Conspecific brood parasitism and anti-parasite strategies in relation to breeding density in female bearded tits

Abstract

Within the animal kingdom conspecific brood parasitism (CBP) is particularly frequent in birds and parasitic individuals mainly save parental costs. Host counterstrategies and their efficiency may be influenced by environmental or socio-ecological factors like breeding density. Breeding density in particular may influence the opportunities for parasitism and the number of parasites. Here we examine the rate and efficiency of parasitism in relation to breeding density in socially monogamous bearded tits Panurus biarmicus. We differentiate between CBP attempts and realized CBP which means genetic representation of parasitic chicks in a brood and relate these to breeding density. Furthermore we investigate anti-parasite strategies of female bearded tits including egg ejection, nest abandonment and manipulation of egg position. Almost 40% of the females suffer from conspecific brood parasitism but there is significant increase with breeding density. Finally, only half of all egg dumping attempts have been successful, which implies high costs for parasites as well. Hence CBP is a 'make the best of a bad job' strategy rather than a strategy to increase reproductive success. Females desert their nests especially when clutch size is larger than normal. Egg ejection occurs less frequently and host females do not move parasitic eggs to less favourable incubation positions.