Birds (Aves) are a group of animals with backbones which evolved from dinosaurs. Technically speaking, they are dinosaurs.
Birds are endothermic. The heat loss from their bodies is slowed down by their 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 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. This is because they are the two main survivors of a once huge group called the Archosaurs.
Modern birds are not descended from Archaeopteryx. According to DNA evidence, modern birds (Neornithes) evolved in the long Upper 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 to 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.p274
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. Diversification occurred around the Cretaceous–Palaeogene extinction event.
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.
In general, birds are effective, and inherit their behaviour almost entirely. The key elements of their life are inherited. It was a great discovery that birds never learn to fly. So it is quite wrong to say, when a chick waves its wings in the nest "It's learning to fly". What the chick is doing is exercising its muscles. They develop the ability to fly automatically (assuming they are species that do fly). And if they are species which migrate, 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 which they use, for example, when they search for food.
Several bird species make and use tools. Some social species pass on some knowledge across generations, a form of culture. Birds are social. They communicate with visual signals, calls and bird songs. Most of their social behaviours are inherited, 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 are sources of eggs, meat, and feathers. In English, domesticated birds are often called poultry, undomesticated birds are called game. Songbirds, parrots and other species are popular as pets. Guano, which is bird manure, 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.