Family conflicts and the evolution of nestling mouth colour

A variety of family conflicts can influence provisioning behaviour at the p asserine nest. There can be sexual and parent-offspring conflict over the a mount of food provided for young, and sibling conflict over how food is all ocated among the brood. Whatever the type of conflict, its resolution may b e determined by nestling begging displays, and its intensity will vary betw een species with variation in % EPY. Begging intensity is therefore predict ed to vary with % EPY. One component of the bagging display, that varies wi dely between species, is nestling mouth colour. Recent empirical work on ca naries and great tits has shown that parents prefer to feed young with redd er mouths, even if offspring naturally possess yellow gapes. I use comparat ive analyses to explain the diversity of nestling mouth colour between spec ies in terms of the various family conflicts. In species where there are hi gh rates of % EPY, and sibling and sexual conflicts are more intense, offsp ring that are reared in well-lit nests display redder mouths. Offspring rea red in dark nests, however, show no such relationship and have yellower mou ths generally. A comparison of Cuculinae species and host nestling mouth co lour showed that cuckoo young have the redder mouths, which might be the re sult of more intense parent-offspring conflict. I suggest that nestling mou th colour reflects the intensity of family battles waged in the past, but o nly 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 desig n. I conclude by discussing how variation in the choice of nest site within species might cause family differences in conflict resolution.