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| Birds origin |
| We try to gather
online data from various sources: Birds (class Aves) are bipedal, warm-blooded, oviparous vertebrate animals. Most scientists believe that birds evolved from theropod dinosaurs. Ranging in size from tiny hummingbirds to the huge Ostrich and Emu, there are between 9,000 and 10,000 known living bird species in the world, making Aves the most diverse class of terrestrial vertebrate. A bird is characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a light but strong skeleton. Most birds have forelimbs modified as wings and can fly. Birds are important sources of food, acquired either through farming or hunting. Numerous species of birds are also used commercially, and some species, particularly songbirds and parrots, are popular pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. Most paleontologists regard birds as the only clad of dinosaurs that survived the Cretaceous (Tertiary extinction event approximately 65.5 Ma). Modern birds are characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations. Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviors including cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertilizer. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them. All modern birds lie within the subclass Neornithes, which has two subdivisions: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds. These two subdivisions are often given the rank of super order, although Livezey & Zusi assigned them "cohort" rank. Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050. Dinosaurs and the origin of birds:- There have been many controversies in the study of the origin of birds. Early disagreements included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors. Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently. In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae. A few scientists suggest that birds are not dinosaurs, but evolved from early archosaurs like Longisquama. Based on fossil and biological evidence, most scientists accept that birds are a specialised sub-group of theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity. The consensus view in contemporary paleontology is that the birds, Aves, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these three form a group called Paraves. The basal dromaeosaur Microraptor has features which may have enabled it to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, and/or may have been able to glide. |
| The Late Jurassic
Archaeopteryx is well-known as one of the first transitional fossils to
be found and it provided support for the theory of evolution in the late
19th century. Archaeopteryx has clearly reptilian characteristics:
teeth, clawed fingers, and a long, lizard-like tail, but it has finely
preserved wings with flight feathers identical to those of modern birds.
It is not considered a direct ancestor of modern birds, but is the
oldest and most primitive member of Aves or Avialae, and it is probably
closely related to the real ancestor. It has even been suggested that
Archaeopteryx was a dinosaur that was no more closely related to birds
than were other dinosaur groups and that Avimimus was more likely to be
the ancestor of all birds than Archaeopteryx. External anatomy of a bird: 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Feet, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 Wattle Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the Snow Petrel's breeding colonies up to 440 kilometres (270 mi) inland in Antarctica. The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics, however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics. Several families of birds have adapted to life both on the world's oceans and in them, with some seabird species coming ashore only to breed and some penguins have been recorded diving up to 300 meters (980 ft). Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the Ring-necked Pheasant, for example, has been introduced around the world as a game bird. Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity. Some species, including Cattle Egret, Yellow-headed Caracara and Galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat. Feathers are a feature unique to birds. They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signaling. There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called puerile. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematic. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex. |