Why do flying birds have feathers growing only in some areas – pterilia, but no feathers on apteria?

1) This position of the feathers lightens the flight weight of the bird.
2) It also facilitates muscle contraction, mobility of the skin, joints, neck.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.