adityasasidhar/Wikipedia_Cleaned huggingface dataset:
A maternal or paternal relative is one who is related through their mother or father, respectively. For example, a maternal uncle is a the subject's mother's brother.An in-law is a relationship that is not by blood, but instead by marriage. The in-law shares the in-law relationships of the spouses relatives. For example, an uncle in-law could be the husband of the subject's Aunt.
Ice can be made in a home refrigerator or freezer. If water is put in a freezer and left for a while, the water gets very cold and will freeze solid, creating ice. Water can be placed into a copper (or other metal) container to cause it freeze into ice faster. Water can freeze faster than in a plastic ice tray.
Salt water needs a lower temperature to freeze than pure water. Most of the salt does not enter the ice, so the resulting ice is less salty than the water. This fact has sometimes been used for desalination. Salty ice is not as strong as frozen pure water. Salt that is added to ice melts it, if it's not too cold: the salt progressively eats into the ice, forming brine which does not freeze at that temperature.
Because ice floats, even large bodies of water which freeze, like some oceans, only form ice on the surface. Most lakes never freeze to the bottom. Even the coldest oceans, like the Arctic Ocean, only freeze on the top, leaving liquid ocean circulating below. Because of this the Earth's oceans are able to redistribute heat. The climate of the Earth has less extremes of heat and cold than it would otherwise. A solid ocean would not circulate heat. But because ice floats on the surface the water beneath can continue to circulate. Ice on the surface stays exposed and readily melts when the temperature rises.
Big bodies of ice on land are called glaciers. Antarctica has most of the world's ice.
Climate change happens all the time. When it is cold it is called an ice age. During ice ages large areas of the Earth are covered in ice, snow and glaciers. The causes of ice ages are complex, and not entirely known, but see Milankovic cycle. Global warming is currently affecting the Earth's ice resources and its causes are also very complex.
Travel is the activity of going from one place to another place. When a person makes the same trip every day to work or school, this type of traveling is usually called "commuting." Some people travel to other cities as part of their job. This is called "business travel." When many people travel to a distant place to stay, it is called "human migration".
Some people travel on their vacation, to visit other towns, cities, or countries. This is tourism. These people stay overnight in hotels, hostels, motels, apartments or bed and breakfast. Some prefer camping. Through the centuries, modes of travel have changed. Some people (travel writers) write about travelling, as in autobiographies or journals. Some write guide books suggesting places to go.
The Cable News Network (CNN) is an American cable news television channel. It was founded in 1980 by Ted Turner. The Cable News Network first aired on television on June 1, 1980. The Cable News Network's first newscast was anchored (hosted) by David Walker and his wife Lois Hart. In its first year CNN hired many political analysts, including Rowland Evans and Robert Novak. On January 1, 1982 CNN launched a 24-hour sister newscast channel with no talk shows or commentary shows called . CNN broadcasts programs from its headquarters at the CNN Center in Atlanta, or from the Deutsche Bank Center in New York City, or from studios in Washington, D.C., and Los Angeles. CNN is owned by Warner Bros. Discovery, and the U.S. news channel is a part of the CNN Global.
A blackboard, also called a chalkboard, is a surface on which chalk is visible. It is used as a surface to write on. It is usually made up of wood or fiber. It is painted black to reflect the white chalks.
Blackboards are often used to help in teaching in school. Blackboards are not used as much now but can be seen in some schools still. The chalk dust got everywhere, and some people are allergic to chalk dust. Now people use whiteboards: they can take modern felt-tip colour markers, and clean easily.
Schoolteachers often use the chalkboard to display things at large. It also makes the learning cooperative between teachers and student. Chalkboards can be black or dark green, in which case they may be called blackboards and greenboards.
Linux or GNU/Linux is a Unix-like operating system (or family of) for computers. The Linux kernel (the basis of the operating system) is free software, meaning people can use it, see how it works, change it, or share it.
Popular Linux distributions include Debian, Fedora, and Ubuntu. Commercial distributions include Red Hat Enterprise Linux and SuSE Linux Enterprise Server. Desktop Linux distributions include a windowing system such as X11 or Wayland, and a desktop environment such as GNOME or KDE Plasma. Distributions intended for servers may omit graphics altogether, and include a solution stack such as LAMP. Anyone may create a distribution for any purpose.
Linus Torvalds shared the Linux kernel on some internet groups for MINIX users. Linus first called the operating system "Freax". The name Freax came from joining up the English words "free" and "freak", and adding an X to the name because Unix has an X in its name. Ari Lemmke, who worked with Linus at the university, was responsible for the servers that Freax was stored on. Ari did not think Freax was a good name, so he called the project "Linux" without asking Linus. Later, Linus agreed that Linux was a better name for his project.
Linux relied on software code from MINIX at first. But, with code from the G.N.U. system available for free, he decided it would be good for Linux if it could use that code, instead of code from MINIX, because MINIX did not let people share or change it how they wanted. The G.N.U. General Public License is a software license that lets people change any part of the code they want to, as long as they share any changes they make with the people they give their software to and allow them to redistribute it for free or for a price . The software from G.N.U. was all licensed under the G.N.U. General Public License, so Linus and the other people who worked on Linux could use it.
Since 1991, thousands of programmers and companies have worked on Linux, including Google.
Tux has now become a symbol for Linux, and sometimes even for open source. He can be seen in many different places and often, when people refer to Linux, they often think about Tux. Tux has even been included in many video games, such as Super Tux (parody of Super Mario Bros.), Tux Racer (where players race Tux down an icy hill) and Pingus (like Lemmings).
Many free software programs that are popular on Windows, such as Pidgin, Mozilla Firefox, LibreOffice, Chromium, VLC and GIMP, are available for Linux. A growing amount of proprietary desktop software can also be used under Linux, such as Steam, Spotify and Skype. CrossOver is a proprietary solution based on the open source Wine project that supports running Windows applications such as Microsoft Office and Adobe Photoshop under Linux.
Version 3.10 of the Linux kernel, released in June 2013, has 15 million lines of code, while the version 4.1, released in June 2015, has grown to over 19.5 million lines of code by almost 14,000 programmers.
People can download Linux from a website on the internet or buy it from a store. Sometimes books and magazines about Linux have a CD. or DVD. with Linux on it. Any certain version of Linux is called a "distribution", or "distro". A Linux version has the Linux kernel, G.N.U. software, and some extra programs that might not be a part of G.N.U. Different versions include different extra programs. The versions used by the most people include:
The most common desktop environment on personal computers is the one given by Microsoft Windows; another common environment is the one given by Apple Mac OS X. Other mostly-used desktop environments for Unix-like operating systems using the are KDE, GNOME, Xfce. There are also other desktop environments, including : Aston, EDE, Graphical Environment Manager, IRIX Interactive Desktop, Sun's Java Desktop System, Mezzo, Project Looking Glass, ROX Desktop, UDE, Xito, XFast.
OpenOffice.org was a free open source office suite. The free software project was hosted by Sun Microsystems and then Oracle Corporation and it was based on the older StarOffice software that was created by Sun Microsystems. It was available for many different operating systems, including Microsoft Windows, Linux, OS X, FreeBSD and Solaris; and was meant to be an alternative to Microsoft Office. It supported Microsoft Office file formats as well as its own OpenDocument format.
The ".org" part of the previous name OpenOffice.org was there because OpenOffice is already trademarked. It was often called "OOo" for short.
In January 2010, Oracle Corporation bought Sun Microsystems, which developed OpenOffice.org until that point. In September 2010, a large part of the developers created The Document Foundation and forked the project into the new LibreOffice project because, among other reasons, they were not certain if the development would be successful under Oracle management. In April 2011, Oracle stopped development of OpenOffice.org and laid off the remaining development team. A few months later, it donated the existing source code and trademarks to the Apache Foundation, which released the first version of Apache OpenOffice.
Dogs are mammals, usually kept as pets or for work on farms or police. Some dogs are trained to be rescue dogs, and join teams such as mountain rescue. They have been bred by humans from their ancestral wolves. They were the first animals to live with humans, as far as we know.
There was a lot of variety among wolves in the Late Pleistocene. The dingo is also a dog, but many dingos have become wild animals again and live independently of humans in the range where they occur (parts of Australia).
Today, some dogs are used as pets, and others are used to help humans do their work. They are popular pets because they are usually playful, friendly, loyal, and listen to humans. Thirty million dogs in the United States are registered as pets. Dogs eat both meat and vegetables, often mixed together and sold in stores as dog food. Dogs often have jobs, including police dogs, army dogs, assistance dogs, fire dogs, messenger dogs, hunting dogs, herding dogs, or rescue dogs.
They are sometimes called "canines" from the Latin word for dog - "canis". Wolves are also canids. A baby dog is called a pup or puppy. A dog is called a puppy until it is about one year old.
Dogs are sometimes referred to as "man's best friend" because they are kept as domestic pets, are usually loyal, and like being around humans. Dogs like to be petted, but only when they can first see the petter's hand before petting; one should never pet a dog from behind.
Dogs can smell and hear better than humans but cannot see well in color because they are color blind. Due to the anatomy of the eye, dogs can see better in dim light than humans. They also have a wider field of vision.
Like wolves, wild dogs travel in groups called packs. Packs of dogs are ordered by rank, and dogs with low rank will submit to other dogs with a higher rank. The highest ranked dog is called the alpha male. A dog in a group helps and cares for others. Domesticated dogs often view their owner as the alpha male.
Different dog breeds have different lifespans. In general, smaller dogs live longer than bigger ones. The size and the breed of the dog change how long the dog lives, on average. Breeds such as the Dachshund usually live for fifteen years, Chihuahuas can reach age twenty. On the other hand, the Great Dane has an average lifespan of six to eight years; some Great Danes have lived for ten years. An American Bulldog lives for around 15 years. Bigger dogs will have smaller lives than smaller dogs because of the pressure on its heart to move around.
All dogs are descended from wolves, by domestication and artificial selection. This is known because DNA genome analysis has been done to discover this. They have been bred by humans. The earliest known fossil of a domestic dog is from 31,700 years ago in Belgium. Dogs have lived with people for at least 30,000 years. In 2013, a study was published that showed that the skull and teeth of a canid dated as 33,000 years, had characteristics closer to a dog than to a wolf. The authors concluded that "this specimen may represent a dog in the very early stages of domestication, i.e., an "incipient" dog." The researchers suggest that it was a line that did not lead to modern dogs. Genetically, this material is closer to that of a modern dog than that of a wolf. Other signs of domestication are that sometimes, dogs were buried together with humans. Evidence of this is a tomb in Bonn, where a man of about 50 years of age, a woman of about 25 years of age, the remains of a dog, plus other artifacts were found. Radiocarbon dating showed that the human bones were between 13.300 and 14.000 years old.
There are at least 800 breeds (kinds) of dogs. Dogs whose parents were the same breed will also be that breed: these dogs are called purebred or pure pedigree dogs. Dogs with parents from different breeds no longer belong to one breed: they are called "mutts", "mixed-breed dogs", "hybrids", or "mongrels". Some of the most popular breeds are sheepdogs, collies, poodles and retrievers. It is becoming popular to breed together two different breeds of dogs and call the new dog's breed a name that is a mixture of the parents' breeds' two names. A puppy with a poodle and a pomeranian as parents might be called a Pomapoo. Instead of being called mutts, these kinds of dogs are known as designer dog breeds. These dogs are normally used for prize shows and designer shows.
Legal blindness is defined as a level of visual acuity low enough to qualify for disability benefits and\or special accommodations. Legal blindness is also defined as being unable to be completely corrected by prescription glasses or contact lenses.
The Arctic is the area around the Earth's North Pole. The Arctic includes parts of Russia, Alaska, Canada, Greenland, Iceland, Lapland and Svalbard as well as the Arctic Ocean. It is an ocean, mostly covered with ice. Most scientists call the area north of the treeline Arctic. Trees will not grow when the temperatures get too cold. The forests of the continents end in the far north or high up a mountain. The place where in the trees stop is called the tree line.
The area north of the treeline is not an empty ice field. In fact, the only large ice-covered land is central Greenland, which is covered year-round by a continental glacier. The land of the Arctic is around the edges of the Arctic Circle and is usually covered with tundra. A tundra is a cold, almost treeless plain covered with moss and sedges (grasslike plants).
Tundras get little rain or snow. However, they are very wet in the summer time because, three feet (1m) or so below the surface, the ground is always frozen. This permafrost is a layer of ice with the hardness of stone that keeps water from soaking into the earth beneath it. Permafrost can be thousands of feet thick. In the summer the ground on top thaws out when the sun shines and the snow that did fall melts. But the melted snow cannot drain into the ground; it just soaks the topsoil. There is no place for the water to go, except to form lakes and puddles on top of the ground until it freezes again or dries up.
In some parts of the Arctic the ground is covered with bright-colored flowers during the short summer. These little plants have very short stems. They must stay close to the ground, because the earth is warmer than the biting winds that blow above it. Even some rocks have plants growing on them. These rock plants are called lichens. Lichens grows on bare rocks. Yellow reindeer moss is really a lichen. It grows slowly, but often grows over six inches (15 cm) tall. It is spongy and full of water. This plant is the summer food for reindeer.
Each summer the ice and snow melt, allowing the animals to find food and the plants to flower. The moisture from the snow stays near the top of the ground's surface. Plant roots can use it for their growth. Even so, in many places the soil is too poor for plant's growth. Less than half of the tundra has plants growing on it.
The lemmings and voles are the rodents of the Arctic. The lemmings have very short tails. They live underground in summer. Because the top soil freezes in the winter, they cannot burrow underground then, so they live under the snow during the colder months. They eat plants and roots. In a year with plenty of food, millions of lemmings will be roaming the tundra. The owls and foxes have more food than they can eat. They have more young in years when there are many lemmings. However, the tundra does not have enough food for millions of lemmings. They rush across the tundra looking for food, and are eaten by predators or starve to death. Some try to swim across lakes or rivers and drown. The few that remain continue to have families.
But the best known of the Lapp people were the nomads who raised reindeer. The Lapps survived in their harsh homeland by domesticating the reindeer. The Lapps were able to get everything they needed from the deer. They ate mostly meat, milk, and cheese. Their clothes were made from reindeer skins and wool. Their tents were also made from deer skins. They are known for the beautifully decorated woollen clothing they made.
Eskimos did not have a government or laws. They learned early in life to help each other to survive. Always sharing food, they usually moved around in small groups looking for food. Sometimes they'd get together in a large group when they hunted big animals such as whales. The men did the hunting and building the homes, and the women cooked, made the clothes, and took care of the children.
Russia has more land in the Arctic than any other nation. The Russian Arctic in Asia and the area just south of it are called Siberia. It is such an empty, harsh place that it has been used as a penal colony for many years. The old Russian rulers and the modern communist Soviets would send criminals and people who disagreed with the government to live there. However, many people were also sent there to remove minerals, harvest trees from forests, and build cities.
Analog circuits are used for signals that have a range of amplitudes. In general, analog circuits measure or control the amplitude of signals. In the early days of electronics, all electronic devices used analog circuits. The frequency of the analog circuit is often measured or controlled in analog signal processing. Even though more digital circuits are now made, analog circuits will always be necessary since the world and its people work in analog ways.
Pulse circuits are used for signals that require rapid pulses of energy. For example, a radar works by using pulse circuits to create and send high powered bursts of radio energy from radar transmitters. Radar antennas are used to send ("transmit") the high powered bursts in the direction the antenna is pointed.
The radar transmitter's pulses or bursts of radio energy hit and bounce back (they are "reflected") from hard or big metallic objects. Hard objects are things like buildings, hills, and mountains. Big things made of metal include aircraft, bridges, or even objects in space, like satellites. The reflected radar energy is detected by radar pulse receivers which use both pulse and digital circuits together. The pulse and digital circuits in radar pulse receivers are used to show the location, distance, and speed of objects which have reflected the radar transmitter's high powered pulses.
where formula_2 is the photon's energy, formula_3 is the Planck constant, and formula_4 is the frequency of the light associated with the photon. This frequency, formula_4, is typically measured in cycles per second, or equivalently, in Hz. The quantum energy of different photons is often used in cameras, and other machines that use visible and higher than visible radiation. This because these photons are energetic enough to ionize atoms.
The Fire Brigades Union (FBU) is the main union for firefighters in the United Kingdom. A union is a group of workers. They make the group to improve pay and working conditions. It was founded in 1918 in the London area as the "Firemans Trade Union". It soon grew to cover the entire country. Today, it has around 50,000 members.
A wheel is a disc or circle-shaped mechanical device. Its main purpose is to allow things to "roll"; in other words, the wheel spins, and object on the wheels moves more easily along the ground. It is a simple machine. The principle behind the wheel is that of mechanical advantage.
Most land vehicles roll on wheels. Wheels are often used in pairs, connected by a rod of wood or metal known as an axle. The wheel and axle turn together. The part of the wheel that attaches to the axle is called the hub.
The wheel with an axle is the basis of many machines, not just vehicles. The potter's wheel, the lathe and the windlass are examples. Many machines have wheels with teeth, known as gears.
Today, wheels are used in cars, carts, airplanes, wheelchairs, bicycles, trains, and skateboards, in addition to many more devices.
The CDC recommends that adults should get 150 minutes of physical activity weekly. Regular exercise has been shown to help high blood pressure, obesity, heart disease, diabetes, and depression, among others. Exercise also helps people feel less tired.
Regular exercise can have a profoundly positive impact on depression, anxiety, and ADHD. It also relieves stress, improves memory, helps sleep better, and boosts overall mood. Studies show that exercise can treat mild to moderate depression as effectively as antidepressant medication but without the side-effects, of course.
7. Why Fitness is famous and necessary?
Even now, there is still widespread illiteracy. There is often a political dimension to this. People who cannot write are easier to control.
Plato ( ) was one of the most important philosophers of all time. Born to wealthy parents in Athens, Greece, Plato was a student of Socrates (who did not write) and, later, became the teacher of Aristotle. Plato started a university in Athens called the Academy where he taught. Plato wrote about many ideas in philosophy that are still talked about today, including political philosophy and the philosophy of language. One modern scholar, Alfred North Whitehead, said that all European philosophy since Plato is a series of footnotes to his works.
Plato wrote his books in the form of conversations called dialogues. In a dialogue, two or more people talk about ideas and sometimes disagree over them. The "Laws" is Plato's longest dialogue and probably his last.
Socrates is usually the main person in Plato's dialogues. Often, Socrates talks with people and tries to see if they believe anything that is illogical. Because of this, some people become angry with Socrates and try to kill him. In Plato's Apology, Socrates is put on trial by these people and is eventually sentenced to death by drinking poison.
Plato is famous for developing the theory of forms. This theory says that everything in our world is imperfect but corresponds to a perfect version of itself that exists in another realm.
One simple way to think about Plato's theory is with numbers. The difference between "chairness" and a regular chair is similar to the difference between the number five and having five apples, oranges, pears, etc.
People who study Plato argue about whether Socrates really said the same things that Plato makes him say, or whether Plato just used Socrates as a character to make the ideas he was talking about seem more important.
There are many dialogues that were written by Plato. This list includes those he probably did write:
A book is now a set of printed sheets of paper held together between two covers. The sheets of paper in a book are called pages. The pages have words written in them and maybe illustrations drawn. The first books were not printed, but written by hand in ink.
The book is a more flexible format than the earlier idea of the scroll. The change from scrolls to books began in the Roman Empire and took many centuries to become complete.
A writer of a book is often called an author. Someone who draws the pictures in a book is called an illustrator. Books can have more than one writer or illustrator.
A book can be a text that is a part of a larger collection of texts. A section of a text may be published as a book so that it only has one writer or only focuses on one subject area. Books written in this way can be understood without reading whole collection of writings. Examples are the Iliad, Odyssey, Bible, Quran and Torah. Encyclopedias often have separate articles written by different people, and are published as separate volumes. Each volume is a book.
"Hardcover" books have hard covers made of cardboard that is covered in cloth or leather and sewn together. "Paperback" books have covers made of stiff paper that is glued together. A small book is a "booklet" and may be stapled together. The words in books can be read aloud and recorded. These are called "audiobooks".
Books may be borrowed from a library or bought from a bookstore. People can make their own books and write in them, and add family photos and drawings. Some books are empty inside, like a diary, an address book, or photo album. These books are meant to be written in. Usually, the word "book" means that the pages inside have words, and often pictures.
Some books are written just for children Some are for entertainment, and some are textbooks for studying something in school, such as math or history.
It is thought that the earliest Indo-European writings may have been carved on beech wood. The Latin word , means a book in the modern sense (bound and with separate leaves). It originally meant 'block of wood'.
These books are novels and short stories. They are stories that have not happened. They are often imagined by the writer. Many books are based on real events from history,but the writer creates imaginary characters or dialogue for the events.
Non-fiction fiction are about true facts or things that have really happened. Some examples are dictionaries, cookbooks, textbooks for learning in school, or a biography (someone's life story).
Between the written manuscript and the book are several inventions. While manuscripts are hand-made, books are now industrial products.
A common type of manuscript was the scroll. It is a long sheet that is rolled up. The sheet could have been made of papyrus (made by the Egyptians, by weaving the inner stems of the papyrus plant and then hammering them together), or parchment or vellum (very thin animal skin, first used by the ancient Greeks), or paper (made from plant fibers, invented by the Chinese). Manuscripts of this kind lasted to the 16th century and beyond. Turning the manuscript into a book required several developments.
In Roman Britain we have examples of a message system which was widespread. It was wax on boards, reusable. Waxed boards could be scratched with messages such as (from one woman to another) "Dear, please come to my birthday party, it won't be the same without you". Many examples have been found near Roman encampments on the border with Scotland.
Johannes Gutenberg was the first person to invent a machine for printing, called the printing press. He made it in the 15th century. This involved more than just a press because it involved the production of a movable metal type that was suitable for the machine process.
Initially, the machines were slow, and needed a muscle power to work. The Industrial Revolution brought steam power, and later electrification.
Today, some of the technologies have been changed, especially those involving illustration and typography. However, books look much the same as they did, with more illustration in color, but basically the same. That is because experience has shown that readers need certain things for pleasurable reading. Graphic design and typography are the practical arts used to make books attractive and useful to readers.
These combinatorial games can be represented by trees, each vertex of which is the game resulting from a particular move from the game directly below it on the tree. These games can be given game values. Finding these game values is of great interests to CG theorists, as is the theoretical concept of game addition. The sum of two games is the game in which each player on her/his turn must move in only one of the two games, leaving the other as it was.
Blue is one of the colors of the rainbow that people can see. It is one of the primary colors (colors that can be mixed with other colors) of light, along with red and green. Apart from indigo and violet, it has the shortest wavelength of colors in the rainbow (about 470 nanometers).
Blue paint can be made by mixing magenta and cyan paint.
It is the color of cold, introversion and openness, by opposition to the red, which is the color of warm, extroversion and the closure. In alchemy, blue is also the color of the warm introvert (blue fire), by opposition to the red, which is the color of the warm extrovert (red fire).
Memory is the personal record of past experiences. It is the brain's ability to store information to use later. Memory is studied in sciences known as "cognitive psychology".
Murder is where one person kills another person with aforethought. If a person does something reckless that makes someone else die, without the intention of killing, it is homicide and may be manslaughter. An accident caused by carelessness may be criminally negligent homicide. Sometimes, a death caused by someone else may not be a crime, for example killing in self-defense. Though, if the killer could've stopped a violent encounter without killing the victim, it's generally "not" a lawful killing.
A person who commits murder is called a murderer. The legal definition of "murder" and "manslaughter" may be different between countries. A killing in war is not usually called "murder" by those who fight in the war, as it's justifiable homicide. Killing in self defense is usually not "murder". Some countries do not even have manslaughter as a legal concept, and only have murder; the countries that do not have manslaughter charge with murder instead. In fact, the United States did not have manslaughter on the books until the 20th century. England was the first country to indoctrinate manslaughter, and the United States was second.
"Assassination" is a word which means murdering a prominent person for a reason. The word is most used when the person who was killed was a celebrity or was a person involved in politics. The people who carry out assassinations are called "assassins" or "Hit-men". An assassin may murder someone for political reasons, for money, or for other reasons, such as favors owed and revenge.
Murder often comes in three degrees: first, second, and third degree murder. First degree murder is considered murder that has been planned out prior to being committed, and this ranges from stalkers to those who assassinate. Second degree murder is considered murder in action, or murder that was committed with intent to kill, but without planning. Third degree murder is when someone kills someone without intent or planning. An example could be in a drunken bar fight. Two men are fighting, both drunk, and one kills another accidentally. While the killer did not intend nor plan to kill the other man, he committed third degree murder by killing him.
Birds (Aves) are a group of animals with backbones. They evolved from dinosaurs. Technically, birds are dinosaurs.
Birds are warm blooded. Their feathers help prevent loss of body heat. Modern birds do not have teeth. They have beaked jaws. Birds lay hard-shelled eggs. They have a high metabolic rate and a strong but lightweight skeleton. Their hearts have four chambers.
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. There are about ten thousand species of birds. More than half of these are passerines, or perching birds.
Birds have wings. The wings develop differently in different species. The only known groups of birds without wings are the extinct moa and the elephant birds. Wings evolved from the forelimbs of birds. They give birds the ability to fly. Over time, many groups of birds evolved with smaller wings. These include 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 have evolved as good swimmers. This is seen in seabirds and some waterbirds.
In general, birds inherit their behaviour almost entirely. The key elements of their lives are inherited. It was a great discovery that birds never learn to fly. It is wrong to say when a chick waves its wings in the nest that "it is learning to fly". What the chick is doing is exercising its muscles. If they are a species that flies, they develop the ability "automatically." If they are species that migrates, that behaviour is also inherited. Many species migrate over great distances each year. Other main features of their life may be inherited, though they can, and do, learn. Birds have good memories, An example of this is when they search for food.
Several bird species make and use tools. Some social species pass on some knowledge across generation. This is a form of culture. Birds are social animals. They communicate with visual signals as well as with calls and bird songs. Most of their social behaviours are inherited. Examples of this include flocking, mobbing of predators and cooperative breeding and hunting,
Most bird species are socially monogamous; they usually mate for one breeding season at a time. They may mate for years. It is rare that these birds will mate for life. Other species are polygynous, in these species, one male will mate with many females. Birds species are rarely polyandrous. In these species, one female will mate with many males. Birds produce offspring by laying eggs. The eggs are fertilised by sexual reproduction. The eggs are often laid in a nest and incubated by the parents. Most birds take care of their offspring after hatching. Some birds, such as chickens, lay eggs even when not fertilised. Unfertilised eggs do not produce offspring.
Birds come in many different colours. These colours can be useful to a bird in two ways. Camouflage help to hide birds from predators. Brighter colours can also be useful to birds. They can be used to identify the bird to others of the same species. Male bird are often brightly coloured while the female is camouflaged. The logic is as follows: the female carries the "precious package" of developing eggs. The male has to defend a territory. His colour and song are used to let others know that "this place is occupied".
Many birds are brown, green or grey. These colours make a bird harder to be seen: they camouflage the bird. 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". 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 because its camouflage is helped by its posture (beak and head pointed upwards). Other birds, including starlings and mynas, are quite dark in colour, but are flecked with little spots that look like raindrops on leaves. Bird may also camouflage their nests.
Some birds are famous for their colour and some are named for it, such as the bluebird, the azure kingfisher, the golden pheasant, the scarlet macaw. The European robin is known popularly as the red 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 goldfinches, hummingbirds, 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 few species of birds (such as the eclectus parrot) where the female is more colourful than the male.
Most birds can fly, and if they do, then the ability is inherited, not learnt. They fly 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 in the air. 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.
Different types of birds have different needs. Their wings have evolved to suit their lifestyle. Large birds of prey, such as eagles, spend a lot of time soaring on the wind. They 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 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. They stock up on food for the long flight. Migrating water birds usually form family groups of 1230 birds. They fly very high, making use of long streams of air that blow from north to south in different seasons. They are 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 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.
Wandering albatross might spend several years without coming to land. They can sleep while gliding. Arctic terns nest every one to three years.
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. Mixed feeding flocks occur, and can help to spot predators.
Some birds do not fly. Flightlessness in birds has evolved many times.
These include running birds like ostriches and emus and ocean-living birds, the large penguin family. Birds on islands have usually lost the power of flight. This is to their advantage because birds with the power of flight can be blown off their island during a storm. The same ability which got them to the island may later take them away in a storm.
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 disgusting in taste. All the same, 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 are a very successful group of birds. They are a clade. They spend half their time on land. Their wings are adapted to life in the sea and have become flippers which let them in swim fast. They catch fish at sea, where they are in danger from seals.
Preening is how birds keeps their feathers in order. They use their beak to position feathers, interlock feather barbules that have become separated, clean plumage, and keep ectoparasites in check.
Feathers in good shape help the bird's insulation, waterproofing and flight. Their condition is vital to the bid's survival.
Their stomach comes next, with two very different parts. One part is like a straight hollow rod (the "proventriculus") 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.
Ninety-five per cent of bird species are socially monogamous. These birds pair for at least the length of the breeding season. In some cases this arrangement lasts until the death of one of the pair. Monogamy clearly helps if females need males' help to raise a brood successfully. It has other practical advantages: the nest is never left without defence. Birds are generally small, and they have many potential enemies.
Some birds mate for life, like married couples. These birds include pigeons, geese, and cranes. Other birds look for new partners each year. 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.
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. The bird is driven by a hormone (estradiol E2) to prepare a place for the eggs to hatch. Birds' nests may be up a tree, in a cliff or on the ground according to species. When filled with eggs they are almost always guarded by one of the pair. In fact it is virtually impossible for the eggs to survive if one of the parents dies.
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 (and reptiles) 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.
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, catching fish, so that they can feed the chicks when they return.
When baby birds hatch, in most types of birds, they are fed by both parents, and sometimes by older aunts 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.
The same species of bird may 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 a bit from year to year and from place to place.
The class Aves is was defined (1990) as all the descendants of the most recent common ancestor of modern birds and "Archaeopteryx lithographica". But "Archaeopteryx" is almost certainly "not" the ancestor of modern birds. The transition to flight happened a number of times. The researchers offered four definitions. Birds can be:
Birds are the closest living relatives of the Crocodilia. This is because they are the two main survivors of a once huge group called the Archosaurs.
Modern birds are "not" descended from "Archaeopteryx" specifically. According to DNA evidence, "modern" birds (Neornithes) evolved in the long Cretaceous period. More recent estimates showed that modern birds originated early in the Upper Cretaceous.
Primitive bird-like dinosaurs are in the broader group Avialae. They have been found back in the mid-Jurassic period, around 170 million years ago. 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.
A leading authority says "Most living birds have fossil representatives in the Cenozoic"... "Key problems remain in understanding bird phylogeny... we seem to understand as little about the relationships among living birds as among Cretaceous birds". A useful source for modern birds is Clements J. 2007. "The Clements Checklist of Birds of the World". Cornel University Press (6th ed).
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.
A report produced by BirdLife International every five years measures the population of birds worldwide. One in every eight types of birds is now "in decline".