This is a natural behavior for nestlings. Sometimes, the aggression is essentially play-fighting, a way for them to practice and hone their skills. Other times, especially when food is scarce, aggression can lead to siblicide. Also, if the parents can sense that one chick is doing better than the other, they may focus all their food, attention, and energy into ensuring that the healthy chick survives. The older chicks may be the only one to survive.