Data from: Variable helper effects, ecological conditions, and the evolution of cooperative breeding in the acorn woodpecker

The ecological conditions leading to delayed dispersal and helping behavior are generally thought to follow one of two contrasting scenarios: that conditions are stable and predictable resulting in young being ecologically forced to remain as helpers (extrinsic constraints and the "habitat saturation" hypothesis), or that conditions are highly variable and unpredictable leading to the need for helpers to raise young, at least when conditions are poor (intrinsic constraints and the "hard life" hypothesis). We investigated how variability in ecological conditions influences the degree to which helpers augment breeder fitness in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), a species in which the acorn crop, territory quality, and prior breeding experience all vary in ways that have important effects on fitness. We found that the relationship between ecological conditions and the probability that birds would remain as helpers was variable, but that helpers generally yielded greater fitness benefits when ecological conditions were favorable, rather than unfavorable, for breeding. These results affirm the importance of extrinsic constraints to delayed dispersal and cooperative breeding in this species, despite this species' dependence on a highly variable and unpredictable acorn crop. Our findings also confirm that helpers can have very different fitness effects depending on conditions, but that those effects are not necessarily greater when breeding conditions are unfavorable.

学界普遍认为，引发延迟扩散（delayed dispersal）与协助抚育行为的生态环境，大致可归为两种对立的情境：其一为环境稳定且可预测，导致幼鸟因生态压力被迫留巢协助亲代，即外在限制（extrinsic constraints）与「栖息地饱和假说（habitat saturation hypothesis）」；其二为环境高度多变且不可预测，使得群体需要协助者抚育幼鸟，尤其在环境条件恶劣时，即内在限制（intrinsic constraints）与「艰难生活假说（hard life hypothesis）」。本研究以合作抚育物种橡树啄木鸟（Melanerpes formicivorus）为研究对象，探讨生态环境的变异性如何影响协助者对繁殖亲代适合度（breeder fitness）的提升程度；该物种的橡子收成、领地质量与过往繁殖经验，均存在对适合度（fitness）具有显著影响的变异模式。研究结果显示，生态环境与鸟类留巢协助的概率之间的关联存在变异性，但总体而言，当繁殖所需的生态环境较为适宜时，协助者能为亲代带来更高的适合度收益，而非环境恶劣时。尽管该物种高度依赖变异性强且不可预测的橡子收成，但本研究结果证实了外在限制对于该物种延迟扩散与合作抚育行为的重要性。本研究结果同时证实，协助者的适合度效应会随环境条件产生显著差异，但这类效应并非必然在繁殖环境恶劣时更强。