Mimicry cannot explain rejection type in a host–brood parasite system

One of the most effective defensive strategies of hosts against brood parasites is rejection, commonly achieved by ejection of the parasitic egg or desertion of the parasitized nest. Nest desertion should be a costlier strategy than egg ejection, because birds must thesn spend additional time and energy renesting, and therefore we still cannot explain why some individuals desert their nests rather than eject parasitic eggs and continue a given breeding attempt. The great reed warbler, Acrocephalus arundinaceus, is a frequent host of the common cuckoo, Cuculus canorus, and is known to use both types of rejection response. By measuring cuckoo egg mimicry, we investigated the hypothesis that the hosts desert if they cannot reliably recognize the cuckoo egg in their nest. We predicted that we would find better mimicry when hosts deserted rather than ejected. However, we did not find a difference in mimicry between these two groups of nests, implying that host females do not desert because they cannot reliably recognize the parasitic egg. We also showed that neither the date in the season nor the age of the host females influenced the type of rejection. Other factors potentially eliciting nest desertion, including host personality, host, inability to eject, excessive clutch reduction and visibility of the cuckoo female at the host nest, are discussed. Finally, we suggest that desertion may persist as a host defensive strategy against brood parasitism because it is not as costly as previously assumed and/or it is beneficial for host females in good physical condition.