Abstract. Colonially nesting Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwestern Nebraska, USA, are commonly parasitized by hematophagous swallow bugs (Hemiptera: Cimicidae: Oeciacus vicarius) and fleas (Siphonaptera: Ceratophyllidae: Ceratophyllus celsus). We examined to what degree these ectoparasites represent a cost of coloniality for Cliff Swallows. The number of swallow bugs per nest increased significantly with Cliff Swallow colony size. Body mass of nestling swallows at 10 d of age declined significantly as the number of bugs per nestling increased. By fumigating half of the nests in some colonies, killing the bugs, and leaving half of the nests as nonfumigated controls, we showed that swallow bugs lower nestling body mass and nestling survivorship in large Cliff Swallow colonies but not in small ones. Bugs cost nestlings, on average, up to 3.4 g in body mass, and reduced survivorship by up to 50%. Parasitism by fleas showed no consistent relationship with colony size during the nestling period but increased significantly with colony size early in the season, when birds were first arriving in the study area. Fleas did not affect nestling body mass or survivorship and thus, unlike swallow bugs, are probably not important costs of coloniality to Cliff Swallows. Field observations and nest fumigation experiments showed that Cliff Swallows apparently assess which nests are heavily infested with swallow bugs early each spring and select parasite-free nests, leading sometimes to alternate-year colony site usage. Cliff Swallows were more likely to construct new nests (rather than reusing old ones) in large colonies than in small colonies, probably in response to heavier infestations of ectoparasites in the existing nests of large colonies.

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Extreme climatic disturbances provide excellent opportunities to study natural selection in wild populations because they may cause measurable directional shifts in character traits. Insectivorous cliff swallows (Petrochelidon pyrrhonota) in the northern Great Plains must often endure periods of cold weather in late spring that reduce food availability, and if cold spells last four or more days, mortality due to starvation may result. We analyzed morphological shifts associated with viability selection, and how patterns of bilateral symmetry were affected by survival selection, during a four-day period of cold weather in 1992 and a six-day period in 1996 in southwestern Nebraska. Birds that died during the cold were compared to those still alive when the severe weather ended. The event in 1992 killed relatively few birds, but the cold spell in 1996 killed thousands of cliff swallows and reduced their population by about 53%. Climatological records suggest that mortality events comparable to that of 1996 have occurred in only one other year since 1875. Larger birds were favored in the 1996 event. Selection was more intense in 1996 than in 1992 because of more stressful conditions in 1996. Directional selection gradient analysis showed that measures of skeletal body size (tarsus length, culmen width and length) and wing length were targets of selection in 1996. Survivors had lower wing and outer tail asymmetry, and wing and tail asymmetry were targets of selection in both events. Mortality patterns did not differ by sex, but older birds suffered heavier mortality; morphological traits generally did not vary with age. Nonsurvivors were not in poorer apparent condition prior to the weather event than survivors, suggesting that selection acted directly on morphology independent of condition. Selection on body size in cliff swallows was more intense than in studies of body size evolution in other bird species. Larger swallows were probably favored in cold weather due to the thermal advantages of large size and the ability to store more fat. Swallows with low asymmetry were favored probably because low asymmetry in wing and tail made foraging more efficient and less costly, conferring survival advantages during cold weather. This population of cliff swallows may have undergone relatively recent body size evolution.

Sizes of most kinds of animal groups vary considerably within a population, with group size often causing direct effects on the fitness of group members. Although the consequences of varying group size have been well studied, the causes of variation in group size remain poorly known for most animals. Groups might vary in size because different individuals perform better in differently sized groups and thus have genetic predispositions to choose large or small groups. We examined whether heritable variation for choice of group size exists in the cliff swallow (Petrochelidon pyrrhonota), a colonial bird that nests in colonies ranging from 2 to 3,700 nests. Parent-offspring regressions showed significant heritabilities for choice of colony size under natural conditions. Partial cross-fostering experiments showed that individuals reared in colonies of sizes different from those of their birth returned to breed the next year in colonies that matched their birth colony in size and actively avoided those similar to their rearing colony, suggesting that choice of colony size is genetically based. Common environmental effects, maternal effects, and philopatry did not explain these results. Variation in group size probably results in part from a polymorphism in genetic preferences within the population, and the range in colony sizes is maintained by natural selection on the type of bird occupying each site.

Sociality is associated with increased risks of parasitism, predation, and social competition, which may interact because social stress can reduce immunity, and parasitized individuals are more likely to fall prey to a predator. A mechanism allowing evolution of sociality in spite of high costs of parasitism is increased investment in antiparasite defenses. Here we show that the impact of parasites on host reproductive success was positively associated with the degree of sociality in the bird family Hirundinidae. However, the cost of parasitism in highly colonial species was countered by high levels of T- and B-cell immune responses. Investment in immune function among colonial species was particularly strong in nestlings, and among social species, this investment was associated with a relatively prolonged period of development, thereby leading to extended exposure to parasites. Thus, highly social species such as certain species of swallows and martins may cope with strong natural selection arising from parasites by heavy investment in immune function at the cost of a long exposure to nest parasites.

Summary1. One way that animals may select breeding sites is by assessing the reproductive success of conspeci®cs in one season and settling the next year in those habitat patches where success collectively had been greatest. This sort of habitat assessment may promote the formation of colonies at high quality sites. 2. We examined whether cli swallows, Petrochelidon pyrrhonota, in south-western Nebraska used conspeci®c breeding performance to choose colony sites. 3. Reproductive success at colony sites varied spatially within seasons and between seasons, and was autocorrelated at a site from one year to the next, but not over longer time intervals. Cli swallows thus met the conditions for potential use of information on conspeci®c breeding performance. 4. Among sites re-used in consecutive years, those with highest collective success in one season showed the greatest rates in colony growth the next season, including the greatest in¯ux of immigrants. 5. The probability of colony-site re-use in successive years increased with collective reproductive success and average breeder body mass (a measure of individual condition) the previous season. 6. Cli swallows probably use conspeci®c breeding performance in selecting colonies. This mechanism is one component of habitat selection that also includes attraction to conspeci®cs and assessment of an individual's own success.

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Abstract. While epidemiological models have suggested that the spread of parasites and infectious diseases often depends critically on the extent of movement by infected individuals between populations, there is little empirical information for any organism on the frequency of between-group parasite transmission or how it varies spatially. The transmission of parasites between discrete social groups or populations may also help determine a host's total parasite or pathogen exposure. We measured the introduction of parasitic bugs (Oeciacus vicarius) into colonies of Cliff Swallows (Petrochelidon pyrrhonota) by transient birds from outside each group. The transmission of bugs increased strongly as the size of a nesting colony increased. More total transients visited the larger colonies, and the direction of change in transient numbers and the numbers of bugs introduced at a site from week to week tended to vary together. Transients at large colonies were more likely to have previously or subsequently visited other large, infested colonies. The greater likelihood of parasites being introduced into larger colonies by transient birds contributes to an increase in parasite load with increased colony size in Cliff Swallows.