Conspecific brood parasitism and egg quality in blue tits Cyanfetes caeruleus

摘要

Laying eggs in nests of unrelated conspecific pairs to parasitize their parental care is a common phenomenon inbirds. In blue tits Cyanistes caeruleus such conspecific brood parasitism (CBP) has never been reported in theliterature. However, in a situation where breeding density was extremely high, we found six nests to beparasitized with eggs of conspecific females. Natural selection may favour elevated competitiveness of parasiteyoung, since the negative consequences of increased sibling competition are incurred on the unrelated hostparents and siblings, and therefore do not act as inclusive fitness costs for the parasites. Parasitizing females couldachieve such a competitive advantage for their offspring by laying larger eggs or eggs with higher concentrationsof testosterone in the yolk. We analyzed these parameters of the six parasitized nests, but did not find thatparasite eggs differ systematically in these aspects from host eggs, nor that parasite eggs showed resemblance tohost eggs. We suggest that a shortage of available nest sites caused some females to use CBP as a best-of-a bad jobstrategy, but that either the occurrence of CBP is too rare to lead to strong selection for egg adjustments or thatparasitizing females are unable to do so.