Abstract Conspecific brood parasitism (CBP) has been reported in more than 100 altricial avian species, yet its frequency and underlying behavioural processes remain poorly understood. We studied CBP in the barn swallow ( Hirundo rustica ), a species in which CBP has been reported. We conducted parentage analysis on 1943 barn swallow broods involving 7827 offspring collected over 12 breeding seasons at five populations, where we found only six cases of mixed maternity (0.3% of broods). By identifying all individuals involved, we determined that only three cases could be reliably interpreted as true CBP. Other three cases resulted most likely from nest take-overs when ‘parasitic’ females laid eggs into their own nests from which they were expelled by other females. Based on a review of the literature on CBP in barn swallow and seven other Hirundinidae, where CBP has also been reported, we conclude that the only reliable evidence for CBP comes from cliff swallows ( Petrochelidon pyrrhonata ). Data on other species do not allow exclusion of non-CBP mechanisms that could generate mixed maternity, thus, failing to present sufficient evidence supporting the occurrence of CBP as an alternative reproductive strategy. Several studies relied on the appearance of two eggs in a nest within one day, however, based on the parentage analysis, we show that CBP only occurred in one out of 11 such cases in our barn swallow data. Our findings highlight that CBP is virtually absent in our barn swallow populations, and has been likely overestimated in other studies. Overall, our results challenge existing beliefs about the frequency of CBP in barn swallow and Hirundinidae in general and provide insights into the circumstances that lead to mixed maternity broods. We emphasize that frequent nest checks and rigorous genetic validation are essential in CBP research to rule out alternative causes of mixed maternity.
Jelínek et al. (2026) studied this question.