Final answer:
The bird structure responsible for stretching and moistening food is the crop, which stores and softens food before digestion. The gizzard grinds the food, compensating for the lack of teeth. These adaptations aid in maintaining efficient digestion and a light body weight for flight.
Step-by-step explanation:
The structure of a bird that causes food to stretch and moisten is called the crop. The crop is a sac-like storage compartment that is an adaptation of the bird's esophagus. It allows birds to quickly ingest food and then later process it securely away from predators. The stored food in the crop is moistened and may stay there for some time before being passed on to the stomach for digestion.The gizzard is another critical component of the avian digestive system. It is a muscular stomach that grinds the food with the help of swallowed stones or 'grit'. This is necessary because birds lack teeth to chew their food, so the gizzard helps to mechanically break down food so that the nutrients can be easily absorbed further down the digestive tract.Birds have unique organ system adaptations for flight, which include a simplified and efficient digestive system to keep their body weight low and their metabolic rate high.