Enhanced Foraging Efficiency Through Information Centers: A Benefit of Coloniality in Cliff Swallows

Abstract

Cliff Swallows (Passeriformes: Hirundo pyrrhonota) in southwestern Nebraska, USA, nest in colonies that serve as information centers in which unsuccessful individuals locate and then follow successful individuals to aerial insect food resources. I investigated to what degree colony size affected the efficiencies at which Cliff Swallows foraged for food, and to what degree information transfer among colony residents might represent a benefit of living in colonies. The number of Cliff Swallows departing from a colony each hour to forage increased significantly with colony size, meaning that individuals did not wait as long to locate appropriate foraging associates in large colonies as in small colonies. Waiting intervals between an individual's arrival at its nest and its departure on its next foraging trip increased as the number of birds nesting in a colony declined. Parental Cliff Swallows in large colonies returned with food for their nestlings more often and brought more food per trip, than did parental Cliff Swallows in small colonies. As a result, nestling body mass at 10 d of age increased significantly with colony size, when the confounding negative effects of ectoparasites on nestling body mass were removed by fumigating nests. Adult Cliff Swallow body mass late in the nesting season during the period of feeding of nestlings increased significantly with colony size even in the presence of blood—sucking ectoparasites (in nonfumigated colonies). Experimental reduction of large Cliff Swallow colonies to the approximate size of small colonies suggested that nestling and adult body mass in these colonies did not vary with colony location, meaning that enhanced foraging efficiency of birds in larger colonies was probably attributable to more efficient transfer of information among the larger number of colony residents and was probably not attributable to differences in local resource characteristics near colonies of different sizes. Enhanced foraging efficiency through information centers is likely a major benefit of coloniality for Cliff Swallows.