In diverse animal taxa, offspring produce signals that serve the function of soliciting parents to provision care. In birds, such ‘begging’ signals may consist of vocal and visual displays that influence parental decisions on allocation of critical food resources among the progeny. According to sex allocation theory, parents may gain fitness benefits from favouring offspring of either sex, depending on ecological conditions or parental quality. However, adaptive allocation of care can occur only if parents can identify the sex of their offspring, which, however, appear to be sexually monomorphic in most bird species. Begging displays may be a vehicle of such sex-specific signals. However, only very few studies have investigated sex differences in begging calls and none has analysed sex differences in a main component of begging displays of passerine birds, that is, mouth coloration. In the present study, we show that male nestling barn swallows, Hirundo rustica, have more brightly coloured mouths than their female broodmates early in the nestling period. Sex differences in mouth coloration disappear later in the nestling period when, however, differences in begging calls develop, confirming the results of a previous study. Thus, begging displays carry sex-specific components whose nature (visual or acoustic) changes during offspring ontogeny and might mediate the differential access of male and female offspring to parental care observed in this species, particularly under adverse rearing conditions.