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Evolution and Classification of Birds:
– Archaeopteryx is considered the oldest known true bird.
– The first classification of birds was developed by Francis Willughby and John Ray in 1676.
– Carl Linnaeus modified the classification system in 1758, categorizing birds as the class Aves.
– Phylogenetic taxonomy places Aves in the clade Theropoda.
– Avialae is often used synonymously with the term bird by researchers.

Origin and Early Evolution of Birds:
– Paraves, †Scansoriopterygidae, Eosinopteryx, Jinfengopteryx, and Aurornis are important in the origin of birds.
– Confuciusornis sanctus is the oldest known bird with a beak.
– Over 40% of modern bird traits evolved during the transition from archosaurs to maniraptoromorphs.
– Paravians and Avialans diversified into various forms during the Cretaceous period.
– The evolution of body size reduction and neotenic characteristics occurred over 40 million years.

Diversification and Evolution of Avialans:
– Oldest paravian fossils are from the late Jurassic period in China.
– Avialans developed features ancestral to modern birds, like hind wings and enlarged claws.
– Enantiornithes and Euornithes were dominant avialan groups during the Cretaceous period.
– Ornithothoraces evolved refined aerodynamics and flight capabilities.
– Avian ancestors evolved a more open pelvis for laying larger eggs.

Modern Bird Diversification and Classification:
– Modern birds are classified into Palaeognathae, Neognathae, Galloanserae, and Neoaves.
– The most recent common ancestor of modern birds likely existed in the Cretaceous period.
– Molecular dating studies suggest a Cretaceous evolutionary radiation for modern birds.
– Approximately 11,000 living bird species are known.
– The relationships among bird orders are still debated, with fossil and molecular evidence helping understand their evolution.

Anatomy, Physiology, and Adaptations of Birds:
– Birds have unique adaptations for flight, with wings being modified forelimbs.
– The digestive and respiratory systems of birds are uniquely adapted for flight.
– Birds excrete uric acid and lack a urinary bladder.
– The skeletal system of birds is lightweight with air-filled cavities.
– The reproductive system of birds varies among different groups, with some having a penis and others not.

Bird (Wikipedia)

Birds are a group of warm-blooded vertebrates constituting the class Aves (/ˈvz/), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are over 11,000 living species, more than half of which are passerine, or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology.

Temporal range:
Late Cretaceouspresent, 72–0 Ma Possible Early Cretaceous or early Late Cretaceous origin based on molecular clock
Red-crested turacoSteller's sea eagleRock doveSouthern cassowaryGentoo penguinBar-throated minlaShoebillGrey crowned craneAnna's hummingbirdRainbow lorikeetGrey heronEurasian eagle-owlWhite-tailed tropicbirdIndian peafowlAtlantic puffinAmerican flamingoBlue-footed boobyKeel-billed toucan
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Sauropsida
Clade: Archosauria
Clade: Avemetatarsalia
Clade: Dinosauria
Clade: Theropoda
Clade: Ornithurae
Class: Aves
Linnaeus, 1758
Extant clades

Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs. Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Birds are descendants of the primitive avialans (whose members include Archaeopteryx) which first appeared during the Late Jurassic. According to recent estimates, modern birds (Neornithes) evolved in the Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-avian dinosaurs.

Many social species pass on knowledge across generations, which is considered a form of culture. Birds are social, communicating with visual signals, calls, and songs, and participating in such behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous, usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 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 birdwatching is an important part of the ecotourism industry.

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