The bright red gape of the nestling common cuckoo Cuculus canorus has often been supposed to act as a supernormal stimulus to elicit provisioning from its foster parents. Parents of three main host species were tested for their response to their own nestlings with artificially reddened gapes. Robins, dunnocks and reed warblers allocated no more food to red-mouthed nestlings than to control nestlings in the same nest, and manipulations of the gape colour of whole broods of reed warblers revealed no effect on provisioning rates. Our data do not support the hypothesis that there is a universal parental preference for redder gapes in opennesting passerines, or that the bright red gape of nestling cuckoos has evolved to exploit parental preferences in these three hosts. We suggest that although mouth colour has little influence on the allocation of feeds resulting from sibling competition and begging intensity in these species, it may have a role under certain conditions.