Reduced parasitism by retaliatory cuckoos selects for hosts that rear cuckoo nestlings

We present a model to investigate why some bird species rear the nestlings of brood parasites in spite of suffering large reductions in their own imme diate fitness. Of particular interest is the case in which hosts rear only the parasite's young, all of their own offspring having been ejected or des troyed by the parasite. We investigate the conditions for the evolution of retaliation by brood parasites against hosts that eject their young, as wel l. as the evolution of nonejection by hosts. Retaliation by cuckoos can evo lve, despite potentially benefiting other brood parasites, if rates of ejec tion by hosts are neither too high nor too low and if depredated nests are reparasitized at a high rate by the depredating cuckoo. The presence of a r etaliatory cuckoo then eases the conditions for the evolution of hosts to a ccept and rear cuckoo offspring. A key condition favoring the evolution of nonejection is that nonejectors enjoy lower rates of parasitism in later cl utches compared to ejectors. This requires that cuckoos reparasitize the cl utches of ejectors at relatively high rates and that nonejectors can rear a clutch of their own following the rearing of a cuckoo nestling. If these c onditions are not met, it pays hosts to eject cuckoo nestlings even if the cuckoo retaliates. The model can explain why nonejection is relatively easy to evolve in cases in which the host young are reared alongside those of t he cuckoo, such as in cowbirds, and shows how hosts can resist invasion by parasitic cuckoos. The model predicts that retaliatory brood parasites such as the cuckoo have good memory for the location and status of nests in the ir territory. Hosts of retaliatory cuckoos whose nestlings destroy the host clutch are predicted to have long breeding seasons or the ability to attem pt more than one clutch per season. Our model of retaliation may have wider applications to host-parasite relationships, virulence, and immunity.