Birds (Aves) are a group of vertebrates which evolved from dinosaurs. They are endothermic, with feathers.

Modern birds are toothless: they have beaked jaws. They lay hard-shelled eggs. They have a high metabolic rate, a four-chambered heart and a strong yet lightweight skeleton.

Birds live all over the world. They range in size from the 5 cm (2 in) bee hummingbird to the 2.70 m (9 ft) ostrich. They are the class of tetrapods with the most living species: about ten thousand. More than half of these are passerines, sometimes known as perching birds.

Birds are the closest living relatives of the Crocodilia. The fossil record shows that birds evolved from feathered theropod dinosaurs.

Modern birds are not descended from Archaeopteryx. According to DNA evidence, modern birds (Neornithes) evolved in the Middle to Upper Cretaceous.[2] More recent estimates, using a new way of calibrating molecular clocks, showed that  modern birds originated early in the Upper Cretaceous. However, diversification occurred around the Cretaceous–Palaeogene extinction event.[3]

The Cretaceous–Palaeogene extinction event 66 million years ago killed off all the non-avian dinosaur lines. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world.[4]

Primitive bird-like dinosaurs are in the broader group Avialae.[5] They have been found back to the mid-Jurassic period, around 170 million years ago.[1] Many of these early "stem-birds", such as Anchiornis, were not yet capable of fully powered flight. Many had primitive characteristics like teeth in their jaws and long bony tails.[2]

Birds have wings which are more or less developed depending on the species. The only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly. Later many groups evolved with reduced wings, such as ratites, penguins and many island species of birds. The digestive and respiratory systems of birds are also adapted for flight. Some bird species in aquatic environments, particularly seabirds and some waterbirds, have evolved as good swimmers.

Some birds, especially crows and parrots, are among the most intelligent animals. Several bird species make and use tools. Many social species pass on knowledge across generations, a form of culture. Many species annually migrate great distances. Birds are social. They communicate with visual signals, calls and bird songs. They have social behaviours such as cooperative breeding and hunting, flocking and mobbing of predators.

Most bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised by sexual reproduction. They are often laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.

Many species of birds are eaten by humans. Domesticated and undomesticated birds (poultry and game) are sources of eggs, meat, and feathers. Songbirds, parrots and other species are popular as pets. Guano is harvested for use as a fertiliser. Birds figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational bird-watching is an important part of the ecotourism industry.

Birds come in a huge range of colours. These colours can be useful to a bird in two ways. Camouflage colours help to hide the bird, and bright colours identify the bird to others of the same species. Often the male is brightly coloured while the female is camouflaged.

Many birds are brown, green or grey. These colours make a bird harder to see; they camouflage the bird.[6] Brown is the most common colour. Brown birds include: sparrows, emus, thrushes, larks, eagles and falcons and the female birds of many species such as: wrens, ducks, blackbirds and peafowls. When a brown bird is in long grass or among tree trunks or rocks, it is camouflaged.[7] Birds that live in long grass often have brown feathers streaked with black which looks like shadows. A bittern is almost invisible in long reeds. Other birds, including starlings and minahs, are quite dark in colour, but are flecked with little spots that look like raindrops on leaves. Bird may also camouflage their nests.

Many birds from hot countries are green or have some green feathers, particularly parrots. Birds that live in green trees often have green backs, even if they have bright-coloured breasts. From the back, the birds are camouflaged. This is very useful when sitting on a nest.[8] The bird's bright-coloured breast is hidden. Budgerigars are bred in different colours such as blue, white and mauve, but in the wild, they are nearly all green and yellow. Even though they fly very well, they normally spend a lot of time on the ground, eating grass seeds. Their yellow and black striped back helps to hide them in the shadows made by long dry grass, while their green breasts are a similar colour to the leaves of gum trees.

Grey birds include most pigeons and doves, cranes, storks and herons. Grey birds are often rock-living birds like pigeons or birds that sit on dead tree trunks looking like a broken branch. Water birds like herons often have a pale grey colour which makes it harder for a fish to notice that the bird is standing, looking down for something to catch. Water birds, no matter what colour they are on top, are often white underneath, so that when a fish looks up, the bird looks like part of the sky.

Black birds include crows, ravens and male blackbirds. Some birds that are dark colours spend quite a lot of time on the ground, hopping around in the shadows under bushes. Among these birds are the male blackbird and the Satin Bowerbird which is not black but very dark blue. Crows and ravens often perch high on bare trees in the winter, where their black shape against the sky looks like the dark bare branches.

Many birds are not camouflaged, but stand out with vivid colours. They are usually male birds whose females are dull and camouflaged. The function of the colours is two-fold.[6] First, the colours help them get mates, and second, the colours identify them to other males of the same species. Many birds are territorial, especially in the nesting season. They give out territory sounds and are easily seen. This lets other males know they will defend their territory. It sends out a "look elsewhere" signal to their competitors.

Some birds are famous for their colour and are named for it, such as the bluebird, the azure kingfisher, the golden pheasant, the scarlet macaw, the violet wren and the robin.

Many other birds are very brightly coloured, in countless combinations. Some of the most colourful birds are quite common, like pheasants, peacocks, domestic fowl and parrots. Colourful small birds include blue tits, the gold finches, humming birds, fairy wrens and bee eaters (which are also called rainbow birds). Some birds, like those of the bird of paradise in Papua New Guinea have such beautiful feathers that they have been hunted for them.

The peafowl is the best example of a display of colour to attract a mate. Also the male domestic fowl and junglefowl have long shiny feathers above his tail and also long neck feathers that may be a different colour to his wings and body. There are only a very few types of birds (like the eclectus parrot) where the female is more colourful than the male.

''Pied birds'' are black and white. Black and white birds include magpies, pied geese, pelicans and Australian magpies (which are not really magpies at all). Pied birds often have brightly coloured beaks and legs of yellow or red. The silver pheasant, with its long white tail striped with fine bars of black, has a brightly coloured face.

King parrot, Australia

Common shelduck

Kingfisher

Flamingo

Golden oriole.

Himalayan bluetail

Malayan banded pitta

Most birds can fly. They do this by pushing through the air with their wings. The curved surfaces of the wings cause air currents (wind) which lift the bird. Flapping keeps the air current moving to create lift and also moves the bird forward.

Some birds can glide on air currents without flapping. Many birds use this method when they are about to land. Some birds can also hover and remain in one place. This method is used by birds of prey such as falcons that are looking for something to eat. Seagulls are also good at hovering, particularly if there is a strong breeze. The most expert hovering birds are tiny hummingbirds which can beat their wings both backwards and forwards and can stay quite still in the air while they dip their long beaks into flowers to feed on the sweet nectar.

A flock of tundra swans fly in V-formation.

This osprey at Kennedy Space Centre is hovering.

A wandering albatross can sleep while flying.

The large broad wings of a vulture allow it to soar without flapping.

The soft feathers of an owl allow it to fly quietly.

Some birds, such as the quail, live mainly on the ground.

A cassowary cannot fly but can defend itself.

Penguin's flippers are good for swimming.

Different types of birds have different needs. Their wings are adapted to suit the way they fly.

Large birds of prey, such as eagles, that spend a lot of time soaring on the wind have wings that are large and broad. The main flight feathers are long and wide. They help the eagle to stay on rising air currents without using much energy, while the eagle looks at the ground below, to find the next meal. When the eagle sees some small creature move, it can close its wings and fall from the sky like a missile, opening its great wings again to slow down as it comes to land. The world's largest eagle, the Philippine eagle has a wingspan of about 2 m (6.7 ft) wide.

Birds that live in grassland areas or open forests and feed on fruit, insects and reptiles often spend a lot of time flying short journeys looking for food and water. They have wings that are shaped in a similar way to eagles, but rounder and not as good for soaring. These include many Australian birds like cockatoos.

Birds such as geese that migrate from one country to another fly very long distances. Their wings are big and strong, because the birds are large and they stock up on food for the long flight. Migrating water birds usually form family groups of 12-30 birds. They fly very high, making use of long streams of air that blow from north to south in different seasons. They are very well organised, often flying in a V pattern. The geese at the back do not have to flap so hard; they are pulled on by the wind of the ones at the front. Every so often, they change the leader so that the front bird, who does most work and sets the pace, can have a rest. Geese and swans are the highest-flying birds, reaching 8,000 metres or more when on migration. Geese often honk loudly while they are flying. It is thought that they do this to support the leader and help the young ones.

Birds that fly very quickly, such as swifts and swallows, have long narrow pointed wings. These birds need great speed because they eat insects, catching most of them while they are flying. These birds also migrate. They often collect in huge flocks of thousands of birds that move together like a whirling cloud.

Birds that live in bushes and branches have triangular wings that help the bird change direction. Many forest birds are expert at getting up speed by flapping and then gliding steadily among the trees, tilting to avoid things as they go. Members of the kingfisher family are expert at this type of flying.

Birds such as owls that hunt at night have wings with soft rounded feathers so that they do not flap loudly. Birds that are awake at night are called nocturnal birds. Birds that are awake during the day are diurnal.

A wandering albatross and Arctic tern might spend several years without coming to land. They can sleep while gliding and have wings which, when they are stretched right out, look like the wings of a jet plane.

Bird like chickens that feed mainly on the ground and only use their wings to fly to safety have small wings.

Flocks of birds can be very highly organised in a way that takes care of all the flock members. Studies of small flocking birds like tree sparrows show that they clearly communicate with each other, as sometimes thousands of birds may fly in close formation and spiral patterns without colliding (or flying into each other).

Two common behaviours in flocking birds are guarding and reconnaissance. When a flock of birds is feeding it is common for one bird to perch on a high place to keep guard over the flock. In the same way, when a flock is asleep, often, one bird will remain awake. It is also common for large flocks to send one or two birds ahead of them when they are flying to a new area. The look-out birds can spy the lie of the land to find food, water and good places to perch.[9]

Some birds do not fly. These include running birds like ostriches and emus and ocean-living birds, the large penguin family.

Ostriches and emus do not need to fly because although they feed and nest on the ground, their great size and their speed is their protection. Some other ground-feeding birds have not been so lucky. Some birds such as the dodo and the kiwi were ground-feeding birds that lived in safety on islands where there was nothing dangerous to eat them. They lost the power of flight. Kiwis are endangered because European settlement to New Zealand brought animals like cats, dogs and rats which kill kiwis and eat their eggs. However, kiwis and also the rare New Zealand ground parrot have survived. In the case of dodos, they were fat and delicious. They were killed and eaten by sailors until there was none left. Other flightless birds which have disappeared are the great auk and the moa.

Penguins spend a great deal of time at sea, where they are in danger from seals. On land, they usually live in areas where there were few dangers, until the arrival of European settlers with dogs and cats. Their wings have adapted to life in the sea and have become flippers which help them in swimming very fast.

Modern birds do not have teeth, and many swallow their prey whole. Nevertheless, they must break up food before it is digested. First of all, along their throat (oesophagus) they have a crop. This stores food items before digestion. That way a bird can eat several items, and then fly off to a quiet spot to digest them.

Their stomach comes next, with two very different parts. One part is like a straight hollow rod which secretes mild hydrochloric acid and an enzyme to break down protein. The other part of the stomach is the gizzard. This is muscular, and grinds up the contents. In herbivorous birds the gizzard contains some gastroliths (small stones or pieces of grit). Bones of fish will mostly be dissolved by the stomach acid. The partly digested and ground-up food now goes to the intestine, where digestion is completed, and most contents are absorbed. Anything indigestible, for example remains of feathers, is regurgitated via the mouth, not the cloaca.

The system is effective, and carnivorous birds can swallow quite large prey. A blue heron can swallow a fish as large as a carp successfully.[10] Raptors eat by holding the prey down with a foot, and tearing it apart with their beak.

Although birds are warm-blooded creatures like mammals, they do not give birth to live young. They lay eggs as reptiles do, but the shell of a bird's egg is hard. The baby bird grows inside the egg, and after a few weeks hatches (breaks out of the egg).

Birds in cold climates usually have a breeding season once a year in the spring. Migratory birds can have two springs and two mating seasons in a year.

When the breeding season arrives, the birds choose partners. Some birds are mated for life, like married couples. These birds include pigeons, geese, and cranes. Other birds look for new partners each year and sometimes a male bird or cock will have several wives.

For birds that choose new mates, part of the breeding season is display. The male bird will do all sorts of things to attract females. These include singing, dancing, showing off the feathers and building a beautiful nest. Some male birds have splendid feathers for attracting females. The most famous is the peacock who can spread the feathers above his tail into a huge fan.

A peacock display

The sarus crane, like most cranes, mates for life and pairs dance together.

Emu nest.

A nest of house sparrows.

Once the birds have found partners, they find a suitable place to lay eggs. The idea of what is a suitable place differs between species, but most build bird nests. Robins will make a beautiful little round nest of woven grass and carefully line it with feathers, bits of fluff and other soft things. Swallows like to nest near other swallows. They make nests from little blobs of clay, often on a beam near the roof of a building where it is well sheltered. Many birds like a hollow tree to nest in. Eagle's nests are often just piles of dead wood on the top of the tallest tree or mountain. Scrub turkeys scratch together a huge pile of leaves that may be 10 metres across. Guillemots lay their eggs on rock shelves with no nest at all. Their eggs are shaped so that they roll around in circles and do not fall off cliffs. A cuckoo does not make its own nest. It lays its egg in the nest of another bird and leaves it for them to care for. The cuckoo eggs are camouflaged to look like the host's eggs.

When the nest has been prepared, the birds mate so that the eggs are fertilised and the chicks will start growing. Unlike mammals, birds only have one opening as the exit hole for body fluids, and for reproduction. The opening is called the cloaca. A female bird, called a hen, has two ovaries, of which the left one usually produces eggs.

Most male birds have no sex organs that can be seen. But inside the male are two testes which produce sperm which is stored in the cloaca. Birds mate by rubbing their cloacas together, although with some birds, particularly large water birds, the male has a sort of a penis inside the cloaca.

Once the hen has mated, she produces fertile eggs which have chicks growing inside them. She lays the eggs in the nest. There might be just one egg or a number of them, called a clutch. Emus might lay as many as fifteen huge dark green eggs in a clutch. After the eggs are laid, they are incubated, or kept warm so the chicks form inside. Most birds stay together for the whole nesting season, and one advantage is that the work is shared. Many birds take turns sitting on the eggs, so that each adult can feed.

This is not always the case. With emus, the male does all the sitting and all the baby-minding. With emperor penguins it is also the male that cares for the egg. There is only one egg, which he keeps on his feet and under his feathers, standing in a big group of males without feeding until the chick is hatched. While the eggs are hatching, the females are at sea, feeding, so that they can care for the chicks when they return.

Some birds put the eggs inside or on top of the mound of leaves and twigs. The mound acts like a compost heap. The decomposition of the rotting leaves causes the temperature to rise. This is heat released by the chemical action of bacterial and fungal respiration. It is the same reaction as that which keeps mammals and birds at a high temperature. The parents leave the mound. When the chicks hatch, they are able to feed themselves.

Many small birds take 2–4 weeks to hatch eggs. Albatrosses take 80 days. During this time the female loses a lot of her body weight.

The quickest hatching time is for the cuckoo. Some types of cuckoos take only 10 days. This means that when they hatch in the nest of their ''foster parents'', the eggs that the parents have laid are not yet ready. Newborn cuckoos are naked, blind and ugly, but they are strong. They get under any eggs that are in the nest and throw them out before they hatch. That means that the cuckoo has the whole care of both parents. Baby cuckoos grow fast and often get bigger than the parents who feed them.

When baby birds hatch, in most types of birds, they are fed by both parents, and sometimes by older aunties as well. Their mouths are open all the time and are often very brightly coloured, which acts as a ''releaser'', a trigger which stimulates the parent to feed them. For birds that eat grain and fruit, the parents eat and partly digest the food for the babies. It is then vomited carefully into the baby's mouth.

A black redstart feeding chicks

Black swan and cygnets

A reed warbler feeding a baby cuckoo

Two sulphur crested cockatoos from a big flock are on the lookout

Many birds, particularly those that mate for life, are very sociable and keep together in a family group which might be anything from 4 or 6 adult birds and their young to a very large flock.

As chicks grow they change the fluffy down that covers them as babies for real feathers. At this stage they are called fledglings. Other family members may help care for fledgling chicks, feeding them and protecting them from attack while parents are feeding. When the fledglings have their new feathers, they come out of the nest to learn to fly. In some types of birds, like pigeons, the parents watch over this and as the young ones get stronger, will give them flying lessons, teaching them how to glide, how to fly in spirals and how to land like an expert.

Most birds are social animals, at least part of the time. They communicate to each other using sounds and displays.

Almost all birds make sounds to communicate. The types of noises that vary greatly. Some birds can sing, and  they are called songbirds or passerines. Examples are robins, larks, canaries, thrushes, nightingales. Corvids are passerines, but they do not sing. Birds that are not songbirds include: pigeons, seagulls, eagles, owls and ducks. Parrots are not songbirds, even though they can be taught to sing human songs.

A favourite songbird, the European robin.

The crow of the rooster is a familiar bird call.

The pied currawong, an outstanding singer.

The jackdaws helped Lorenz to understand bird communication.

All birds make noises (''bird vocalisation''), but not all sing. Songbirds are passerines, many of which have beautiful melodic songs. Songs have different functions. Danger cries are different from territorial songs and mating calls are a third type. Fledgling may also have different calls from adults. Recognition calls for partners are quite common.

As to where the song comes from, there are three kinds of species:

Most singing birds that are kept as pets, like canaries, have several tunes and some variations.

The same species of bird will sing different songs in different regions. A good example of this is the currawong. This is an Australia bird which is like a black and white crow. In the autumn, families get together in large flocks and do a lot of singing. Currawongs from some areas sing much more complex songs than others. Generally, currawongs from the Blue Mountains are the finest singers. The song of the currawong can be sung as a solo, but is often performed as a choir. One bird will take the lead and sing "Warble-warble-warble-warble!" All the other birds will join in and sing "Wooooooo!". When all the birds know the song, the choir will sing the "Warble" part and the soloist will sing the "Woo!". The song changes from year to year and from place to place.

The Austrian naturalist Konrad Lorenz studied the way in which birds communicate, or talk to each other. He found that each type of bird had a number of sounds which they made automatically, when ever they felt a certain way. Every sound had an action that went with it. So, if the bird was frightened, it acted frightened and made a frightened sound. This told the other birds around it that something frightening was happening.

If a flock of birds were flying over a field, they would be calling "Fly! Fly!" But a hungry bird, seeing something good to eat down below might start calling "Food! Food!" If other birds were also hungry, they would make the same call until more birds were calling "Food! Food!" than "Fly! Fly!". At this point, the mind of the flock would be changed. Some of the birds would start to yell "Fly downwards! Fly downwards!" as they sank from the sky, until the whole flock was all noisily calling the same thing.

These communication sounds are often short hard sounds like: chirps, squeaks, squawks and twitters. Sometimes the calls are longer and more musical. They include the "Rookety-coo" sound of a pigeon and the "Cockadoodledoo!" of a rooster. The bird cannot change these sounds. They always make them in the same way. The bird is locked into making each sound every time a particular idea comes into its head. The connection between how they feel and how they call is innate: they are born with it. Some calls in some species are learnt. Then, it is the tendency to learn which is inherited.

Konrad Lorenz noticed that when birds sing, they often use a lot of their regular calls as part of the song. Lorenz had a flock of jackdaws which were scattered during World War II. One day, an old bird returned. For many months she sat on the chimney singing her song, but in the song she kept making the call which Lorenz knew meant "Come home! Come home!" One day, to the great surprise of Lorenz, a male bird flew from a passing flock and joined her on the chimney. Lorenz was sure that it was her long-lost "husband" who had found his way home at last.[11]

Palaeontologists have found some exceptional places (lagerstätten) where fossils of early birds are found. The preservation is so good that on the best examples impressions of their feathers can be seen, and sometimes even the remains of meals they have eaten. From these remains we know that birds evolved from small carnivorous dinosaurs (theropods) in the Jurassic period.[12] They radiated into a huge variety in the Lower Cretaceous. At the same time, their direct competitors, the pterosaurs, dwindled in numbers and variety, and became extinct at the end of the Mesozoic.

Birds are classified by taxonomists as 'Aves' (Avialae). Birds are the only living descendants of dinosaurs (strictly speaking, they are dinosaurs). Birds and Crocodilia are the only living members of the once-dominant Archosaur reptiles.

The class Aves is now defined as all the descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[13]

Archaeopteryx, from the Upper Jurassic (some 150–145 million years ago), is the earliest bird which could fly. It is famous, because it was one of the first important fossils found after Charles Darwin published his ideas about evolution in the 19th century. By modern standards, Archaeopteryx could not fly very well.[14] Other early fossil birds are, for example, Confuciusornis, Anchiornis huxlei and other Paraves.

Many fossils of early birds and small dinosaurs have been discovered in the Liaoning Province of Northeast China. The fossils show that most small theropod dinosaurs had feathers. These deposits have preserved them so well that the impressions of their feathers can be clearly seen. This leads us to think that feathers evolved first as heat insulation and only later for flight. The origin of birds lies in these small feathered dinosaurs.

Palaeontologists now agree that birds evolved from Maniraptora group of dinosaurs. This explains why one might say birds are living dinosaurs.

Canaries are often kept as pets for their beautiful songs.

The African grey parrot is a renowned talker.

Blue-winged teal Ducks used to be shot for sport.

In many countries storks are thought to bring good luck.

Some birds are eaten as food. Most usually it is the chicken and its eggs, but people often also eat geese, pheasants, turkeys and ducks. Other birds are sometimes eaten are: emus, ostriches, pigeons, grouse, quails, doves, woodcocks and even songbirds. Some species have died out because they have been hunted for food, for example the dodo and the passenger pigeon.

Many species have learned how to get food from people. The number of birds of these species has grown because of it. Seagulls and crows find food from garbage dumps. The feral pigeon (Columba livia), sparrows (Passer domesticus and starlings (Sturnus vulgaris) live in large numbers in towns and cities all over the world.

Sometimes people also use working birds. For example, homing pigeons carry messages. Nowadays people sometimes race them for sport. People also use falcons for hunting, and cormorants for fishing. In the past, people in mines often used a canary to see if there were bad gas methane in the air.

People often have colorful birds such as parrots and mynahs as pets. These intelligent birds are popular because they can copy human talking. Because of this, some people trap birds and take them to other countries to sell. This is not usually allowed these days. Most pet birds are specially bred and are sold in pet shops.

People can catch some bird diseases, for example: psittacosis, salmonellosis, campylobacteriosis, Newcastle's disease, mycobacteriosis, influenza, giardiasis and cryptosporiadiosis. In 2005, there was an epidemic of bird influenza spreading through some parts of the world, often called avian flu.

Some people have birdboxes in their gardens to give birds a place to nest and bird tables where birds can get food and water in very cold or very dry weather. This lets people see some small birds close up which are normally hidden away in bushes and trees.



Blue tit

Male house sparrow

Male chaffinch

White-breasted nuthatch

A report produced by BirdLife International every five years measures the population of birds worldwide. In 2018 the number of bird species has decreased by 40%.One in every eight types of birds is now almost extinct.

The report highlighted the reduction of the number of Snowy Owls, Atlantic Puffin, European Turtle-Dove andseveral species of vultures.[15]