Parasitism is always costly to the host.

Recent studies have examined the cost of raising parasitic cuckoos and highlighted the importance of "no extra cost" in explaining the low levels of defense in hosts. To clarify the reasons for parasitization in typical hosts, we present a simple model to explore the immediate and future costs of parasitism in shaping the evolution of defense behavior in hosts. Our results suggest that any cost of parasitization is maladaptive to the host and learned egg recognition is always favored to overcome these costs. Furthermore, although facing a potential cost of mis-imprinting, learned nestling recognition may still evolve when there is a non-zero immediate cost from raising a parasitic nestling. Therefore, we contend that "no extra cost" does not provide sufficient evidence to explain the low levels of defense behavior in hosts per se.