Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense. The first is through dispersal, and the second is by avoiding familiar group members as mates.  The screamers (Anhimidae), some jacanas (Jacana, Hydrophasianus), the spur-winged goose (Plectropterus), the torrent duck (Merganetta) and nine species of lapwing (Vanellus) use a sharp spur on the wing as a weapon. , Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this. The other part of the stomach is the gizzard. Anything indigestible, for example remains of feathers, is regurgitated via the mouth, not the cloaca. Hill, Richard W. (2012) Animal Physiology/ Richard W. Hill, Gordon A. Wyse, Margaret Anderson. In: K. Padian, ed. They are usually laid in a nest and incubated by the parents. Swans, for instance, may strike with the bony spurs and bite when defending eggs or young. It sends out a "look elsewhere" signal to their competitors. Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones. , The length and nature of parental care varies widely amongst different orders and species. , The consensus view in contemporary palaeontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Some calls in some species are learnt.  The highest bird diversity occurs in tropical regions. The digestive and respiratory systems of birds are also uniquely adapted for flight. Male blue tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers. Seagulls are also good at hovering, particularly if there is a strong breeze. Many birds are not camouflaged, but stand out with vivid colours. Species that are unable to defend feeding territories, such as seabirds and swifts, often breed in colonies instead; this is thought to offer protection from predators. Birds are a group of warm-blooded vertebrates constituting the class Aves /ˈeɪviːz/, characterized 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.  Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. For other uses, see, Gauthier, J., and de Queiroz, K. (2001). From these remains we know that birds evolved from small carnivorous dinosaurs (theropods) in the Jurassic period. This is very useful when sitting on a nest.  Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Those where the song is entirely learnt, and the bird often copies sounds from its.  Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as sulids, kingfishers and terns plunge dive after their prey.  Some birds also use mechanical sounds for auditory communication.  At the other extreme, many seabirds have extended periods of parental care, the longest being that of the great frigatebird, whose chicks take up to six months to fledge and are fed by the parents for up to an additional 14 months. Unlike mammals, birds only have one opening as the exit hole for body fluids, and for reproduction. People often have colorful birds such as parrots and mynahs as pets. They lay hard-shelled eggs. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Because of this, some people trap birds and take them to other countries to sell.  When not copulating, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. It is the same reaction as that which keeps mammals and birds at a high temperature. , Altitudinal migration is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. As chicks grow they change the fluffy down that covers them as babies for real feathers. Guillemots lay their eggs on rock shelves with no nest at all. , All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Nicholls, (Eds). Their stomach comes next, with two very different parts. They are endothermic, with feathers.  As scientists have discovered more theropods 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 many small theropod feathered dinosaurs, contribute to this ambiguity. an infra-red scope on burrow-nesting sooty shearwaters, The Enduring Symbolism of Doves, From Ancient Icon to Biblical Mainstay", "Generative landscapes: the step mountain motif in Tiwanaku iconography", "NJIT professor finds nothing cuckoo in serenading our feathered friends", "Composers' Appropriation of Pied Butcherbird Song: Henry Tate's "undersong of Australia" Comes of Age", "BirdLife International announces more Critically Endangered birds than ever before", "How many bird extinctions have we prevented? And fish-eaters to tree-dwelling forms and seed-eaters blood to be related to sperm competition aquatic environments particularly. Legs of yellow or red of eggs, so that they do not produce offspring laying! Nevertheless, they were fat and delicious birds do not flap loudly have adapted to suit the in! 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