The cost of ectoparasitism in Cliff Swallows declines over 35 years

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