genus Hirundo is grouped within the ‘‘mud-nesting” swallows together
with the crag martins (Ptyonoprogne), house martins (Delichon),
cliff swallows (Petrochelidon) and red-rumped swallows
(Cecropis). The most taxonomically complete phylogenetic survey
(Sheldon et al., 2005) resolved many of the species relationships
within Hirundo, but some nodes remained weakly resolved and
two Hirundo species were not compared in that study.
Adult wing lengths overlap (Roselaar, 1988) and the degree of variation in the underparts
is considerable. Adults with unbroken black pectoral bands are most likely to be
nominate rustica, and, in Asia, birds with almost uniform chestnut underparts to be
tytleri but the latter intergrades with gutturalis (although not perhaps with the nominate
form in the north of its range).