Early evolution of birds

See also: List of fossil birds

Aves  Archaeopteryx  Pygostylia  Confuciusornithidae  Ornithothoraces  Enantiornithes  Ornithurae  Hesperornithiformes Neornithes Basal bird phylogeny simplified after Chiappe, 2007[20]

Birds diversified into a wide variety of forms during the Cretaceous Period.^[20] Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,^[20] the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia.

The first large, diverse lineage of short-tailed birds to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.^[20] More advanced lineages also specialised in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").^[21]
One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Despite their extreme specializations, the Hesperornithiformes represent some of the closest relatives of modern birds.^{[20]
Diversification of modern birds See also: Sibley-Ahlquist taxonomy and dinosaur classification
Containing all modern birds, the subclass Neornithes is, due to the discovery of Vegavis, now known to have evolved into some basic lineages by the end of the Cretaceous[22] and is split into two superorders, the Palaeognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. It is agreed that the Neornithes evolved in the Cretaceous, and that the split between the Galloanseri from other Neognathes occurred before the K–T extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.[23] This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.[23][24]}

^{The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds,[25] although it is frequently debated and constantly revised. Most evidence seems to suggest that the assignment of orders is accurate,[26] but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.}

Modern bird orders: Classification

See also: List of birds

Neornithes   Palaeognathae  Struthioniformes Tinamiformes  Neognathae    Other birds (Neoaves) Galloanserae  Anseriformes Galliformes Basal divergences of modern birds based on Sibley-Ahlquist taxonomy

Cladogram showing the most recent classification of Neoaves, based on several phylogenetic studies.^[27]

This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This list uses the traditional classification (the so-called Clements order), revised by the Sibley-Monroe classification. The list of birds gives a more detailed summary of the orders, including families.

Subclass Neornithes
The subclass Neornithes has two superorders –

Superorder Palaeognathae:
The name of the superorder is derived from 'paleognath', the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. The Palaeognathae consists of two orders which comprise 49 existing species.

• Struthioniformes—ostriches, emus, kiwis, and allies
• Tinamiformes—tinamous

Superorder Neognathae:
The superorder Neognathae comprises 27 orders which have a total of nearly ten thousand species. The Neognathae have undergone adaptive radiation to produce the staggering diversity of form (especially of the bill and feet), function, and behavior that are seen today.
The orders comprising the Neognathae are:

• Anseriformes—waterfowl
• Galliformes—fowl
• Charadriiformes—gulls, button-quails, plovers and allies
• Gaviiformes—loons
• Podicipediformes—grebes
• Procellariiformes—albatrosses, petrels, and allies
• Sphenisciformes—penguins
• Pelecaniformes—pelicans and allies
• Phaethontiformes—tropicbirds
• Ciconiiformes—storks and allies
• Cathartiformes—New World vultures
• Phoenicopteriformes—flamingos
• Falconiformes—falcons, eagles, hawks and allies
• Gruiformes—cranes and allies
• Pteroclidiformes—sandgrouse
• Columbiformes—doves and pigeons
• Psittaciformes—parrots and allies
• Cuculiformes—cuckoos and turacos
• Opisthocomiformes—hoatzin
• Strigiformes—owls
• Caprimulgiformes—nightjars and allies
• Apodiformes—swifts and hummingbirds
• Coraciiformes—kingfishers and allies
• Piciformes—woodpeckers and allies
• Trogoniformes—trogons
• Coliiformes—mousebirds
• Passeriformes—passerines

The radically different Sibley-Monroe classification (Sibley-Ahlquist taxonomy), based on molecular data, found widespread adoption in a few aspects, as recent molecular, fossil, and anatomical evidence supported the Galloanserae.^[23]
Distribution

See also: Lists of birds by region

The range of the House Sparrow has expanded dramatically due to human activities.[28]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.[29] 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.[30] 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[31] and some penguins have been recorded diving up to 300 metres (980 ft).[32]

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.[33] Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity.[34] Some species, including Cattle Egret,[35] Yellow-headed Caracara[36] and Galah,[37] have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.