Birds  

Aves is the latin name for the birds - feathered, winged, bipedal, warm-blooded, egg-laying, vertebrate animals with evolutionary origins among the reptiles. The taxon has been historically treated as equal to fish, amphibia, reptiles and mammals, but in order to make classifications reflect evolutionary history, they are now more usually regarded as falling inside the Reptilia. Around 10,000 living species makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm Bee Hummingbird to the 2.75 m Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 160 million years (Ma) ago. Birds are the only clade of dinosaurs to have survived the Cretaceous–Paleogene extinction event 65.5 Ma ago.Modern birds are characterised 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 living species of birds have wings. Wings are evolved forelimbs, and most bird species can fly; exceptions include the ostriches, emus and relatives, penguins, and some endemic island species. Birds also have unique digestive and respiratory systems that are well suited to their flying needs. 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 transmit knowledge across generations. Many species undertake long distance annual migrations, and many more perform shorter irregular movements.Many species are social and communicate using visual signals and through calls and songs, and participate in social behaviours, 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, and rarely for life. Other species have polygynous (\"many females\") or, rarely, polyandrous (\"many males\") breeding systems. 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 fertiliser. 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.

Feather parts reattach: birds
 

Feather filaments of birds connect to each other with interlocking hooks.

 
  "A central shaft carries on either side a hundred or so filaments; each filament is similarly fringed with about a hundred smaller filaments or barbules. In downy feathers, this structure produces a soft, air-trapping fluffiness and, therefore, superb insulation. Flight feathers have an additional feature. Their barbules overlap those of neighbouring filaments and hook them onto one another so that they are united into a continuous vane. There are several hundred such hooks on a single barbule, a million or so in a single feather; and a bird the size of a swan has about twenty-five thousand feathers." (Attenborough 1979:173)

"Disarranged feathers are carefully repositioned. Those that have become bedraggled or have broken vanes are renovated by careful combing with the beak. As the filaments pass through the mandibles and are pressed together, the hooks on the barbules reengage like teeth of a zip-fastener to make a smooth and continuous surface again." (Attenborough 1979:179)
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Preening waterproofs feathers: birds
 

The uropygial gland of birds protects them from water penetration, fungi, and bacteria by producing preen waxes.

 
  "In addition to the stratum corneum barrier, glandular lipids are deposited exteriorly to the epidermis in both mammals and birds (Hadley, 1991)…In birds, 'preen waxes' from the uropygial gland are spread over feathers to prevent water penetration and ingress of bacteria and fungi. Uropygial secretions contain a complex mixture of lipids in which wax esters usually predominate…In birds and mammals, plumage and pelage appear to impede significantly the passage of water vapor from skin to atmosphere, although the skin remains the principal barrier to TEWL [transepidermal water loss] (Cena and Clark, 1979; Webster et al., 1985). In pigeons, for example, plumage contributes 5–20% of total resistance to water loss through the integument, and the plumage and boundary layer together account for 6–26% of total resistance to water vapor diffusion (Webster et al., 1985). Therefore, adjustments of plumage or pelage and seasonal shedding patterns are potential means of adjusting rates of TEWL." (Lillywhite 2006:219)
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Shape of feather shafts protect from wind: birds
 

The shafts of feathers and petioles of leaves protect from wind by having non-circular cross sections.

 
  "In cross section, feathers look like grooved petioles upside down. Again, that makes functional sense. If an elongated structure must have a groove to raise EI/GJ ('twistiness-to-bendiness ratio'), the groove should be on the side that's loaded in tension. That location won't increase the structure's tendency to buckle, since tensile loading is nearly shape-indifferent. A leaf blade bends its petiole downward; its aerodynamic loading bends a feather upward--leaf blades hang from the ends of their petioles; flying birds hang from bases of their wing feathers." (Vogel 2003:385)
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