Intraspecific Brood Parasitism in Two Tit Parus species: Occurrence and Responses to Experimental Parasitism

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In many bird species females parasitize the parental investment of other pairs by laying eggs in their nests. Such intraspecific brood parasitism is considered rare among altricial birds, but this may result from methodological difficulties in detecting and quantifying this behaviour. In this study, we investigated first whether intraspecific brood parasitism occurs in Blue Tits Parus caeruleus and Great Tits P. major, using a combination of frequent nest inspections, DNA profiling and a survey of the literature. No evidence for parasitism was found. Next, we investigated whether the absence of brood parasitism could be explained by efficient anti-parasite strategies of the hosts. We experimentally parasitized 75 Blue Tit and 51 Great Tit nests before and during egg-laying. We found no evidence for rejection of the parasitic eggs. Thus the absence of brood parasitism is not a product of undetected parasitism. Although intense female-female aggression may deter females that try to lay eggs in other nests, we suggest that in both species intraspecific brood parasitism has not evolved because females do not benefit from laying eggs in the nests of others.