Parent and offspring behaviours are expected to act as both the agents and targets of selection. This may generate parent-offspring coadaptation in which parent and offspring behaviours become genetically correlated in a way that increases inclusive fitness. Cross-fostering has been used to study parent-offspring coadaptation, with the prediction that offspring raised by non-relatives, or parents raising non-relatives, should suffer fitness costs. Using long-term data from more than 400 partially crossed broods of blue tits (Cyanistes...