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. When two or more individuals cooperate to provision a shared brood, each carer may be able to maximize their payoffs by coordinating their provisioning visits with others. Such coordination is widespread, yet its fitness consequences have rarely been explored. Here, we use a long-term study of long-tailed tits Aegithalos caudatus , a cooperatively breeding bird with coordinated care to test hypotheses for two measures of coordination: turn-taking (alternation) and synchrony of provisioning visits. First, we find support for the hypothesis that turn-taking mitigates conflict between carers because, in cooperative groups, provisioning rates and offspring recruitment increased with the level of alternation. Second, our results support the hypothesis that synchronous nest visits reduce nest conspicuousness and the risk of nest predation, especially when provisioning rates are high. We conclude that alternation and synchrony both confer fitness benefits on carers and despite being intrinsically linked, they appear to serve independent functions. Natural selection dictates that individuals should seek to maximize their genetic contribution to future generations. To achieve this goal, many animals provide parental care, which increases the survival and fitness of their offspring. However, providing care is costly so when multiple individuals care for the same young, as is commonplace in birds, each individual should prefer to contribute less and for other carers to invest more. Offspring will receive suboptimal care if carers cannot resolve this conflict. Taking turns to provision a brood (alternation) is hypothesized to mitigate this conflict, allowing each carer to contribute similar amounts so that the optimal level of care for offspring is achieved. Similarly, carers may also optimize their care by provisioning at the same time (synchrony), which is hypothesized to reduce the period of activity near nests and, hence, decrease the risk of advertising the location of a nest to predators. Both forms of carer coordination are widely reported, but their functions have received little attention. Here, we tested these hypotheses by investigating the relationships between coordination and offspring fitness in long-tailed tits, finding that alternation was associated with increased provisioning and offspring recruitment, while greater synchrony of visits corresponded with increased brood survival.
