Abstract
When two or more individuals cooperate to provision a shared brood, each carer may be able to maximize their payoffs by coordinating provisioning in relation to what others are doing. This investment "game" is not simply a matter of how much to invest but also of the relative timing of investment. Recent studies propose that temporal coordination of care in the forms of alternation (i.e., turn-taking) and synchrony (i.e., provisioning together) function to mitigate conflict between carers and reduce brood predation risk, respectively. Such coordination is widespread in biparental and cooperatively breeding birds, yet the fitness consequences have rarely been empirically tested. Here, we use a long-term study of long-tailed tits Aegithalos caudatus, a facultative cooperatively breeding bird with active coordination of care, to assess the support for these hypothesized functions for coordination of provisioning visits. First, we found evidence that turn-taking mitigates conflict between carers because, in cooperative groups, provisioning rates and offspring recruitment increased with the level of active alternation exhibited by carers and with the associated increase in provisioning rate parity between carers. In contrast, offspring recruitment did not increase with alternation in biparental nests, although it was positively correlated with parity of provisioning between carers, which is predicted to result from conflict mitigation. Second, synchronous nest visits were associated with a reduced probability of nest predation and thus increased brood survival, especially when provisioning rates were high. We attribute this effect to synchrony reducing carer activity near the nest. We conclude that temporal coordination of provisioning visits in the forms of alternation and synchrony both confer fitness benefits on carers and despite being intrinsically linked, these different kinds of coordination appear to serve different functions.