Most animal groups vary extensively in size. Because individuals in certain sizes of groups often have higher apparent fitness than those in other groups, why wide group size variation persists in most populations remains unexplained. We used a 30-y markrecapture study of colonially breeding cliff swallows (Petrochelidon pyrrhonota) to show that the survival advantages of different colony sizes fluctuated among years. Colony size was under both stabilizing and directional selection in different years, and reversals in the sign of directional selection regularly occurred. Directional selection was predicted in part by drought conditions: birds in larger colonies tended to be favored in cooler and wetter years, and birds in smaller colonies in hotter and drier years. Oscillating selection on colony size likely reflected annual differences in food availability and the consequent importance of information transfer, and/or the level of ectoparasitism, with the net benefit of sociality varying under these different conditions. Averaged across years, there was no net directional change in selection on colony size. The wide range in cliff swallow group size is probably maintained by fluctuating survival selection and represents the first case, to our knowledge, in which fitness advantages of different group sizes regularly oscillate over time in a natural vertebrate population.cliff swallow | coloniality | group size | natural selection | social behavior M ost social animals exhibit wide variation in group size (1-3), with the smallest and largest groups often differing in size by several orders of magnitude. Natural group size variation has proven useful in studying both the benefits of sociality and some of the costs, such as the spread of disease among groups (4-6). Surprisingly, in almost all species, some group sizes occur that are clearly disadvantageous relative to others (7,8). Individuals in the less successful group sizes should be selected against (9), especially given known heritable preferences for group sizes (10-12), genetic differences among individuals in different-sized groups (13, 14), and behavioral specializations for particular group sizes (15, 16). What maintains size variation (and the associated variation in individual behavior) in the face of apparent group size-related fitness costs remains one of the most perplexing, but unresolved, problems in behavioral biology (17)(18)(19).One mechanism that can maintain long-term stasis in trait distributions such as group size is temporally fluctuating selection, in which selection alternately favors traits in one direction and then the opposite direction (20)(21)(22). Although perhaps common in nature (23), only a relatively few studies have demonstrated statistically significant reversals in selection direction (24,25), and whether fluctuating selection is a common evolutionary process remains unclear. In the case of group size, fluctuating selection could hypothetically generate a long-term stasis in group size distributions if the sign of directional...

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.

The swallow bug (Oeciacus vicarius) is the only known vector for Buggy Creek virus (BCRV), an alphavirus that circulates in cliff swallows (Petrochelidon pyrrhonota) and house sparrows (Passer domesticus) in North America. We discovered ants (Crematogaster lineolata and Formica spp.) preying on swallow bugs at cliff swallow colonies in western Nebraska, U.S.A. Ants reduced the numbers of visible bugs on active swallow nests by 74-90%, relative to nests in the same colony without ants. Ant predation on bugs had no effect on the reproductive success of cliff swallows inhabiting the nests where ants foraged. Ants represent an effective and presumably benign way of controlling swallow bugs at nests in some colonies. They may constitute an alternative to insecticide use at sites where ecologists wish to remove the effects of swallow bugs on cliff swallows or house sparrows. By reducing bug numbers, ant presence may also lessen BCRV transmission at the spatial foci (bird colony sites) where epizootics occur. The effect of ants on swallow bugs should be accounted for in studying variation among sites in vector abundance.

The swallow bug, Oeciacus vicarius Horvath, is a hematophagous ectoparasite of the cliff swallow, Petrochelidon pyrrhonota Vieillot, and is closely related to bed bugs (Cimex spp.). Evolution of insecticide resistance has been documented for bed bugs but not studied in Oeciacus. For periods of 17 and 32 yr, two cliff swallow colonies in western Nebraska were treated during the summer breeding season using the organophosphate insecticide Dibrom. Despite continual treatments, O. vicarius has been observed frequently within these colonies. We evaluated the efficacy of Dibrom 8 on O. vicarius during the 2016 season at two treated colonies and four that had never experienced treatment. Dibrom 8 was found to be effective in 100% of trials, with immobilization within minutes and death within 72 h, for individuals from all colonies. In control treatments (water), individuals collected from treated colonies exhibited greater survival than individuals from untreated colonies, and those from active colonies (bugs fed) had greater survival than those from inactive colonies (bugs unfed). A residual effect was observed in both lab and field trials: 100% mortality occurred in the lab after exposure to filter paper substrates treated both 5 and 10 d earlier, and in the field, nests treated once early in the season had O. vicarius counts 43 d later that were <1% of those from untreated nests within the same colony. We hypothesize that the lack of resistance results from the limited potential for resistance allele fixation due to outbreeding and frequent immigration of insecticide-naïve individuals.

No abstract