The cost of ectoparasitism in Cliff Swallows declines over 35 years

Abstract

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 parasitefree 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.

Social foraging and the associated benefits of group‐living in Cliff Swallows decrease over 40 years

“…Despite the observed decline in adult survival over time at fumigated colonies, the opposite pattern was observed at nonfumigated sites (Brown et al, 2021). The reason for the difference is unclear but may include the relaxation of parasite-driven selection on birds at colony sites that are perennially fumigated (Brown et al, 2021). Regardless of cause, when averaged across nonfumigated and fumigated sites, adult survival in general does not seem to be declining in the study area as a whole.…”

“…3. Despite the observed decline in adult survival over time at fumigated colonies, the opposite pattern was observed at nonfumigated sites (Brown et al, 2021). The reason for the difference is unclear but may include the relaxation of parasite-driven selection on birds at colony sites that are perennially fumigated (Brown et al, 2021).…”

“…For analyses involving specific nests, only those from the fumigated sections of split colonies were used in this study. The fumigation methodology is given in Brown (1986, 1996) and Brown et al (2021). Naled works primarily as a contact insecticide, although for semantic convenience we use the term "fumigation."…”

“…The years included in this study were generally the periods of 1983-1991 and 2015-2022 when comparable data were collected in the same ways; in most cases no observations were available in the intervening years (see Brown et al, 2021). A continuous-year data set…”

Birds of a Feather Flock Together: Extent of Long-Term Consistency of Colony-Size Choice in Cliff Swallows

“…This may be an artifact of longer-interval birds necessarily being captured for the last time in the later years of the study, when net changes tended to be more positive (Figure 9). Birds using sites that were never fumigated tended to shift to smaller colonies over their lifetimes, although this pattern weakened during the study (Figure 10), possibly because the cost of ectoparasitism to cliff swallows has waned over time under natural conditions (Brown et al, 2021). In contrast, cliff swallows always using fumigated sites shifted to larger colonies over their lifetimes (Figure 10), again consistent with the amelioration of the cost of parasitism at fumigated sites that removes constraints on occupying large colonies there each year.…”