Why does the yolk get reduced as baby bird starts growing?

The yolk sac of a baby bird is a temporary structure that provides nourishment to the developing embryo and hatchling. It is attached to the bird's abdomen and is filled with a rich source of nutrients, including proteins, fats, vitamins, and minerals. As the baby bird grows and develops, it begins to absorb the nutrients from the yolk sac. This process is facilitated by the yolk stalk, a narrow tube that connects the yolk sac to the bird's digestive system. As the bird's digestive system becomes more fully developed, it becomes increasingly capable of digesting food and absorbing nutrients from other sources, such as the food that the parent birds provide. As the bird continues to grow, its body increases in size and its internal organs develop, allowing it to become more independent. Consequently, the yolk sac逐渐减小, eventually becoming fully absorbed into the bird's body.