A variety of family conflicts can influence provisioning behaviour at the passerine nest. There can be sexual and parent-offspring conflict over the amount of food provided for young, and sibling conflict over how food is allocated among the brood. Whatever the type of conflict, its resolution may be determined by nestling begging displays, and its intensity will vary between species with variation in % EPY. Begging intensity is therefore predicted to vary with % EPY. One component of the begging display, that varies widely between species, is nestling mouth colour. Recent empirical work on canaries and great tits has shown that parents prefer to feed young with redder mouths, even if offspring naturally possess yellow gapes. I use comparative analyses to explain the diversity of nestling mouth colour between species in terms of the various family conflicts. In species where there are high rates of % EPY, and sibling and sexual conflicts are more intense, offspring that are reared in well-lit nests display redder mouths. Offspring reared in dark nests, however, show no such relationship and have yellower mouths generally. A comparison of Cuculinae species and host nestling mouth colour showed that cuckoo young have the redder mouths, which might be the result of more intense parent-offspring conflict. I suggest that nestling mouth colour reflects the intensity of family battles waged in the past, but only at nests where there is sufficient light for such visual displays to be perceived by parents. The diversity of nestling mouth colour can therefore be explained by both 'strategic' and 'tactical' influences on signal design. I conclude by discussing how variation in the choice of nest site within species might cause family differences in conflict resolution.