Review of the Natural Nesting of the Barn Swallow in North America

Abstract

A review of barn swallow (Hirundo rustica) nestings in natural sites reveals that few reported natural nest sites are presently active in North America. A shift from natural sites to human sites took place in early colonial days. The size distribution of nesting groups found today appears to be similar to that in precolonial days when natural sites were used exclusively. The number of potential nest sites has greatly increased with the proliferation of human structures, allowing the species to occupy previously unsuitable habitats and increase its range. The result has been an increase in both colony density and overall abundance of barn swallows. INTRODUCTION The barn swallow breeds widely in Asia, Europe, North Africa and North America (Mayr and Greenway, 1960); however, in North America at least, surprisingly few reports are of natural nesting sites. Barn swallows now construct nearly all nests on human structures such as houses, barns, sheds, tunnels, culverts, mine shafts and wells (pers. observ., unpublished reports and numerous published references). Little information exists, however, on the size of nesting groups in natural sites. Thus, it has been difficult to determine whether the sizes of nesting groups now commonly seen on human structures (cf, Bent, 1942; Snapp, 1976; Shields, 1984) are similar to the sizes of nesting groups when only natural sites were available, or whether nesting group sizes are now larger. Detailed information on the types of natural sites used in the past is also scarce. Little is known about the timing of the shift to human sites, whether both types of sites were used simultaneously, or whether natural nesting remains widespread. Data relevant to these aspects of the breeding biology of the barn swallow are presented here