Data from: Great spotted cuckoo nestlings have no antipredatory effect on magpie or carrion crow host nests in southern Spain

Host defences against cuckoo parasitism and cuckoo trickeries to overcome them are a classic example of antagonistic coevolution. Recently it has been reported that this relationship may turn to be mutualistic in the case of the carrion crow (Corvus corone) and its brood parasite, the great spotted cuckoo (Clamator glandarius), given that experimentally and naturally parasitized nests were depredated at a lower rate than non-parasitized nests. This result was interpreted as a consequence of the antipredatory properties of a fetid cloacal secretion produced by cuckoo nestlings, which presumably deters predators from parasitized host nests. This potential defensive mechanism would therefore explain the detected higher fledgling success of parasitized nests during breeding seasons with high predation risk. Here, in a different study population, we explored the expected benefits in terms of reduced nest predation in naturally and experimentally parasitized nests of two different host species, carrion crows and magpies (Pica pica). During the incubation phase non-parasitized nests were depredated more frequently than parasitized nests. However, during the nestling phase, parasitized nests were not depredated at a lower rate than non-parasitized nests, neither in magpie nor in carrion crow nests, and experimental translocation of great spotted cuckoo hatchlings did not reveal causal effects between parasitism state and predation rate of host nests. Therefore, our results do not fit expectations and, thus, do not support the fascinating possibility that great spotted cuckoo nestlings could have an antipredatory effect for host nestlings, at least in our study area. We also discuss different possibilities that may conciliate these with previous results, but also several alternative explanations, including the lack of generalizability of the previously documented mutualistic association.宿主对杜鹃寄生的防御行为，以及杜鹃为突破宿主防御演化出的欺骗策略，是拮抗协同演化（antagonistic coevolution）的经典范例。近期有研究表明，在腐食鸦（Corvus corone）与其巢寄生者大斑凤头鹃（Clamator glandarius）的互作关系中，这种拮抗关系可能转向互利共生——该结论的依据是，实验性与自然被寄生的鸟巢被捕食率均低于未被寄生的鸟巢。此前研究将该现象归因于杜鹃雏鸟分泌的恶臭泄殖腔分泌物所具备的反捕食特性，推测该分泌物可使捕食者远离被寄生的宿主鸟巢。这一潜在防御机制也解释了在捕食压力较高的繁殖季，被寄生鸟巢的雏鸟出飞成功率更高的观测结果。本研究在另一独立研究种群中，针对腐食鸦和喜鹊（Pica pica）两个宿主物种的自然与实验被寄生鸟巢，探究了巢捕食率降低所带来的预期收益。结果显示，在孵化期，未被寄生的鸟巢被捕食频率高于被寄生鸟巢；但在育雏期，无论是喜鹊巢还是腐食鸦巢，被寄生鸟巢的捕食率均未低于未被寄生鸟巢。此外，对大斑凤头鹃雏鸟开展的实验性移植并未揭示寄生状态与宿主鸟巢捕食率之间存在因果关联。因此，本研究结果与预期不符，未支持‘大斑凤头鹃雏鸟可对宿主雏鸟产生反捕食保护作用’这一颇具吸引力的假说，至少在本研究的研究区域内是如此。我们还讨论了几种可调和本研究与既往研究结果的可能性，以及多种替代解释，其中包括此前记录的互利共生关联并不具备普适性。