possessing four types of cone cells each with a distinctive maximal absorption peak. In some birds, the maximal absorption peak of the cone cell responsible for the shortest wavelength extends to the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

(UV) range, making them UV-sensitive. Pigeons have an additional pigment and are therefore pentachromatic.

The four spectrally distinct cone pigments are derived from the protein opsin

Opsin

Opsins are a group of light-sensitive 35–55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical...

, linked to a small molecule called retinal

Retinal

Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A . Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision...

, which is closely related to vitamin A

Vitamin A

Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...

. When the pigment absorbs light the retinal changes shape and alters the membrane potential of the cone cell affecting neurones in the ganglia layer of the retina. Each neurone in the ganglion layer may processes information from a number of photoreceptor cells, and may in turn trigger a nerve impulse

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

to relay information along the optic nerve for further processing in specialised visual centres in the brain. The more intense a light, the more photons are absorbed by the visual pigments, the greater the excitation of each cone, and the brighter the light appears.

By far the most abundant cone pigment in every bird species examined is the long-wavelength form of iodopsin, which absorbs at wavelengths near 570 nm. This is roughly the spectral region occupied by the red- and green-sensitive pigments in the primate retina, and this visual pigment dominates the colour sensitivity of birds. In penguin

Penguin

Penguins are a group of aquatic, flightless birds living almost exclusively in the southern hemisphere, especially in Antarctica. Highly adapted for life in the water, penguins have countershaded dark and white plumage, and their wings have become flippers...

s, this pigment appears to have shifted its absorption peak to 543 nm, presumably an adaptation to a blue aquatic environment.

The information conveyed by a single cone is limited: by itself, the cell cannot tell the brain which wavelength of light caused its excitation. A visual pigment may absorb two wavelengths equally, but even though their photons are of different energies, the cone cannot tell them apart, because they both cause the retinal to change shape and thus trigger the same impulse. For the brain to see colour, it must compare the responses of two or more classes of cones containing different visual pigments, so the four pigments in birds give increased discrimination.

Each cone of a bird or reptile contains a coloured oil droplet; these no longer exist in mammals. The droplets, which contain high concentrations of carotenoid

Carotenoid

Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

s, are placed so that light passes through before reaching the visual pigment. They act as ﬁlters, removing some wavelengths and narrowing the absorption spectra of the pigments. This reduces the response overlap between pigments and increases the number of colours that a bird can discern. Six types of cone oil droplets have been identified; five of these have carotenoid mixtures that absorb at different wavelengths and intensities, and the sixth type has no pigments. The cone pigments with the lowest maximal absorption peak, including those that are UV-sensitive, possess the 'clear' or 'transparent' type of oil droplets with little spectral tuning effect.

The colours and distributions of retinal oil droplets vary considerably among species, and is more dependent on the ecological niche utilised (hunter, fisher, herbivore) than gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

tic relationships. As examples, diurnal hunters like the Barn Swallow

Barn Swallow

The Barn Swallow is the most widespread species of swallow in the world. It is a distinctive passerine bird with blue upperparts, a long, deeply forked tail and curved, pointed wings. It is found in Europe, Asia, Africa and the Americas...

and birds of prey have few coloured droplets, whereas the surface fishing Common Tern

Common Tern

The Common Tern is a seabird of the tern family Sternidae. This bird has a circumpolar distribution, breeding in temperate and sub-Arctic regions of Europe, Asia and east and central North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. It is sometimes...

has a large number of red and yellow droplets in the dorsal retina. The evidence suggests that oil droplets respond to natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

faster than the cone's visual pigments. Even within the range of wavelengths that are visible to humans, passerine birds can detect colour differences that humans do not register. This finer discrimination, together with the ability to see ultraviolet light, means that many species show sexual dichromatism that is visible to birds but not humans.

Migratory songbirds use the Earth’s magnetic field, stars, the Sun, and polarised light patterns to determine their migratory direction. An American study showed that migratory Savannah Sparrow

Savannah Sparrow

The Savannah Sparrow is a small American sparrow. It is the only widely accepted member of the genus Passerculus...

s used polarised light from an area of sky near the horizon to recalibrate their magnetic navigation system at both sunrise and sunset. This suggested that skylight polarisation patterns are the primary calibration reference for all migratory songbirds. However, it appears that birds may be responding to secondary indicators of the angle of polarisation, and may not be actually capable of directly detecting polarisation direction in the absence of these cues.

Ultraviolet

Some birds can perceive ultraviolet light, which is involved in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

reflective patches on their feathers. Male Blue Tit

Blue Tit

The Blue Tit is a 10.5 to 12 cm long passerine bird in the tit family Paridae. It is a widespread and common resident breeder throughout temperate and subarctic Europe and western Asia in deciduous or mixed woodlands...

s have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers. Male Blue Grosbeak

Blue Grosbeak

Blue Grosbeak , is a medium-sized seed-eating bird in the same family as the Northern Cardinal, "tropical" or New World buntings, and "cardinal-grosbeaks" or New World grosbeaks....

s with the most, brightest and most UV-shifted blue in their plumage are larger, hold the most extensive territories with abundant prey, and feed their offspring more frequently than other males do.

The bill’s appearance is important in the interactions of the Blackbird. Although the UV component seems unimportant in interactions between territory-holding males, where the degree of orange is the main factor, the female responds more strongly to males with bills with good UV-reflectiveness.

A UV receptor may give an animal an advantage in foraging for food. The waxy surfaces of many fruits and berries reﬂect UV light that might advertise their presence. Common Kestrel

Common Kestrel

The Common Kestrel is a bird of prey species belonging to the kestrel group of the falcon family Falconidae. It is also known as the European Kestrel, Eurasian Kestrel, or Old World Kestrel. In Britain, where no other brown falcon occurs, it is generally just called "the kestrel".This species...

s are able to locate the trails of vole

Vole

A vole is a small rodent resembling a mouse but with a stouter body, a shorter hairy tail, a slightly rounder head, smaller ears and eyes, and differently formed molars . There are approximately 155 species of voles. They are sometimes known as meadow mice or field mice in North America...

s visually. These small rodents lay scent trails of urine and faeces that reﬂect UV light, making them visible to the kestrels, particularly in the spring before the scent marks are covered by vegetation.

Movement

Birds can resolve rapid movements better than humans, for whom flickering at a rate greater than 50 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

appears as continuous movement. Humans cannot therefore distinguish individual ﬂashes of a ﬂuorescent light bulb oscillating at 60 Hz, but Budgerigar

Budgerigar

The Budgerigar , also known as Common Pet Parakeet or Shell Parakeet informally nicknamed the budgie, is a small, long-tailed, seed-eating parrot, and the only species in the Australian genus Melopsittacus...

s and chicken

Chicken

The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

s have flicker thresholds of more than 100 Hz. A Cooper's Hawk

Cooper's Hawk

Cooper's Hawk is a medium-sized hawk native to the North American continent and found from Canada to Mexico. As in many birds of prey, the male is smaller than the female...

can pursue agile prey through woodland and avoid branches and other objects at high speed; to humans such a chase would appear as a blur.

Birds can also detect slow moving objects. The movement of the sun and the constellations across the sky is imperceptible to humans, but detected by birds. The ability to detect these movements allows migrating birds to properly orient themselves.

To obtain steady images while flying or when perched on a swaying branch, birds hold the head as steady as possible with compensating reflexes. Maintaining a steady image is especially relevant for birds of prey.

Edges and shapes

When an object is partially blocked by another, humans unconsciously tend to make up for it and complete the shapes (See Amodal perception

Amodal perception

Amodal perception is the term used to describe the perception of the whole of a physical structure when only parts of it affect the sensory receptors...

). It has however been demonstrated that pigeons do not complete occluded shapes. A study based on altering the grey level of a perch that was coloured differently from the background showed that budgerigar

Budgerigar

s do not detect edges based on colours.

Magnetic fields

The perception of magnetic fields by migratory birds has been suggested to be light dependent. Birds move their head to detect the orientation of the magnetic field, and studies on the neural pathways have suggested that birds may be able to "see" the magnetic fields. The right eye of a migratory bird contains photoreceptive proteins called cryptochrome

Cryptochrome

Cryptochromes are a class of blue light-sensitive flavoproteins found in plants and animals. Cryptochromes are involved in the circadian rhythms of plants and animals, and in the sensing of magnetic fields in a number of species...

s. Light excites these molecules to produce unpaired electrons that interact with the Earth's magnetic field, thus providing directional information.

Diurnal birds of prey

The visual ability of birds of prey is legendary, and the keenness of their eyesight is due to a variety of factors. Raptors have large eyes for their size, 1.4 times greater than the average for birds of the same weight, and the eye is tube-shaped to produce a larger retinal image. The retina has a large number of receptors per square millimetre, which determines the degree of visual acuity. The more receptors an animal has, the higher its ability to distinguish individual objects at a distance, especially when, as in raptors, each receptor is typically attached to a single ganglion. Many raptors have foveas with far more rods and cones than the human fovea (65,000/mm² in American Kestrel, 38,000 in humans) and this provides these birds with spectacular long distance vision. The fovea itself can also be lens-shaped, increasing the effective density of receptors further. This combination of factors gives Buteo
Buteo
Buteo is a genus of medium to fairly large, wide-ranging raptors with a robust body and broad wings. In the Old World, members of this genus are called "buzzards", but "hawk" is used in North America...

buzzards distance vision 6 to 8 times better than humans.

The forward-facing eyes of a bird of prey give binocular vision, which is assisted by a double fovea. The raptor's adaptations for optimum visual resolution (an American Kestrel

American Kestrel

The American Kestrel , sometimes colloquially known as the Sparrow Hawk, is a small falcon, and the only kestrel found in the Americas. It is the most common falcon in North America, and is found in a wide variety of habitats. At long, it is also the smallest falcon in North America...

can see a 2–mm insect from the top of an 18–m tree) has a disadvantage in that its vision is poor in low light level, and it must roost at night. Raptors may have to pursue mobile prey in the lower part of their visual field, and therefore do not have the lower field myopia adaptation demonstrated by many other birds. Scavenging birds like vulture

Vulture

Vulture is the name given to two groups of convergently evolved scavenging birds, the New World Vultures including the well-known Californian and Andean Condors, and the Old World Vultures including the birds which are seen scavenging on carcasses of dead animals on African plains...

s do not need such sharp vision, so a condor

Condor

Condor is the name for two species of New World vultures, each in a monotypic genus. They are the largest flying land birds in the Western Hemisphere.They are:* The Andean Condor which inhabits the Andean mountains....

has only a single fovea with about 35,000 receptors mm²

Raptors lack coloured oil drops in the cones, and probably have similar colour perception to humans, and lack the ability to detect polarised light. The generally brown, grey and white plumage of this group, and the absence of colour displays in courtship suggests that colour is relatively unimportant to these birds.

In most raptors a prominent eye ridge and its feathers extends above and in front of the eye. This "eyebrow" gives birds of prey their distinctive stare. The ridge physically protects the eye from wind, dust, and debris and shields it from excessive glare. The Osprey

Osprey

The Osprey , sometimes known as the sea hawk or fish eagle, is a diurnal, fish-eating bird of prey. It is a large raptor, reaching more than in length and across the wings...

lacks this ridge, although the arrangement of the feathers above its eyes serves a similar function; it also possesses dark feathers in front of the eye which probably serve to reduce the glare from the water surface when the bird is hunting for its staple diet of fish.

Nocturnal birds

Owls have very large eyes for their size, 2.2 times greater than the average for birds of the same weight, and positioned at the front of the head. The eyes have a field overlap of 50–70%, giving better binocular vision

Binocular vision

than for diurnal birds of prey (overlap 30–50%). The Tawny Owl's retina

Retina

has about 56,000 light-sensitive rods

Rod cell

per square millimetre (36 million per square inch); although earlier claims that it could see in the infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

part of the spectrum

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

have been dismissed.

Adaptations to night vision include the large size of the eye, its tubular shape, large numbers of closely packed retinal rods, and an absence of cones, since colour vision is unnecessary at night. There are few coloured oil drops, which would reduce the light intensity, but the retina contains a reflective layer, the tapetum lucidum

Tapetum lucidum

The tapetum lucidum is a layer of tissue in the eye of many vertebrate animals....

. This increases the amount of light each photosensitive cell receives, allowing the bird to see better in low light conditions. Owls normally have only one fovea, and that is poorly developed except in diurnal hunters like the Short-eared Owl

Short-eared Owl

The Short-eared Owl is a species of typical owl . In Scotland this species of owl is often referred to as a cataface, grass owl or short-horned hootlet. Owls belonging to genus Asio are known as the eared owls, as they have tufts of feathers resembling mammalian ears. These "ear" tufts may or may...

.

Besides owls, bat hawks

Bat Hawk

The Bat Hawk is a raptor found in sub-Saharan Africa and south Asia to New Guinea. It is named for its diet, which consists mainly of bats. It requires open space in which to hunt, but will live anywhere from dense rainforest to semi-arid veld.-Description:The Bat Hawk is a slender, medium-sized...

, frogmouths and nightjars also display good night vision. Some bird species nest deep in cave systems which are too dark for vision, and find their way to the nest with a simple form of echolocation

Animal echolocation

Echolocation, also called biosonar, is the biological sonar used by several kinds of animals.Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects...

. The Oilbird

Oilbird

The Oilbird , also known as Guácharo, is a bird found in the northern areas of South America . They are nocturnal feeders on the fruits of the Oil Palm and tropical laurels, and are the only nocturnal fruit eating birds in the world...

is the only nocturnal bird to echolocate, but several Aerodramus
Aerodramus
Aerodramus is a genus of small, dark, cave-nesting birds in the Collocaliini tribe of the swift family. Its members are confined to tropical and subtropical regions in southern Asia, Oceania and northeastern Australia...

swiftlets also utilise this technique, with one species, Atiu Swiftlet

Atiu Swiftlet

The Atiu Swiftlet is a species of bird in the swift family and endemic to Atiu in the Cook Islands.Its natural habitats are the islands fernlands and mixed horticultural areas over which it feeds and in makatea limestone caves within which it nests.-External links:*...

, also using echolocation outside its caves.

Water birds

Seabirds such as tern

Tern

s and gull

Gull

Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...

s that feed at the surface or plunge for food have red oil droplets in the cones of their retina

Retina

s. This improves contrast and sharpens distance vision, especially in hazy conditions. Birds that have to look through an air/water interface have more deeply coloured carotenoid

Carotenoid

pigment

Pigment

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

s in the oil drops than other species.

This helps them to locate shoals of fish, although it is uncertain whether they are sighting the phytoplankton

Phytoplankton

Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

on which the fish feed, or other feeding birds.

Birds that fish by stealth from above the water have to correct for refraction particularly when the fish are observed at an angle. Reef Herons and Little Egret

Little Egret

The Little Egret is a small white heron. It is the Old World counterpart to the very similar New World Snowy Egret.-Subspecies:Depending on authority, two or three subspecies of Little Egret are currently accepted....

s appear to be able to make the corrections needed when capturing fish and are more successful in catching fish when strikes are made at an acute angle and this higher success may be due to the inability of the fish to detect their predators.

Birds that pursue fish under water like auk

Auk

An auk is a bird of the family Alcidae in the order Charadriiformes. Auks are superficially similar to penguins due to their black-and-white colours, their upright posture and some of their habits...

s and divers

Loon

The loons or divers are a group of aquatic birds found in many parts of North America and northern Eurasia...

have far fewer red oil droplets, but they have special flexible lenses and use the nictitating membrane as an additional lens. This allows greater optical accommodation for good vision in air and water. Cormorants have a greater range of visual accommodation

Accommodation (eye)

Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

, at 50 dioptre

Dioptre

A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at metre...

s, than any other bird, but the kingfishers are considered to have the best all-round (air and water) vision.

Tubenosed

Procellariiformes

Procellariiformes is an order of seabirds that comprises four families: the albatrosses, petrels and shearwaters, storm petrels, and diving petrels...

seabirds, which come ashore only to breed and spend most of their life wandering close to the surface of the oceans, have a long narrow area of visual sensitivity on the retina This region, the area giganto cellularis, has been found in the Manx Shearwater

Manx Shearwater

The Manx Shearwater is a medium-sized shearwater in the seabird family Procellariidae. The scientific name of this species records a name shift: Manx Shearwaters were called Manks Puffins in the 17th century. Puffin is an Anglo-Norman word for the cured carcasses of nestling shearwaters...

, Kerguelen Petrel

Kerguelen Petrel

The Kerguelen Petrel is a small slate-grey seabird in the family Procellariidae. The species has been described as a "taxonomic oddball", being placed for a long time in Pterodroma before being split out in 1942 into its own genus Lugensa...

, Great Shearwater

Great Shearwater

The Great Shearwater is a large shearwater in the seabird family Procellariidae. Its relationships are unclear. It belongs in the group of large species that could be separated as genus Ardenna ; within these, it might be allied with the other black-billed, blunt-tailed species Short-tailed...

, Broad-billed Prion

Broad-billed Prion

The Broad-billed Prion, Pachyptila vittata, is a small seabird, but the largest Prion, with grey upperparts plumage, and white underparts. It has many other names that have been used such as Blue-billed Dove-petrel, Broad-billed Dove-petrel, Long-billed Prion, Common Prion, Icebird, and...

and Common Diving-petrel. It is characterised by the presence of ganglion cells which are regularly arrayed and larger than those found in the rest of the retina, and morphologically appear similar to the cells of the retina in cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s. The location and cellular morphology of this novel area suggests a function in the detection of items in a small binocular ﬁeld projecting below and around the bill. It is not concerned primarily with high spatial resolution, but may assist in the detection of prey near the sea surface as a bird flies low over it.

The Manx Shearwater

Manx Shearwater

, like many other seabirds, visits its breeding colonies at night to reduce the chances of attack by aerial predators. Two aspects of its optical structure suggest that the eye of this species is adapted to vision at night. In the shearwater's eyes the lens does most of the bending of light necessary to produce a focused image on the retina. The cornea, the outer covering of the eye, is relative flat and so of low refractive

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

power. In a diurnal bird like the pigeon, the reverse is true; the cornea is highly curved and is the principal refractive component. The ration of refraction by the lens to that by the cornea is 1.6 for the shearwater and 0.4 for the pigeon; the figure for the shearwater is consistent with that for a range of different nocturnal bird and mammal.

The shorter focal length of shearwater eyes give them a smaller, but brighter, image than is the case for pigeons, so the latter has sharper daytime vision. Although the Manx Shearwater has adaptations for night vision, the effect is small, and it is likely that these birds also use smell and hearing to locate their nests.

It used to be thought that penguin

Penguin

s were short-sighted on land. Although the cornea is flat and adapted to swimming underwater, the lens is very strong and can compensate for the reduced corneal focusing when out of water. Almost the opposite solution is used by the Hooded Merganser

Hooded Merganser

The Hooded Merganser is a small duck and is the only member of the genus Lophodytes.Hooded Mergansers have a crest at the back of the head which can be expanded or contracted. In adult males, this crest has a large white patch, the head is black and the sides of the duck are reddish-brown...

which can bulge part of the lens through the iris when submerged.

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