Negotiation between parents over care: reversible compensation during incubation

Parental care is often beneficial for the young but costly for the caregiving parent. Because both parents benefit from care via the offspring, whereas they pay the costs individually, a conflict is expected about how much care each parent should provide. How do parents settle this conflict? We addressed this question by reducing nest temperatures during incubation in the Kentish plover Charadrius alexandrinus, a small ground-nesting shorebird in which the parents share incubation. By cooling the clutch using a remote-controlled device built under the nest, we experimentally increased the workload of either the male or the female in random order and recorded the behavioral responses of the targeted parent and its mate. Unlike most previous manipulations of parental effort, our manipulation sought to measure a parent's response to an increase, not a shortfall, in the partner's contribution. The manipulation was also short term and reversed between the members of a given pair. We found that there is a trade-off between the efforts of parents because increased (or reduced) effort by the targeted parent was associated with decreased (or increased) effort by its mate, respectively. This result is consistent with theoretical models that predict compensation as a response to changed parental effort of the mate. We also found that compensation was consistent between treatments when the male or the female of a given pair was targeted. Furthermore, our results support the notion that parents adjust their effort in response to their mate's behavior in real time, that is, they negotiate parental roles.