Final answer:
The structural adaptation of a bird's respiratory system that is part of its physiology is the air sacs. These air sacs are a critical part of the flow-through respiratory system, which provides efficient respiration and is an essential adaptation for flight.
Step-by-step explanation:
Bird Respiratory System Adaptation
The structural adaptation of a bird's respiratory system that is part of its physiology is the air sacs. Unlike other animals that may have swim bladders (a) or gills (b), birds possess a unique flow-through respiratory system. This system includes several air sacs that ensure air flows unidirectionally from the posterior sacs into the lungs, then into the anterior air sacs. These sacs contribute to the unidirectional airflow, making birds lighter and providing efficient respiration—critical for flight.
Birds also have a gizzard (c) which is part of their digestive system and helps grind food as birds do not have teeth. However, the gizzard is not involved in their respiration. Thus, the correct answer to the question is (d) air sacs, which are a vital part of their respiratory system and a major adaptation for flight.
Furthermore, this respiratory system is well-adapted to meet the high metabolic rate required for flight. The Organ System Adaptations for Flight are crucial for birds, providing the necessary oxygen delivery system to support their active lifestyle.