Egg patterning is not a reliable indicator of intraspecific brood parasitism in the blue tit Cyanistes caeruleus

A recent study reported the incidence of intraspecific brood parasitism (IBP) in the blue tit, based on differences in pigmentation between eggs within clutches and the appearance of multiple eggs within a nest in a single day. Here we present data from another population of blue tits Cyanistes caeruleus in which we show that such evidence is not necessarily indicative of IBP. As in many other studies of birds, we found that the patterning and degree of pigmentation on eggs is individually highly characteristic of a female. As in the previous study of blue tits, a minority of clutches (about 3% of 904 clutches) contained one, two or three eggs that were distinctly different from the other eggs in the clutch, either in size or patterning, but not both. Molecular genetic analyses of parentage in a sample of these clutches with suspected IBP demonstrated unequivocally that all eggs within each clutch had in fact been laid by a single female, in all cases the social parent. We also found no evidence of IBP from paternity analyses of a sample of 3,529 offspring from 427 broods in this population, adding to previous evidence of the absence of IBP in this species. These findings cast doubt on the utility of purely observational approaches to provide evidence of intraspecific brood parasitism; our study suggests that egg morphology and records of laying patterns are inadequate to enable the identification of IBP. © 2009 Journal of Avian Biology.