The three "Delichon" species are members of the swallow family of birds, and are classed as members of the Hirundininae subfamily which comprises all swallows and martins except the very distinctive river martins. DNA studies suggest that there are three major groupings within the Hirundininae, broadly correlating with the type of nest built. The groups are the "core martins" including burrowing species like the sand martin, the "nest-adopters", which are birds like the tree swallow that utilise natural cavities, and the "mud nest builders". The "Delichon" species construct a closed mud nest and therefore belong to the latter group; they appear to be intermediate between the "Hirundo" and "Ptyonoprogne" species that make open cup nests, and the "Cecropis" and "Petrochelidon" swallows, which have retort-like closed nests with an entrance tunnel. The genetic evidence suggests a close relationship between "Hirundo" and "Delichon", which is further supported by the frequency of interbreeding between two widespread species, the barn swallow and the common house martin, despite being their being in different genera. The suggested taxonomic sequence of the mud-building swallows has been recommended by at least two European taxonomic committees.