The cuckoo is a bird that never incubates eggs on its own, but throws them into the nests of other birds. Everyone knows about this, but the reason for this behavior is known to few.
This bird belongs to the insectivorous birds and among them it is the largest. A lot of food is required to feed an adult. It simply won't work for adults to feed the chicks as well. Hunting a bird for prey is quite difficult, and this is due to its size. The bigger she is, the harder it is for her to maneuver in the air, and more food is needed.
It is much easier for those birds that feed on grain to feed themselves and raise offspring, to make stocks. The cuckoo does not have such an opportunity, so she throws eggs into other people's nests.
How do birds with very small sizes cope with cuckoos
The mother cuckoo lays only one egg in her chosen nest. The cuckoo is born and all the other eggs are simply thrown out of the nest. As a result, the parent birds feed only one chick, and they are able to do this.
This process is modeled by nature. If this species of birds did not throw eggs, and other relatives did not hatch and feed other people's chicks, then we would not have remembered about the cuckoos, since they would simply have died out.