Colonial Breeding in the Barn Swallow (Hirundo rustica) and Its Adaptive Significance

Abstract

Four factors could account for the presence of coloniality in the barn swallow: predation, starvation, social stimulation and nest-site availability. Neither the number of pairs that collected in a colony nor the degree of within-colony synchrony affected reproductive success by consistently increasing or decreasing losses due to predation or food shortage. Colony size also did not influence the degree of synchrony, occurrence of 2nd broods or replacement of destroyed clutches, as might be expected if social stimulation were important. However, the number of pairs in a colony rose in parallel with the increase in the size of the building and/or the number of entrances to the building. Therefore, the implication is that barn swallow colonies represent passive aggregations of breeding birds and do not actively recruit additional pairs. The lack of close synchrony and the absence of colony-size effects on reproductive success indicate that coloniality in the barn swallow has not reached the level of breeding density where pressures other than nest-site availability begin to exert an influence on the development of social breeding patterns.