We studied the advantages and disadvantages of Bank Swallow (Riparia riparia) coloniality in 1972 and 1973 by examining 54 colonies, ranging in size from 2 to 451 active nests, near Ann Arbor, Michigan USA. Four disadvantages were investigated: (1) increased competition for nest burrows and nest building materials, (2) increased competition for mates and matings, (3) increased possibilities of misdirected parental care because of either brood parasitism or the mixing up of unrelated young, and (4) increased transmission of ectoparasites. Physical interference in reproductive functions and the possibility of intraspecific killing of offspring were also considered. The intensity of the various forms of competition increased with increasing colony size, though not always directly. Flea infestation also increased with increasing colony size. Intraspecific brood parasitism was not observed, and parent Bank Swallows began discriminating between their own and unrelated offspring at the time when the young first began to mix. Three hypotheses to explain the maintenance of coloniality were tested: (1) shortage of suitable nesting habitats, (2) advantages associated with social foraging, and (3) reduced predation on adults, young, or eggs. Shortage of suitable habitat could not be demonstrated. Parents did not appear to feed in groups, and survivorship of nestlings during cold weather and weight of nestlings at 10 days of age both suggested that competition for food increased with increasing colony size. Although there was no relationship between colony size and amount of nocturnal predation, adult birds mobbed diurnal predators. Using a stuffed weasel, we studied such mobbing responses. Our data suggest that diurnal predators at larger colonies are (1) detected more quickly, (2) mobbed by greater numbers of birds, and (3) subjected to more vocal commotion than are predators at smaller colonies. Further, we demonstrated that mobbing is at least sometimes effective in deterring avian predators. We suggest that reduced predation on eggs and young, resulting from both group defense and "selfish herd" effects, is an important advantage of Bank Swallow coloniality.