Common cuckoos (Cuculus canorus) affect the bacterial diversity of the eggshells of their great reed warbler (Acrocephalus arundinaceus) hosts

The common cuckoo (Cuculus canorus) is an avian brood parasite, laying its eggs in the nests of other bird species, where these hosts incubate the parasitic eggs, feed and rear the nestlings. The appearance of a cuckoo egg in a host nest may change the bacterial community in the nest. This may have consequences on the hatchability of host eggs, even when hosts reject the parasitic egg, typically within six days after parasitism. The present study revealed the bacterial community of cuckoo eggshells and those of the great reed warbler (Acrocephalus arundinaceus), one of the main hosts of cuckoos. We compared host eggs from non-parasitized clutches, as well as host and cuckoo eggs from parasitized clutches. As incubation may change bacterial assemblages on eggshells, we compared these egg types in two stages: the egg-laying stage, when incubation has not been started, and the mid-incubation stage (ca. on days 5–7 in incubation), where heat from the incubating female dries eggshells. Our results obtained by the 16S rRNA gene sequencing technique showed that fresh host and cuckoo eggs had partially different bacterial communities, but they became more similar during incubation in parasitized nests. Cluster analysis revealed that fresh cuckoo eggs and incubated host eggs in unparasitized nests (where no cuckoo effect could have happened) were the most dissimilar from the other groups of eggs. Cuckoo eggs did not reduce the hatchability of great reed warbler eggs. Our results on the cuckoo-great reed warbler relationship supported the idea that brood parasites may change bacterial microbiota in the host nest. Further studies should reveal how bacterial communities of cuckoo eggshells may vary by host-specific races (gentes) of cuckoos.

普通杜鹃（Cuculus canorus）属于鸟类巢寄生者，会将卵产于其他鸟类的巢窝中，由宿主代为孵化、喂食并抚育其雏鸟。宿主巢窝内出现杜鹃卵，可能会改变巢内的细菌群落，即便宿主通常会在寄生发生后的6天内将寄生卵移除，这仍可能对宿主卵的孵化率产生影响。本研究分析了杜鹃卵壳以及其主要宿主之一的大苇莺（Acrocephalus arundinaceus）的卵壳细菌群落，我们分别对比了未被寄生卵窝中的宿主卵，以及被寄生卵窝中的宿主卵与杜鹃卵。考虑到孵化过程会改变卵壳表面的细菌群落结构，我们在两个阶段对各类卵样进行了对比：一是产卵阶段（尚未启动孵化），二是孵化中期（约孵化第5至7日，此时孵化亲鸟的体温会使卵壳逐渐干燥）。本研究通过16S rRNA基因测序技术（16S rRNA gene sequencing）获得的结果显示，新鲜宿主卵与杜鹃卵的细菌群落存在部分差异，但在被寄生巢窝中经过孵化后，二者的群落组成逐渐趋于相似。聚类分析结果表明，未被寄生巢窝中的新鲜杜鹃卵与孵化后的宿主卵（未受杜鹃寄生影响的组别）与其他各组卵样的差异最为显著。实验结果还显示，杜鹃卵并未降低大苇莺卵的孵化率。本研究关于杜鹃-大苇莺寄生关系的实验结果，支持了“巢寄生者可能改变宿主巢内细菌微生物群”这一观点。未来的研究可进一步探讨，不同宿主专属宗群（gentes）的杜鹃，其卵壳细菌群落的变异规律。