The tails of juveniles average slightly shorter than older birds. Progne subis subis - is the familiar martin in the eastern half of the continent. It is midway in size between the other two subspecies. Progne subis hesperia - is the martin of the desert southwest. This is the smallest of the three subspecies and nests in cavities of the Saguaro cacti.
Females are much paler than subis on the forehead and underparts.
Progne subis arboricola - is the martin of the Rocky Mountains and Pacific Northwest. This is the largest of the three subspecies and females are also lighter on the forehead and underparts than the nominate race subis. Personally, I feel that over time there may have been breeding between small pockets of P.s.arboricola from the southern Rocky mountain areas, and the desert race of P.s.hesperia in areas where desert and montane habitats occur within the same geographic area, thus producing offspring which are the same size as P.s.subis and yet are not truly of that race. Most martin authorities today recognize these martins as belonging to P.s.subis . Martins also relish winged termites and seek swarms of them in the Spring.