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 co-evolution. Throughout Europe, the magpie <i>Pica pica</i> has been invariably reported as the primary host of the great spotted cuckoo <i>Clamator glandarius</i>, whereas the carrion crow <i>Corvus corone</i> is the secondary one. However, we found that this pattern reversed in northern Spain, where up to 70% of carrion crow nests were parasitized <i>versus </i>20% of magpie nests. In southern Spain, conversely, parasitism increased proportionally in both hosts (up to approximately 90% of available nests) throughout the three 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 <i>gentes </i>at the two 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 towards the crows. The co-evolutionary scenario therefore includes a three-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）本应选择收益最优的宿主物种，但局部环境条件可能会影响其宿主选择决策，进而形成协同进化的地理镶嵌格局。在欧洲全域范围内，喜鹊（magpie，*Pica pica*）始终被记录为大斑杜鹃（great spotted cuckoo，*Clamator glandarius*）的首要宿主，而小嘴乌鸦（carrion crow，*Corvus corone*）为其次要宿主。然而本研究发现，这一经典宿主偏好模式在西班牙北部发生反转：该区域内高达70%的小嘴乌鸦巢穴被寄生，而喜鹊巢穴的寄生率仅为20%。与之相反，西班牙南部在三年研究周期内，两种宿主的寄生率均呈比例攀升，最高可达可用巢穴总数的约90%。

令人意外的是，两个研究区域内喜鹊均能为杜鹃提供最佳的繁殖成效，这与北部地区杜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化种群的可能性，进而否定了"两地宿主宗（gentes）的占比差异可解释观测到的宿主选择变异"这一假说。进一步分析显示，西班牙南部的喜鹊巢穴相较于北部更易抵达且分布更为分散；而在北部区域，杜鹃会优先选择隐蔽性更低、更为孤立的巢穴。我们据此提出，北部的生境条件制约了杜鹃对喜鹊的寄生行为，进而驱使杜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：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种间互作。