The reason why this bird lacks a mouth is: pecking injury, lack of vitamins and calcium.

1, pecking injury: Pearl birds will damage their mouths because of excessive force or hard food when pecking. In this case, the mouth injury will generally heal quickly, but it is necessary to pay attention to the bird's diet to ensure adequate nutrition intake.

2. Lack of vitamins and calcium: The oral defects of pearl birds are caused by lack of vitamins and calcium. These nutrients are essential to the normal growth and development of birds. If birds lack these nutrients, their mouths will be defective or abnormal.