Egg-recognition abilities in non-incubating males: implications for the evolution of anti-parasitic host defenses

Abstract

In the field of brood parasitism, it has been traditionally assumed that only the incubating sex rejects parasitic eggs, but this assumption has been rarely explored despite its important implications for the evolutionary relationship between brood parasites and hosts. Here, we used information on previous egg-rejection experiments to explore the recognition abilities of both males and females of Eurasian blackbirds Turdus merula towards experimental eggs with a variable degree of mimicry. We found that both sexes recognized non-mimetic eggs, supporting the idea that visits to the nest can favor the evolution of rejection abilities. In contrast, only females recognized mimetic eggs, indicating that although recognition abilities can evolve in both sexes, they are subsequently refined in females probably due to their more frequent interaction with parasitic eggs. Clutch size affected nest attendance since females, but not males, spent more time at the nest and visited it more frequently in larger clutches. Finally, our recordings showed that blackbird males are able not only to recognize, but also to eject parasitic eggs. Our results provide new insights into the main anti-parasitic defense in birds, egg rejection, and highlight the need of considering the role of the non-incubating sex in egg-rejection studies. Given the high costs associated to avian brood parasitism, both sexes are expected to evolve anti-parasitic defenses. However, in those species in which only females incubate, females have traditionally been assumed to be the responsible for egg rejection. Here, using the Eurasian blackbird (Turdus merula), we investigated the existence of egg-rejection abilities in non-incubating males and compared them to those exhibited by females. We found that males can recognize non-mimetic eggs, although their recognition abilities were less fine-tuned compared to females, who also recognized mimetic eggs. Even though females were the responsible for most documented egg-ejection events, recordings confirmed that males could also be involved in egg ejection, which could have important implication for the evolution of anti-parasitic defenses in host populations.