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 body size, and bug age
ratios did not change during the study.  Between-colony
transmission of bugs shifted toward greater immigration into smaller
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.