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Swallows and Martins

by Mike Scully

This article first appeared in the April 2003 issue of the San Antonio Audubon News.

I saw my first Barn Swallows (Hirundo rustica) last week, and like most of us, felt a feeling of quiet joy at the return of this beautiful bird. Certainly I am not alone in this; most humans probably see this species at one season or another. The Barn Swallow breeds all across the Northern Hemisphere and winters over much of the Southern Hemisphere. In addition, a bewildering variety of closely related species are native to Africa (where the genus probably originated) and Australia and New Zealand both host the Welcome Swallow (H. neoxena), which appears to me to be a Barn Swallow in all but name.

Less common but also circumpolar in distribution is the Bank Swallow (Riparia riparia), which like the Barn Swallow is our sole representative of an Old World genus. Theory has it that the ancestors of our modern swallows originally excavated burrows much as the Bank Swallow and our own rough-winged swallows (Stelgidopteryx spp.) do today. Based upon DNA and behavioral studies, the members of the genus Riparia are regarded as being closely allied to our endemic swallow genera containing such familiar species as the Northern Rough-winged Swallow (S. serripennis), the Tree Swallow (Tachycineta bicolor) and the Purple Martin (Progne subis) .

Moving from burrows to the use of pre-existing tree holes as most of our New World swallows do would seem a simple behavioral modification. Interestingly, in the Old World, very few species of swallow nest in such cavities, perhaps because of competition with such Old World families as the Sturnidae (starlings). Indeed, those noxious imports the European Starling (Sturnus vulgaris) and the House Sparrow (Passer domesticus) have had a catastrophic effect on our native cavity-nesting swallows wherever their ranges have overlapped.

Like the swallow family itself, the practice of constructing adhesive nests out of mud appears to have originated in Africa (where the greatest diversity of both total species and mud nest-making swallows still exist). Only four species of mud nest-making swallows occur in the Americas, while more than forty species exist across the Old World. Within the family, the complexity of mud nests so constructed varies from simple open cups after the manner of the Barn Swallow, to fully enclosed cavities with tubular entrances like that of our Cliff Swallow (Petrochelidon pyrronota).

Swallows as a group have been subject to much taxonomical revision in recent years, most of those species constructing simple cup nests are now placed in the original genus Hirundo, while most of those constructing more complex enclosed nests have been placed in the genus Petrochelidon. Representatives of both genera occur here in San Antonio; the Barn Swallow in the genus Hirundo, and the Cliff Swallow and Cave Swallow (P. fulva) in the genus Petrochelidon.

One advantage of constructing enclosed nests appears to be that it allows those species which construct them to nest in dense colonies while minimizing competition and confusion between adjacent breeding pairs. One Cliff Swallow colony in Nebraska, consisting of over 3,000 attached nests on a single structure, was reportedly the highest concentration of breeding birds ever recorded. Close relatives of our Cliff Swallow are found across the Old World, including the similar South African Cliff Swallow (P. spilodera).

Within the genus Petrochelidon, the Cave Swallow is a notable exception to the rule, constructing a simple nest cup much like that of a Barn Swallow. Perhaps as a result, as the Cave Swallow has extended its range, hybrids have been recorded between it and the Barn Swallow from locations where the two species have nested in close proximity.



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