The evolution of nestling discrimination by hosts of parasitic birds: Why is rejection so rare?

Abstract

Question: Why do hosts of parasitic birds defend against parasitic eggs but not nestlings? Data incorporated: Reported cases of parasitic chick discrimination or mimicry and all previously published explanations for the rarity of these phenomena. Method of analysis: Contrasting the predictions of previous hypotheses and fitting available data from both parasitic and non-parasitic birds to assess the relative validity of each explanation. Results: None of the previously suggested hypotheses appears to provide a general explanation for the scarcity of chick discrimination. Various cognitive and behavioural traits potentially usable for discrimination of parasitic chicks are present in virtually all avian taxa, including host lineages, yet these traits are not used to reject parasites. Thus, low selection pressure imposed by rare parasites is the most likely general explanation for the absence of these adaptations in the context of brood parasitism. Based on this, I predict that nestling discrimination and mimicry should predominantly evolve in hosts that are forced to accept parasite eggs because of the close match between parasitic and host eggs. This is likely to occur due to egg mimicry or phylogenetic and physical constraints. I demonstrate that available evidence is in line with this rare parasite hypothesis. Conclusion: A host's own behaviour may play a crucial role in retarding the escalation of the arms-race to the nestling stage. © 2006 Tomáš Grim.