Synchronous provisioning increases brood survival in cooperatively breeding pied babblers

Abstract

Summary1. Behavioural synchrony typically involves trade-offs. In the context of foraging, for example, synchrony may be suboptimal when individuals have different energy requirements but yield net benefits in terms of increased foraging success or decreased predation risk. 2. Behavioural synchrony may also be advantageous when individuals collaborate to achieve a common goal, such as raising young. For example, in several bird species, provisioners synchronize nest-feeding visits. However, despite the apparent prevalence of provisioning synchrony, it is not known whether it is adaptive or what its function might be. 3. Here, we propose a novel explanation for provisioning synchrony: it increases brood survival by decreasing the number of temporally separate nest visits and accordingly the chance that the nest will be detected by predators. Using cooperatively breeding pied babblers, we showed experimentally that provisioners synchronized nest visits by waiting for another provisioner before returning to the nest. Brood survival increased with provisioning synchrony. Provisioners were more likely to synchronize feeding visits for older nestlings as they were louder and possibly more conspicuous to predators. Finally, provisioners in large groups were more likely to wait for other provisioners and synchronized a higher proportion of all visits than those in smaller groups. Thus, provisioning synchrony may be one mechanism by which large groups increase brood survival in this species. 4. This study highlights a novel strategy that birds use to increase the survival of young and demonstrates the advantages of coordinated behaviour in social species.

The Adaptive Significance of Provisioning and Foraging Coordination between Breeding Partners

“…Pair synchrony, and therefore the equity in partners' work rate, increased rather than decreased with experimental brood size, which suggests that partners coped with additional offspring demand by increasing cooperation rather than intensifying sexual conflict. This pattern is also consistent with the hypotheses that nest visit synchrony is an adaptive response to counter offspring competition (Shen et al 2010), nest predation risk (Raihani et al 2010), or to improve the assessment of other individuals' care (Doutrelant and Covas 2007;McDonald et al 2008), as suggested for cooperative breeders. Accordingly, even though nest visit synchrony followed foraging synchrony, it also responded directly to the brood size manipulation, unlike the response to nestling age, which was mostly driven by synchrony while foraging.…”

“…Indeed, better partner coordination has long been suspected to contribute to the increase in reproductive success with time observed in many species with long-term pair bonds ("mate familiarity effect," reviewed in Black 1996). In addition, as emphasized more recently in several cooperatively breeding birds, caregivers synchronize their visits to the nest (i.e., visit the nest together rather than independently; Doutrelant and Covas 2007;McDonald et al 2008;Raihani et al 2010;Shen et al 2010), and this synchronization appears to improve nestling growth and survival (Raihani et al 2010;Shen et al 2010). Specifically, synchronizing provisioning visits may improve nestling growth by facilitating food partitioning among nest mates (Shen et al 2010), perhaps because more food is brought to the nest at once, or because partners can better assess and adjust to each other's food allocation to individual nestlings.…”

“…Specifically, synchronizing provisioning visits may improve nestling growth by facilitating food partitioning among nest mates (Shen et al 2010), perhaps because more food is brought to the nest at once, or because partners can better assess and adjust to each other's food allocation to individual nestlings. In addition, pooling nest visits should reduce the likelihood of the nest being detected by a predator (Raihani et al 2010) by reducing the number of bouts of parental activity and noisy chick begging at the nest (Martin et al 2000). These insightful studies were nonetheless correlational, so no causal effect could be established.…”

“…Yet carers could potentially decrease the cost of nestling provisioning by foraging together, when foraging in groups increases foraging efficiency (reviewed in Beauchamp 1998). Accordingly, most bird species where nest visit synchrony has been reported also forage socially in groups (Doutrelant and Covas 2007;McDonald et al 2008;Raihani et al 2010;Mariette and Griffith 2012a), which could result in them returning to the nest together. In addition, it is not known whether nest visit synchrony is flexible or whether it merely reflects partners' intrinsic ability to synchronize their activities.…”

Synchronised provisioning at the nest: parental coordination over care in a socially monogamous species

“…Although widely neglected by those studying socially monogamous species with biparental care, those studying cooperatively breeding birds have understandably devoted more effort to understanding the more cooperative elements of parental care. For example, in several cooperatively breeding avian species synchronized feeding visits by helpers-at-the-nest have been observed and discussed (Doutrelant & Covas, 2007; McDonald et al, 2008; Raihani et al, 2010; Nomano et al, 2013). A number of adaptive explanations for this coordination have been proposed, such as a reduction in activity around the nest to reduce exposure to predators (Raihani et al, 2010); signaling of investment to other group members (Doutrelant & Covas, 2007; McDonald et al, 2008; Nomano et al, 2013); to enhance the distribution of food amongst offspring (Shen et al, 2010); or improve information amongst parents (Johnstone & Hinde, 2006).…”