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 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 years, 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 ...