Research_data_growth_experiment.xlsx

Common cuckoo (<i>Cuculus canorus</i>) females rely on host species to raise their young and therefore should benefit from targeting high-quality individuals that maximise their fitness. Empirical evidence for individual host selection is, however, mixed with some studies suggesting random choice. Nevertheless, it is possible that the lack of consistent evidence for host selection may be because spatio-temporal variation in host availability has rarely been accounted for, or because the implications of host choice on fitness outcomes have not been tested experimentally. Here we combined long-term monitoring data with an experiment to examine whether cuckoo females parasitising Common redstarts (<i>Phoenicurus phoenicurus</i>) target individual hosts of higher quality to optimise their nestlings´ growth. We first explored the scope for cuckoos to choose, finding spatial and temporal variation in host nest availability and host quality (using completed clutch size as a proxy). Cuckoos may choose hosts at different spatial scales, so we next investigated whether parasitism varied with host quality (i) across the study area and (ii) among nests within putative breeding areas; however, we found no evidence that redstarts laying larger clutches were more likely to be parasitised. Finally, we conducted a cross-fostering experiment to disrupt the cuckoo’s choice of nest. Moving cuckoo eggs to non-parasitised nests and between parasitised nests had no effect on morphometric growth of nestlings (mass, tarsus and wing length) but those raised by foster parents differing in quality (i.e. smaller/larger clutch size) from the original nest tended to grow faster but smaller in asymptotic mass and tarsus length. Together these results suggest that the potential fitness benefits of choosing high quality hosts do not compensate possible costs of searching for hosts in this system, although using different proxies for individual quality could help resolve the complexities inherent to understanding host choice.

大杜鹃（Common cuckoo，<i>Cuculus canorus</i>）的雌性个体依赖宿主物种抚育幼雏，因此应倾向于选择高质量宿主以最大化自身适合度（fitness）。然而，关于个体宿主选择的实证证据莫衷一是，部分研究认为其宿主选择行为呈随机性。尽管如此，宿主选择缺乏一致性证据的原因可能在于：研究极少考虑宿主可获得性的时空变异，或未通过实验检验宿主选择对适合度结果的影响。
本研究结合长期监测数据与控制实验，探究寄生于普通红尾鸲（Common redstart，<i>Phoenicurus phoenicurus</i>）的大杜鹃雌性是否会选择更高质量的个体宿主，以优化其雏鸟的生长发育。我们首先分析了大杜鹃的宿主选择空间，发现宿主巢的可获得性与宿主质量（以完成的窝卵数（clutch size）作为替代指标）存在时空变异。由于大杜鹃可能在不同空间尺度上开展宿主选择，我们接下来分别在（i）整个研究区域内，以及（ii）潜在繁殖区域内的不同巢群之间，检验了寄生率是否随宿主质量变化；但结果并未发现产卵窝数更多的红尾鸲更易被寄生的证据。
最后，我们开展了交叉抚育实验（cross-fostering experiment）以干扰大杜鹃的巢选择：将大杜鹃卵转移至未被寄生的巢中，以及在已被寄生的巢之间进行转移，结果显示该操作对雏鸟的形态生长指标（morphometric growth，包括体重、跗跖长与翼长）无显著影响；但由与原巢窝卵数不同（即更小/更大窝卵数）的养父母抚育的雏鸟，其生长速度相对更快，但渐近体重（asymptotic mass）与跗跖长更小。
综合来看，本研究结果表明，在该研究系统中，选择高质量宿主所能带来的潜在适合度收益，无法抵消搜寻宿主所需付出的潜在成本；不过，采用不同的个体质量替代指标，或有助于厘清宿主选择研究中固有的复杂性。