Why do birds not have respiratory trouble at the time of flying at high altitude ?

Because flying takes a tremendous amount of energy, birds need to get lots of oxygen. The extra air sacs in their lungs help them to keep fresh air flowing in, unlike mammals, where the “old” air within the trachea and mouth (which has low oxygen content and high CO² levels) in inhaled each time. Birds eliminate this “anatomical dead space” problem by using air sacs.