Tracking variation in hosts' responses to parasitism across space in a metapopulation is critical to assess the current status of parasitism/anti-parasitism in a host-parasite system, which is also helpful to infer its coevolutionary history. The barn swallow Hirundo rustica is a tractable bird species to understand potential fine-tuned adaptations to avian brood parasitism across small geographic scales, both in the context of variation in its foreign-egg rejection pattern, and its widespread distribution in cuckoo-free urban areas and in cuckoo-present rural habitats, including reedbeds. Here we tested whether variation in co-occurrence between the host and its rare brood parasite, the common cuckoo Cuculus canorus (i.e., high level of co-occurrence in reed habitats, low in town habitats) at the metapopulation level predicts patterns of antiparasitic egg rejection behaviors in barn swallows in response to different types of model and natural eggs. Contrary to our predictions, higher parasite detectability in the reed habitat did not translate into higher parasitism rate in the reed habitat and, in turn, we also found similarly low egg rejection rates across both sampled habitat types. These patterns implied a lack of fine-tuned increase of egg rejection rate in the reed-breeding population of barn swallows as a response to the increased perceived cuckoo encounter rate, perhaps because higher potential parasitism threat did not transfer into greater actual parasitism rate. It remains to be assessed whether the lack of small-scale geographic variation in barn swallows' egg rejection rates persists because this species responds to selection by parasitism as a spatially contiguous evolutionary unit.

中文翻译：

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在普通杜鹃的局部丰度可变的谷仓燕子中缺乏微调的鸡蛋排斥调整

跟踪宿主对跨空间寄生反应的变异对于评估宿主 - 寄生虫系统中寄生/抗寄生的当前状态至关重要，这也有助于推断其共同进化历史。谷仓燕子 Hirundo rustica 是一种易驯服的鸟类物种，可以在小地理尺度上了解对鸟类寄生寄生的潜在微调适应，无论是在其异卵排斥模式的变化背景下，还是在无杜鹃的城市地区广泛分布的背景下以及存在杜鹃的农村栖息地，包括芦苇丛。在这里，我们测试了寄主与其罕见的寄生寄生物——普通杜鹃 Cuculus canorus（即，在芦苇栖息地中的高度共生，低在城镇栖息地）在复合种群水平预测谷仓燕子对不同类型的模型和天然鸡蛋的抗寄生虫卵排斥行为模式。与我们的预测相反，芦苇栖息地中较高的寄生虫可检测性并没有转化为芦苇栖息地中更高的寄生率，反过来，我们还发现两种采样栖息地类型的卵排斥率同样低。这些模式表明，作为对感知杜鹃遭遇率增加的反应，在芦苇繁殖的家燕种群中，鸡蛋排斥率缺乏微调增加，这可能是因为更高的潜在寄生威胁并没有转化为更高的实际寄生率。谷仓燕子是否缺乏小规模的地理变异仍有待评估