WHY DO ALL HOST SPECIES NOT SHOW DEFENSE AGAINST AVIAN BROOD PARASITISM - EVOLUTIONARY LAG OR EQUILIBRIUM

Avian brood parasitism reduces the reproductive success of hosts and i s therefore expected to select for host defenses against parasitism, s uch as an ability to reject parasitic eggs. Field studies have shown t hat some hosts recognize and reject parasitism, whereas others do not, and the degree of the defense varies from population to population. O ne long-standing debate concentrates on the differences in the distrib ution of host defenses observed in hosts parasitized by the brown-head ed cowbird and the common cuckoo. The cowbird's hosts show either few or nearly perfect defenses, whereas the cuckoo's hosts have defenses v arying from none to complete, with most falling in between the two ext remes. To explore the mechanisms underlying this pattern, I constructe d a mathematical model in which host defense is assumed to be genetica lly determined and analyzed how the host defense is established under parasitic pressure. The model shows that differences in the defense-le vel distribution can be attributed to the difference in the parasite's breeding strategy, generalized or specialized: hosts parasitized by g eneralists show perfect, none, or intermediate levels of the defense d epending on the host abundances, whereas hosts parasitized by speciali sts always exhibit either none or intermediate levels of the defense i f the parasite lacks counterdefenses such as egg mimicry. This result provides a testable explanation for the existence of accepter species oi the brown-headed cowbird, which might reconcile the previously conf licting hypotheses.