The part labeled III is used for

The part labeled III is used for grinding food particles. This part refers to the gizzard, which is a muscular stomach that aids in the mechanical breakdown of food. In birds, the gizzard is lined with tough, keratinous plates that grind food particles, such as seeds and insects, into smaller pieces that can be more easily digested. The grinding action of the gizzard helps to increase the surface area of the food, making it easier for digestive enzymes to break down and absorb the nutrients. Therefore, the correct answer is "grinding food particles."