Data from: Spatiotemporal variation of host use in a brood parasite: the role of the environment

Avian brood parasites should target the most profitable host species, but current conditions might locally influence their choice, producing geographic mosaics of coevolution. Throughout Europe, the magpie Pica pica has been invariably reported as the primary host of the great spotted cuckoo Clamator glandarius, whereas the carrion crow Corvus corone is the secondary one. However, we found that this pattern reversed in northern Spain, where up to 70% of carrion crow nests were parasitized versus 20% of magpie nests. In southern Spain, conversely, parasitism increased proportionally in both hosts (up to approximately 90% of available nests) throughout the 3 years of study. Surprisingly, magpies provided the best reproductive output for cuckoos in both areas, in contrast with cuckoo’s preference for the crow host in the north. Genetic data ruled out the presence of different host-specific races in this brood parasite, dismissing the hypothesis that a prevalence of different gentes at the 2 sites explained the observed variability in host choice. Instead, we found that magpie nests in the south were easier to reach and more scattered than in the north, where cuckoos preferentially targeted nests that were less concealed and more isolated. We suggest that the habitat constraints parasitism on magpies in the north, driving cuckoo host choice toward the crows. The coevolutionary scenario therefore includes a 3-way interaction, where the pressure that the parasite puts on a host species in a given place critically depends on the environmentally mediated interaction between the same parasite and a different host.鸟类巢寄生者（Avian brood parasites）理应选择收益最高的宿主物种，但当前的局部环境条件可能会影响其宿主选择，进而形成协同进化的地理镶嵌格局。在欧洲全域范围内，学界一贯将喜鹊（Pica pica）列为大斑凤头鹃（Clamator glandarius）的主要宿主，而小嘴乌鸦（Corvus corone）则为其次要宿主。但本研究发现，这一模式在西班牙北部出现逆转：该区域小嘴乌鸦的巢寄生率高达70%，而喜鹊的巢寄生率仅为20%。与之相反，在西班牙南部的3年研究周期中，两种宿主的巢寄生率均呈比例增长，最终可达可用巢总数的90%左右。令人意外的是，在两个研究区域中，喜鹊均能为大斑凤头鹃提供最优的繁殖产出，这与北部凤头鹃偏好小嘴乌鸦宿主的现象形成鲜明反差。遗传数据分析排除了该巢寄生者存在不同宿主专属宗的可能性，推翻了“两地分布有不同宿主宗类群是宿主选择差异成因”的假说。进一步研究发现，相较于北部，西班牙南部的喜鹊巢更易被触及且分布更为分散；而在北部区域，凤头鹃会优先选择隐蔽性更低、位置更孤立的巢。我们推测，生境条件约束了北部凤头鹃对喜鹊巢的寄生行为，进而驱使凤头鹃将宿主选择转向小嘴乌鸦。因此，该协同进化场景包含三方互作关系：在特定区域内，寄生者对某一宿主物种施加的选择压力，关键取决于该寄生者与另一宿主之间由环境介导的互作关系。