How do biparental species optimally provision young when begging is honest?

Offspring transmit signals to parents to communicate their resource demands. Parents interpret these signals and should adjust provisioning efforts to meet offspring demands but only to the point at which the benefits of enhanced offspring quality stops exceeding the increased costs to future reproduction. We investigated both proximate behavioral mechanisms in these interactions and ultimate-level decisions for total parental investment in streaked shearwaters Calonectris leucomelas by recording begging calls, monitoring parental attendance, and altering states of chicks by supplementing food. In our study, chicks seemed to honestly communicate satiety and body condition via begging. The parents, however, did not downwardly adjust feeding rates, meal sizes delivered to chicks, and total investment in nests in which chicks were regularly supplementary-fed partial meals. But on nights when both parents visited the nest, the second-arriving parents recognized that chicks had already received a full meal because they reduced the food they gave to chicks and also lengthened their subsequent foraging trip. Our findings therefore suggest that although chick begging appeared to reflect need, parents only responded to variation in begging that indicated that chicks had already received a full meal. In a simulation, we show that this strategy prevents parents from exceeding the optimal amount of parental investment. Their insensitivity to slightly reduced begging after partial meals caused them to exceed optimal investment in supplementary-fed nests, suggesting that parental investment is largely regulated by responses to feeding rate of the other parent rather than being fine-tuned to cues about body condition of chicks.