The advantages and disadvantages of coloniality in swallows, Hirundo rustica, were studied in 1971–1975 and 1977–1985. Nest sites were not limiting the local population density, and swallows tended to aggregate actively at specific sites. Colony size was positively correlated with local food abundance and the amount of lee for prevailing westerly winds. The amount of food per individual swallow decreased with colony size, and nestling feeding rates were lower for colonial than for solitary swallows. Social foraging effects, measured as individual feeding success in relation to the number of simultaneously foraging conspecifics, were not important. Extra-pair copulations and chases of female swallows by male nonmates of the colony were common in colonies. The percentage of time males spent mate guarding, the frequency of intraspecific nest parasitism and the percentage of time spent nest guarding during egg laying increased with colony size. Infanticide by unmated males occurred more frequently in larger colonies, being an important mortality factor. Mite infection causing nestling mortality and change of nest sites between first and second clutches increased with colony size. Nest predation averaged only 1·2%. A stuffed owl predator was detected more quickly in larger colonies. Predation on fledglings and adult swallows may have been more important in large colonies. Old mated and young unmated males benefited from breeding in colonies. Young mated males and females may not benefit from colonial breeding. The asynchronous arrival pattern may leave some early-arriving swallows in a deteriorating breeding condition because the local food resource has to be divided between more individuals. All males should stay in their colony because a shortage of females means that these males may not obtain a mate at all, if they move to a new colony. Many of the females may likewise be making the best of a bad job when breeding in a colony.