The "Ptyonoprogne" 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 sequence studies suggest that there are three major groupings within the Hirundininae, broadly correlating with the type of nest built. These 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 "Ptyonoprogne" species construct open mud nests and therefore belong to the last group. "Hirundo" species also build open nests, "Delichon" house martins have a closed nest, and the "Cecropis" and "Petrochelidon" swallows have retort-like closed nests with an entrance tunnel.