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Abstract Animals that feed socially can sometimes better locate prey, often by transferring information about food that is patchy, dense, and temporally and spatially unpredictable. Information transfer is a potential benefit of living in breeding colonies where unsuccessful foragers can more readily locate successful ones and thereby improve feeding efficiency. Most studies on social foraging have been short term, and how long‐term environmental change affects both foraging strategies and the associated benefits of coloniality is generally unknown. In the colonial Cliff Swallow (Petrochelidon pyrrhonota), we examined how social foraging, information transfer, and feeding ecology changed over a 40‐year period in western Nebraska. Relative to the 1980s, Cliff Swallows in 2016–2022 were more likely to forage solitarily or in smaller groups, spent less time foraging, were more successful as solitaries, fed in more variable locations, and engaged less in information transfer at the colony site. The total mass of insects brought back to nestlings per parental visit declined over the study. The diversity of insect families captured increased over time, and some insect taxa dropped out of the diet, although the three most common insect families remained the same over the decades. Nestling Cliff Swallow body mass at 10 days of age and the number of nestlings surviving per nest declined more sharply with colony size in 2015–2022 than in 1984–1991 at sites where the confounding effects of ectoparasites were removed. Adult body mass during the provisioning of nestlings was lower in more recent years, but the change did not vary with colony size. The reason(s) for the reduction in social foraging and information transfer over time is unclear, but the consequence is that colonial nesting may no longer offer the same fitness advantages for Cliff Swallows as in the 1980s. The results illustrate the flexibility of foraging behavior and dynamic shifts in the potential selective pressures for group living. 

Explaining why animal groups vary in size is a fundamental problem in behavioral ecology. One hypothesis is that life-history differences among individuals lead to sorting of phenotypes into groups of different sizes where each individual does best. This hypothesis predicts that individuals should be relatively consistent in their use of particular group sizes across time. Little is known about whether animals’ choice of group size is repeatable across their lives, especially in long-lived species. We studied consistency in choice of breeding-colony size in colonially nesting cliff swallows ( Petrochelidon pyrrhonota ) in western Nebraska, United States, over a 32-year period, following 6,296 birds for at least four breeding seasons. Formal repeatability of size choice for the population was about 0.41. About 45% of individuals were relatively consistent in choice of colony size, while about 40% varied widely in the colony size they occupied. Birds using the smaller and larger colonies appeared more consistent in size use than birds occupying more intermediate sized colonies. Consistency in colony size was also influenced by whether a bird used the same physical colony site each year and whether the site had been fumigated to remove ectoparasites. The difference between the final and initial colony sizes for an individual, a measure of the net change in its colony size over its life, did not significantly depart from 0 for the dataset as a whole. However, different year-cohorts did show significant net change in colony size, both positive and negative, that may have reflected fluctuating selection on colony size among years based on climatic conditions. The results support phenotypic sorting as an explanation for group size variation, although cliff swallows also likely use past experience at a given site and the extent of ectoparasitism to select breeding colonies. 

Abstract The survival of insects that are dormant in winter may either increase or decrease as a consequence of elevated winter temperatures under climate change. Warming can be deleterious when metabolism of the overwintering life stages increases to the point that energy reserves are exhausted before postoverwintering reemergence. We examined experimentally how overwintering survival of swallow bugs (Hemiptera: Cimicidae: Cimex vicarius Horvath), an ectoparasite primarily of cliff swallows (Passeriformes: Hirundinidae: Petrochelidon pyrrhonota Vieillot), was affected by a 3°C rise in mean daily temperature for populations in Oklahoma, Nebraska, and North Dakota. Adult and nymphal swallow bugs exposed to elevated temperature had an average reduction of approximately 31% in overwintering survival (from July/August to April/May), relative to controls exposed to current region-specific ambient-like conditions. Adult males in both groups survived less well in Nebraska and North Dakota than adult males in Oklahoma, but there was no consistent latitudinal effect of the elevated heat treatment. Our results indicate that projected increases in mean temperature in the Great Plains by 2050 could result in fewer swallow bugs surviving the winter and thus a reduced population size upon the arrival of their primary host in the spring, potentially affecting cliff swallow reproductive success, site use, and breeding phenology. Global climate change may alter the dynamics of host–parasite systems by reducing overall parasite abundance.