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one of the areas of the skin of birds that are covered with contour feathers. Contour feathers evenly cover the entire body only in penguins, ratites, and Palamedea. In all other birds pterylae alternate with apteriae. The distribution and shape of the pterylae are a taxonomic character. The development of pterylae and apteriae is connected with thermoregulation and with facilitating movement of separate areas of the skin that are covered with feathers.