In biparental birds, the relative contribution of the sexes to parental care can be viewed as a co-operative equilibrium that reflects the relative costs and benefits to each parent. If there are asymmetries in these costs or benefits, then any changes to the cost of care could result in a corresponding adjustment to their relative contribution. Incubation is a parental activity, shared in many species, which is costly both in terms of energy expenditure and time. In this study we manipulated the cost of incubation for pairs of European starlings (Sturnus vulgaris) by experimentally warming selected clutches to examine how this affected attendance by each parent. We found that total nest attendance did not differ between heated and control nests, although there was some evidence among heated nests that attendance declined with increasing effectiveness of the heater. Furthermore, relative male contribution was greater at heated than control nests resulting from the net effect of females tending to reduce, and males increase, attendance. We suggest that this shift in relative attendance may have been observed because females have a more developed brood patch and are more sensitive and responsive to clutch temperature than males. Consequently, females tended to reduce attendance at heated nests while males, with less reliable information on the clutch's thermal status, increased attendance to compensate for the reduction by the female. We also found that females at heated nests were lighter than at control nests, possibly because they were able to shed the additional fat reserves, a characteristic of incubating birds, earlier than females at control nests. We suggest that adjustment of clutch temperature in biparental species provides a valuable approach to investigating factors, including functional differences, asymmetries in brood value, and parental negotiation rules, that shape the roles of the sexes in incubation.