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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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