Data from: Joint care can outweigh costs of nonkin competition in communal breeders

Competition between offspring can greatly influence offspring fitness and parental investment decisions, especially in communal breeders where unrelated competitors have less incentive to concede resources. Given the potential for escalated conflict, it remains unclear what mechanisms facilitate the evolution of communal breeding among unrelated females. Resolving this question requires simultaneous consideration of offspring in noncommunal and communal nurseries, but such comparisons are missing. In the Seychelles warbler Acrocephalus sechellensis, we compare nestling pairs from communal nests (2 mothers) and noncommunal nests (1 mother) with singleton nestlings. Our results indicate that increased provisioning rate can act as a mechanism to mitigate the costs of offspring rivalry among nonkin. Increased provisioning in communal broods, as a consequence of having 2 female parents, mitigates any elevated costs of offspring rivalry among nonkin: per-capita provisioning and survival was equal in communal broods and singletons, but lower in noncommunal broods. Individual offspring costs were also more divergent in noncommunal broods, likely because resource limitation exacerbates differences in competitive ability between nestlings. It is typically assumed that offspring rivalry among nonkin will be more costly because offspring are not driven by kin selection to concede resources to their competitors. Our findings are correlational and require further corroboration, but may help explain the evolutionary maintenance of communal breeding by providing a mechanism by which communal breeders can avoid these costs.子代间的竞争可极大影响子代适合度与亲本投资决策，在非亲缘竞争者缺乏资源让步动力的集体繁育类群中尤为如此。鉴于冲突存在升级的可能，目前学界仍未明确非亲缘雌性间集体繁育的演化机制。解答该问题需要同时对比非公共育幼区与公共育幼区的子代，但此类对比研究目前仍属空白。针对塞舌尔莺（Acrocephalus sechellensis），本研究对比了公共巢（由2只雌鸟参与育雏）与非公共巢（仅1只雌鸟育雏）的雏鸟对，以及单只雏鸟的相关指标。研究结果表明，提升育雏投喂率可作为缓解非亲缘子代间竞争成本的机制：由于拥有2只雌性亲本，公共育雏群的投喂率有所提升，抵消了非亲缘子代间竞争所带来的额外成本——公共育雏群与单只雏鸟的单位个体投喂量与存活率均无显著差异，而非公共育雏群的上述两项指标均更低。非公共育雏群内的子代个体间适合度差异也更为显著，这可能是因为资源限制加剧了雏鸟间竞争能力的差距。学界通常认为，非亲缘子代间的竞争成本更高，因为亲缘选择不会驱动子代向竞争者让步资源。本研究结果属于相关性分析，尚需进一步验证，但该发现通过提出集体繁育类群可规避此类成本的演化机制，或有助于解释集体繁育的进化维持机制。