Final answer:
Birds have adapted lightweight bodies with specialized respiratory and digestive systems to sustain the high energy demands of flight. They digest food quickly and need to eat often to maintain energy. Their unique respiratory system allows efficient oxygen uptake, particularly important at high altitudes.
Step-by-step explanation:
Birds must maintain light bodies to facilitate flight, which is a high-energy activity demanding efficient metabolic processes. As such, birds' respiratory and digestive systems are highly specialized. To support the energy required for flight, birds need to digest their food rapidly and consequently eat more often to replenish energy reserves. Their respiratory system is adapted to extract a significant amount of oxygen to meet the high metabolic demands, particularly necessary since many species fly at high altitudes where oxygen concentration is low.
Avian respiration is unique: birds have evolved a respiratory system that is extremely efficient. The system includes air sacs that ensure a continuous flow of air through the lungs, allowing for a higher rate of gas exchange which is critical in maintaining sufficient levels of oxygen. Moreover, the design of their feathers and hollow bones reduces weight, while flight feathers are crucial for the mechanics of flying. Organ systems in birds are thus highly adapted for flight, enabling the endurance needed for long distances and the agility required for quick maneuvers.