For flight any animal must be strong and yet light weight. Many bird bones are hollow and reinforced internally with struts. Massive breast muscles power the downward motion of the wing; smaller ones move it up. Feathers are important for flight and they also supply the bird with insulation.
Birds use their eyes constantly, spotting obstacles with them, locating food, judging distance and navigating by the stars. Their vision is among the best in the animal kingdom. Another sense important in flight is that of balance and position. The fluid filled inner ear acts as a balancing organ, working like a gyroscope (device for measuring orientation) to judge deviations from the horizontal and to help birds maintain level flight.
Air currents always changing in direction and force, move across the layer of feathers. In the skin at the base of each feather are nerve endings. These nerve endings are sensitive to feather movements caused by air currents. Birds have the ability to move each feather independently by contracting muscles near the base. Thus the sense of touch helps birds adjust feather position, allowing them to exploit air currents in the best possible way.
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