Favored parent-offspring trait combinations? On the interplay of parental and offspring traits

The expression of key traits like parental provisioning and offspring begging is confounded by the parent-offspring conflict in species with parental care, with offspring seeking greater parental investment than parents are willing to provide. Given the reciprocal interplay of these traits, selection has likely favored specific parent-offspring trait combinations, and it has been a longstanding question which party benefits most from this linkage. This will become apparent in a mismatch situation, which we here experimentally created by reciprocally cross-fostering blue tit broods. We hypothesized that offspring fledgling mass and their excreted corticosterone metabolite (CM) levels (reflecting stress) should vary with the rate of (foster) parental care (if provisioning is under full parental control), with offspring begging (if offspring is fully in control), or an interaction of both traits reflecting stable end points along a power continuum. We found a significant interaction effect, that is, highly demanding broods reached lowest fledgling mass when raised by low providing parents (due to partly unrewarded costly begging) but highest fledgling mass when raised by high providers. This pattern, however, was not reflected in offspring CM levels. Parental provisioning of the foster parents affected CM levels again in interaction with the offspring's begging level, but this pattern may rather reflect intensity and frequency of sibling competition. Taken together, our results suggest that an adjustment of the offspring's phenotype to the post-hatching social environment is primarily beneficial for highly demanding offspring and that parents have the upper hand (but probably not full control) over provisioning.