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Aves:
Aves are bipedal, warm-blooded, oviparous vertebrate animals characterized primarily by feathers, forelimbs modified as wings, and (in most) hollow bones. Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu. Depending on the taxonomic viewpoint, there are about 8,800–10,200 living bird species (and about 120–130 that have become extinct in the span of human history) in the world, making them the most diverse class of terrestrial vertebrates.Birds feed on nectar, plants, seeds, insects, fish, mammals, carrion, or other birds. Most birds are diurnal, or active during the day, but some birds, such as the owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night. Many birds migrate long distances to utilise optimum habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross). Some, such as Common Swifts, stay aloft for days at a time, even sleeping on the wing. Common characteristics of birds include a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, a 4-chambered heart, and a light but strong skeleton. Most birds are characterised by flight, though the ratites are flightless, and several other species, particularly on islands, have also lost this ability. Flightless birds include the penguins, ostrich, kiwi, and the extinct Dodo. Flightless species are vulnerable to extinction when humans or the mammals they introduce arrive in their habitat. The Great Auk, flightless rails, and the moa of New Zealand, for example, all became extinct due to human influence. Birds are among the most extensively studied of all animal groups. Hundreds of academic journals and thousands of scientists are devoted to bird research, while amateur enthusiasts (called birdwatchers, twitchers or, more commonly, birders) probably number in the millions. The earliest known species of this class is Archaeopteryx lithographica, from the Late Jurassic period. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria. Phylogenetically, Aves is usually defined as all descendants of the most recent common ancestor of modern birds (or of a specific modern bird species like Passer domesticus), and Archaeopteryx. Modern birds are divided into two superorders, the Paleognathae (mostly flightless birds like ostriches), and the wildly diverse Neognathae, containing all other birds. Bird Orders:
Relationships between bird orders according the Sibley-Ahlquist taxonomy. "Galloanseri" is now considered a superorder Galloanserae. This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. The list of birds gives a more detailed summary of these, including families. SUBCLASS NEORNITHES:
Paleognathae: Struthioniformes, Ostrich, emus, kiwis, and allies, Tinamiformes etc. Neognathae: Anseriformes, waterfowl
Galliformes, fowl
Gaviiformes, loons Podicipediformes, grebes Procellariiformes, albatrosses, petrels, and allies Sphenisciformes, penguins Pelecaniformes, pelicans and allies Ciconiiformes, storks and allies Phoenicopteriformes, flamingos Falconiformes, falcons, eagles, hawks and allies Gruiformes, cranes and allies
Charadriiformes, gulls, button-quail, plovers and allies Pteroclidiformes, sandgrouse
Columbiformes, doves and pigeons Psittaciformes, parrots and allies Cuculiformes, cuckoos, turacos, hoatzin Strigiformes, owls Caprimulgiformes, nightjars and allies Apodiformes, swifts and hummingbirds Coraciiformes, kingfishers Piciformes, woodpeckers and allies Trogoniformes, trogons Coliiformes, mousebirds
Passeriformes, passerines
This is the traditional classification (the so-called Clements order). A radically different classification based on molecular data has been developed (the so-called Sibley-Monroe classification or Sibley-Ahlquist taxonomy). This has influenced taxonomical thinking considerably, with the Galloanserae proving well-supported by recent molecular, fossil and anatomical evidence. Extinct bird orders:
A wide variety of bird groups became extinct during the Mesozoic era and left no modern descendants. These include the orders Archaeopterygiformes, Confuciusornithiformes, toothed seabirds like the Hesperornithes and Ichthyornithes, and the diverse subclass Enantiornithes ("opposite birds"). There is significant evidence that birds evolved from theropod dinosaurs, specifically, that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes less so. Recent discoveries in northeast China (Liaoning Province), demonstrating that many small theropod dinosaurs had feathers, contribute to this ambiguity. The basal bird Archaeopteryx, from the Jurassic, is well-known as one of the first "missing links" to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Confuciusornis is another early bird; it lived in the Early Cretaceous. Both may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt if this was a bird ancestor. Other Mesozoic birds include the Enantiornithes, Yanornis, Ichthyornis, Gansus and the Hesperornithiformes, a group of flightless divers resembling grebes and loons. The recently discovered dromaeosaur Cryptovolans was capable of powered flight, possessed a sternal keel and had ribs with uncinate processes. In fact, Cryptovolans makes a better "bird" than Archaeopteryx which is missing some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is currently inconclusive, but digs continue to unearth fossils (especially in China) of the strange feathered dromaeosaurs. At any rate, it is fairly certain that avian flight existed in the mid-Jurassic and was "tried out" in several lineages and variants by the mid-Cretaceous. Although ornithischian (bird-hipped) dinosaurs share the same hip structure as birds, birds actually originated from the saurischian (lizard-hipped) dinosaurs (if the dinosaurian origin theory is correct), and thus arrived at their hip structure condition independently. In fact, the bird-like hip structure also developed a third time among a peculiar group of theropods, the Therizinosauridae. An alternate theory to the dinosaurian origin of birds, espoused by a few scientists (most notably Lary Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama, a theory which is contested by most other scientists in paleontology, and by experts in feather development and evolution such as R.O. Prum. Modern birds are classified in Neornithes, which are now known to have evolved into some basic lineages by the end of the Cretaceous (see Vegavis). The Neornithes are split into the Paleognathae and Neognathae. The paleognaths include the tinamous (found only in Central and South America) and the ratites. The ratites are large flightless birds, and include ostriches, cassowaries, kiwis and emus (though some scientists suspect that the ratites represent an artificial grouping of birds which have independently lost the ability to fly in a number of unrelated lineages). The basal divergence from the remaining Neognathes was that of the Galloanseri, the superorder containing the Anseriformes (ducks, geese and swans), and the Galliformes (the pheasants, grouse, and their allies). See the chart for more information. The classification of birds is a contentious issue. Sibley & Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds (although frequently debated and constantly revised). A preponderance of evidence seems to suggest that the modern bird orders constitute accurate taxa. However, scientists are not in agreement as to the relationships between the orders; 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. Bird Anatomy:
Birds have a body plan that shows so many unusual adaptations (mostly aiding flight) that birds have earned their own unique class in the vertebrate phylum. The skeleton consists of bones which are very light. They have large pneumatic cavities which connect with the respiratory system. The skull bones are fused and do not show sutures. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and sternum is keeled for the attachment of flight muscles, except in the flightless bird orders. The forelimbs are modified into the wings.
Unlike mammals, birds don't urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine as we do, they excrete it in the form of uric acid. They also excrete creatine rather than creatinine as in mammals. Uric acid has a very low solubility in water, so it emerges as a white paste. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose hole for birds: their wastes come out of it, they have sex by putting their cloacas together, and females lay eggs out of it.
Birds have one of the most complex lung system of all organisms. Air enters the bird and immediately 75% of the air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. When the bird exhales the air from the posterior air sac is forced into the lungs; thus birds receive a supply of air during both inhalation and exhalation. The gas exchange then takes place in the capillaries. The heart has four chambers and the right aortic arch gives rise to systemic aorta (unlike in the mammals where the left arch is involved). Postcava receives blood from the limbs via the renal portal system.
Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes, situated at the lower end of the trachea where it bifurcates. The digestive system of the bird is unique and has a crop for storage and a gizzard for grinding food. Most are highly adapted for rapid digestion, an adaptation to flight. Migratory birds have the additional ability to reduce parts of the intestines prior to migration.
The nervous system, relative to the bird's size, is actually quite large. The most developed part of the brain is the one that controls the flight related function while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. A bird's eyes are developed for taking off, spotting landmarks, hunting and feeding. Birds with eyes on the side of their head have a wide vision field while birds with eyes on the front of their heads like owls have binocular vision and can measure depth.
Most birds have poor sense of smell except in the Kiwis and vultures. The visual system is highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water. Some species also have dual fovea. Retina has a fan shaped blood supply system called the pecten. The ear lacks external pinnae but is covered by feathers. The inner ear has a cochlea but is not spiral as in mammals.
Nesting:
Eggs:
All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay coloured eggs, even if, like the tits they are hole-nesters. The brown or red protoporphyrin markings on passerine eggs reduce brittleness and are a substitute for calcium when that element is in short supply. The colour of individual eggs is genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ). The eggs are laid in a nest, which may be anything from a bare cliff ledge or ground scrape to elaboratey decorated structures such as those of the oropendolas.
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