Cuckoo birds will put their eggs in other birds' nests and mix them together. On the surface, they are very similar. Without careful observation, you really can't see it, so many mother birds don't know that their eggs have been changed and are still trying to hatch. However, under the hatching of the mother bird, the cuckoo's children are often the first to hatch. The little cuckoo is as overbearing as its mother's temper, and will grab the mother's food as soon as she is born. Even other eggs in the nest will be squeezed out, and some hatched birds can't compete with the little cuckoo, and eventually they will starve to death because of lack of food.
But some mother birds still think that the little cuckoo is their own child and their own flesh and blood, so they take care of it wholeheartedly in feeding. As the little cuckoo grows bigger and bigger, the mother bird will catch more food to feed them, but in the end, the little magpie will grow bigger than the mother bird, and when the feathers grow, the little magpie will fly away.
Most birds don't kill these young birds. Some mothers are not sure if they are their own children, but they dare not take the risk to kill them, because if they kill them wrong, they will probably hurt their real children. Of course, this is not absolute. Some birds that are big or overbearing like cuckoos are not allowed to put their children in their nests. Of course, if it is found, it will be killed and driven away.