Eurasian coots, a species of migratory water bird, may hatch up to nine chicks. But under normal circumstances, food is in short supply. The parent birds feed the baby birds tiny shrimp for the first three days after hatching. Then, mama coot turns into Mommy Dearest. A baby bird begs for food, as usual—but, with no warning, the parents "punish" it, biting the chick hard on its tiny head. The parents do this to all the chicks in turn. Eventually, one chick is singled out for special torture, and abused until it stops begging for food and starves to death.
This process is repeated until only two or three chicks survive.
Pelicans hatch three chicks, but under normal circumstances, only one survives. Instead of the parent birds doling out death, it's the siblings—the two larger birds pluck at the smallest with their sharp beaks and knock it out of the nest. Then the conspirators turn on each other until only one chick is left.
Sir David Attenborough himself acknowledges that this might be a bit cruel, by human standards. But, he assures us, it's all for the best—in especially good years, a pelican or coot can raise an extra chick or two. So torturing baby birds to death serves the purpose of increasing the genetic fitness of the parents by a little bit.
Does that really make it okay? -Sarah Perry - Every Cradle Is A Grave