Birds, like all other animals, are in a race to reproduce successfully as many times as possible. Several possible advantages of brood parasitism have been suggested. One definite advantage is the energy saved by the parents. Without the taxing process of rearing the young, the parent bird is free to use that energy to produce more eggs. In Africa, parasitic cuckoos lay an average of one egg more than non-parasitic birds.
Some birds lay each egg in a different nest.
This could be a means of reducing the effects of predation on the baby birds. If a bird's eggs are all in different places, predation on one nest would not kill all the offspring of the parent. As the name suggests, brood parasitism is detrimental to the host and its own chicks. The host-parasite conflict is one that is not well understood. Natural selection keeps this type of parenting in low numbers.