Why does an egg have a yolk?

Unlike us, birds form outside of their mother’s womb. In a fertilized egg, the yolk is home to the developing bird. Because the baby is encased in the eggshell, the hen can’t feed her young before it hatches. So the yolk serves as a food source, filled with vitamins, calcium, and other nutrients important for proper growth. As the baby animal grows, the yolk shrinks.