Avian brood parasitism provides an ideal system to study co-evolutionaryarms race between hosts and parasites because both the parties are tightlylinked by a conflicting way of reprodcution. Although we observesophisticated adaptations and counteradaptations in their behaviors/traits,some of them still seem maladaptive. I review various levels of theseadaptive behaviors/traits and then present several models for severalspecific topics to explain why these variations are achieved.