Data from: Male and female helper effects on maternal investment and adult survival in red-winged fairy-wrens

Despite its importance for the evolution of cooperative breeding, it has proven difficult to determine whether helpers improve their recipients’ fitness. Helpers affect fitness in multiple ways, both positive and negative, but their effects can also be concealed through reduced maternal investment. Furthermore, determining the direction of causation is difficult, as helper presence may indicate a productive territory, rather than high productivity indicating an effect of help. In cooperatively breeding red-winged fairy-wrens (Malurus elegans) groups reduce care when they have male helpers, but groups with female helpers do not, so nestlings receive more food. Thus our predictions vary with helper sex rather than helper number, and by studying within-group changes with regard to group composition we separate phenotypically plastic responses from among-group correlations. Females did not reduce egg size in response to an increasing number of female helpers. However, more male or female helpers allowed females to lay larger clutches and more female helpers reduced re-nesting intervals. There was mixed support for a benefit of load lightening: Helpers, but not breeders, gained survival benefits with increasing number of male helpers. However, helper survival decreased with the number of female helpers, suggesting that increased competition counterbalanced these male helper benefits. We also found consistent among-group differences, which would have erroneously been interpreted as helper effects had we not disentangled the within-group changes with regard to group composition. This study highlights the importance of assessing carers’ benefits in relation to both group composition and size, and of investigating the within-individual plastic response of helper effects.

尽管合作繁殖（cooperative breeding）的演化离不开协助者（helpers）的作用，但学界始终难以证实协助者能否提升其照料对象的适合度（fitness）。协助者对适合度的影响兼具正负两面，且这些效应可能因母体投入（maternal investment）的减少而被掩盖。此外，确定因果关系方向颇具难度，因为协助者的存在或许只是领地生产力较高的体现，而非协助者本身提升了繁殖成效。
针对红翅细尾鹩莺（Malurus elegans）的合作繁殖研究显示，当繁殖群存在雄性协助者时，会减少育雏投入；而存在雌性协助者的繁殖群则无此现象，因此雏鸟能获得更多食物。据此，我们的预测随协助者的性别而非数量发生变化，通过分析与种群组成相关的组内变化，我们得以将表型可塑性响应（phenotypically plastic responses）与组间相关性区分开来。
雌性个体并不会因雌性协助者数量增加而缩减卵的大小。不过，更多的雄性或雌性协助者可让雌性提升窝卵数，且更多雌性协助者会缩短重新筑巢的间隔。关于减负（load lightening）收益的研究结果存在分歧：协助者（而非亲鸟）会随雄性协助者数量增加而获得更高的存活收益；但雌性协助者的存活率会随其数量增多而下降，这表明竞争加剧抵消了雄性协助者带来的收益。
我们还发现了稳定存在的组间差异——若未针对种群组成拆解组内变化，这些差异会被错误地归因于协助者的效应。本研究强调，评估协助者的收益时需同时考虑种群组成与种群规模，且需探究协助者效应的个体内可塑性响应。