Females brood parasites usually destroy 1 host egg. Occasionally whole nests are destroyed, causing the host to renest. Young wydah's gape patterns (and feeding postures and calls match those of host. Wrong gape pattern wont be fed). Pied-crested Cuckoo lay thick-shelled egg from above nest this damages one of host's eggs. Young parasites may eject host eggs or young or actually murder them (honey-guides have sharp mandibular hooks). In the Indian Koel (a cuckoo), male approaches House Crow nest calling loudly. When the incubating crow gives chase, the female koel slips in to lay egg. Some parasites have protrusible cloaca so they can lay eggs in holes that are too small for the adults. Others lay eggs very quickly (1-2 seconds apart in European Cuckoo).
The evolution of parasitism: a) take over nest of other species but still taking care of own young (Bay-winged Cowbird); b) leave host to rear young or intra-specific brood parasitism, such as in starlings and tree swallows.