Due to the ability to fly, birds have acquired a streamlined body shape, which reduces its resistance to the oncoming air flow. So, their body is ovoid; the upper and lower jaw of the skull is extended forward, forming a beak. The whole body is covered with feathers, which overlap each other in tiles and form a streamlined surface of the feather cover. Lightening of body weight in birds is achieved by the absence of teeth, bladder, right ovary in females, the presence of air sacs in the body cavity and air cavities in the tubular bones, as well as thin strong bones and thin dry skin.
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