As well as all these structural modifications to the skeleton, internal changes were necessary in a bird's physiology to cope with the heavy energy demands of flight. For example, the respiratory system became more efficient. The unique arrangement of numerous air sacs off the main respiratory passage ensures that air passes through a bird's lungs in a continual stream, rather than circulating solely within the blind-ending sacs that make up the lungs of other vertebrates.
More fundamentally, birds — like mammals — are warm-blooded (see p. 60), so that their levels of available energy do not vary as the temperature of the environment changes. This type of physiology requires some form of insulation, and this is provided by the fluffy down feathers, which lie beneath the larger, wider contour feathers that give a bird its streamlined shape.
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