How does evolution against brood parasites reshape or modify an instinctive behaviour in birds?

Egg retrieval is regarded as an ancestral behaviour that evolved in birds as an adaptation to eggs accidentally rolling out of nests and a fixed pattern of instinctive behaviour. By contrast, egg recognition is a later and specific adaptation that arises under selection pressure from brood parasitism, through which eggs likely belonging to brood parasites are rejected from the nest. Furthermore, egg recognition is flexible and can be enhanced through learning. We performed an empirical study on 12 populations of seven species in the tit family (Paridae), which revealed that mimetic eggs were retrieved more frequently than nonmimetic eggs across species; however, the rejection patterns were completely contrasting. Furthermore, species with a stronger recognition capacity of nonmimetic eggs retrieved eggs with lower probability; however, they rejected them with higher probability. This study also revealed that although egg retrieval is possibly a fixed pattern of instinctive behaviour, it can be modified to become a flexible reaction that adapts to subsequent evolution. Moreover, egg recognition, a specific adaptation to brood parasitism, gradually reshapes or modifies egg retrieval behaviour during evolution in a predictable manner, resulting in birds with higher levels of egg recognition being less willing to retrieve an egg from outside their nests.