Birds are vertebrates that have feathers, warm blood, and lay eggs externally. Birds are one of the lineages of reptiles; in fact, they probably form a lineage from within the dinosaurs. Because "reptile" is not a coherent group if mammals and birds are excluded, and because "dinosaur" is not a coherent group if birds are excluded, many scientists include birds and mammals with the reptiles and include birds with the dinosaurs. There are almost twice as many bird species (more than 9,000) as mammal species.

The single most recognizable feature of birds is feathers. Feathers are complex and lightweight structures consisting of a central shaft with a vane consisting of barbs and hooks. Flight feathers are very specialized, with the shaft off-center in a manner that permits aerodynamic efficiency. Other kinds of feathers are less complex. Down feathers, for example, are small and function mostly in holding in body heat. Since simpler feather structures are ineffective for flight, the evolution of feathers probably began with simple feathers that held in body heat, from which more complex flight feathers evolved. Feathers also repel water, especially when birds preen them, applying water-repellent materials from glands.

Birds have many other features that adapt them to flight. In fact, the entire body of the bird appears adapted to flight, especially by the reduction of weight. The evolutionary origin of birds from among reptiles is evident from modern characteristics and from fossil evidence. . .

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