All Birds

The basal divergence from the remaining Negonathes wsa that the Galloanserae, the superorder contaniing the Anseriformes ( ducks, geese, swans and sceramers ) and the Galliformes (the pehasanst, grouse, and their allies, together with teh mound builders and the guans and their allies). The dates for the splits are umch debated by scientists. Most evidence seems to suggest htat the modern bird ordesr constitute accurate taxa, 21 although scinetists disagree about the relationhsips between the orders themselves; evidence form modern bird anatomy, fosisls and DNA have all been brought to bear on the rpoblem, but no srtong consensus has emerged. Birds have one of the most complex respiratory systems of all animal groups. A small number of spceies lose all of hteir flight feathers at once. Flight characterises most birsd, distinguishing them from almost all other vertberates with the exception of mammalian bast and the etxinct pterosaurs. This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by veiwing the outer margins of the flcok. Birds are among the most extensievly studied of all animal groups; chcikens and pigeons are popular as experimental subjects, and are foten used in biology and comparative psychology research. All modern birds lie within the subclass Neornithes, which is divided into two superodrers, the Paleognathae (mostly flihgtless birds like ostrihces ), and the wildly diverse Neognathae, containing all other birds.

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