Host–parasite dynamics often vary over time, brought about by changes in the parasite’s virulence or the host’s ability to resist or tolerate the parasite. Although virulence evolution in microparasites is well studied, we know little about temporal change in the pathogenicity of macroparasites such as blood‐feeding insects. Using data collected over 35 yr, we report a reduction in pathogenicity of the hematophagous swallow bug (Cimex vicarius) on its Cliff Swallow (Petrochelidon pyrrhonota) host. Relative to experimentally fumigated, parasite‐free nests, the negative effects of bugs on nestling swallow body mass and survival were less in the later years of the study than in the earlier years, and the negative relationship between nestling body mass and bug abundance became weaker over time. The survival of adult birds exposed to swallow bugs increased throughout the study, while survival of birds from parasite‐free nests decreased over time. Swallow bug abundance per nest, bug body size, and bug age ratios did not change during the study. Between‐colony transmission of bugs showed less immigration into larger colonies than in earlier studies, but there was no net change in transmission. Cliff Swallows did not reduce their exposure to bugs over time by being more likely to avoid infested nest or colony sites. Parents increased the number of food deliveries to their offspring over time in the presence of parasites, but the total amount of food delivered was unchanged. The reduced cost of swallow bug ectoparasitism does not seem related to changes in parasite narrow‐sense virulence, the host’s avoidance of parasites, the presence of alternative hosts for bugs, or climate‐driven phenological mismatches. The results probably reflect the Cliff Swallow’s evolving of greater tolerance to swallow bugs, brought about by the bird’s shift from natural cliff nesting sites to artificial structures that may harbor more bugs than natural cliffs. This study shows that hosts can respond relatively rapidly to high levels of parasitism, and provides support for models that suggest the evolution of tolerance should be expected in some host–parasite systems.

中文翻译：

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悬崖燕子的外生殖器炎的成本在过去35年中呈下降趋势

寄主－寄生虫的动态通常随时间变化，这是由于寄生虫的毒力或寄主抵抗或耐受该寄生虫的能力的变化所引起的。尽管对微寄生虫的毒力进化进行了深入研究，但我们对诸如吸血昆虫等大寄生虫的致病性的时间变化知之甚少。使用超过35年收集的数据，我们报告了其吞食燕子（Petrochelidon pyrrhonota）的食血燕子臭虫（Cimex vicarius）的致病性降低） 主持人。相对于经过实验熏蒸的无寄生虫巢，虫子对雏鸟吞咽体重和存活的负面影响要比前几年要小，并且雏鸟体重和虫子丰度之间的负面关系变得更弱随着时间的推移。在整个研究中，暴露于燕子的成年鸟类的存活率增加，而无寄生虫巢的鸟类的存活率随时间下降。在研究期间，每个巢的燕子虫丰度，虫体大小和虫龄比没有变化。与以前的研究相比，虫子在殖民地之间的传播显示出其向较大菌落的迁移较少，但传播没有净变化。悬崖燕子更可能避免出没巢穴或殖民地，因此不会随着时间的流逝减少虫子的暴露。在寄生虫的存在下，父母随着时间的流逝向后代提供的食物数量增加了，但交付的食物总量没有变化。吞咽的昆虫外生寄生虫的成本降低似乎与寄生虫狭义感毒力的变化，寄主避免寄生虫，存在虫子的替代寄主或气候驱动的物候失配无关。结果可能反映了悬崖燕子对燕子虫具有更大的耐受性，这是由于鸟类从天然悬崖筑巢地转移到可能比天然悬崖容纳更多虫子的人工结构所造成的。这项研究表明宿主可以对高水平的寄生虫作出相对迅速的反应，并为某些在某些宿主-寄生虫系统中应预期耐受性发展的模型提供了支持。