Costs associated with recognition and rejection of parasitic eggs in two European passerines

Many avian brood parasite hosts accept parasitic eggs even though successful parasitism frequently is detrimental to their own reproduction. Such behaviour seems suboptimal, but has been explained by the existence of opposing selective pressures operating against the evolution of appropriate host defence. Costs associated with rejection and recognition of eggs are central topics in this respect. Here, we report the occurrence of such costs in two European passerines (chaffinches and blackcaps) that are good rejecters of foreign eggs, even though the common cuckoo does not presently use them as hosts. Since high rejection rates are maintained in the absence of parasitism, we predicted that few recognition errors are made by these species. We tested this prediction by monitoring the occurrence of such errors in both experimentally parasitised and non-parasitised host clutches. We found support for the prediction, as our results show that recognition errors are at best rare events in these two species. We discuss the role of intraspecific brood parasitism as well as other explanations for the retention of a high rejection rate in these species. Various studies have reported mixed support for the occurrence of recognition errors among cuckoo hosts, and we consider other explanations for the existence of both acceptors and rejecters of foreign eggs in host populations.