Why do foster parents feed a young cuckoo even after it grows larger than them?

Foster parents continue to feed a young cuckoo even after it grows significantly larger than them because they perceive the nestling as their own offspring. This behavior is a result of the cuckoo's evolution as a brood parasite, which involves several sophisticated adaptations to ensure the survival of its young.12

Once a cuckoo egg is accepted and incubated by the host, the newly hatched cuckoo often eliminates competition by evicting the host's own eggs or nestlings from the nest. By monopolizing the nest, the young cuckoo secures the full attention and food supply provided by the foster parents. Because the foster parents are biologically programmed to care for the contents of their nest, they continue to provide for the rapidly growing cuckoo, which often dwarfs them by the time it is ready to fledge. This interaction is a classic example of coevolution, where the parasite has evolved to exploit the parental instincts of the host species.123