the american cliff swallow (petrochelidon pyrrhonota) is a member of the passerine bird family hirundinidae  the swallows and martins .
the scientific name is derived from ancient greek .
petrochelidon is from petros , "rock", and khelidon , "swallow", and pyrrhonota from purrhos , "flame-coloured", and -notos "-backed" .
it breeds in north america and is migratory , wintering in western south america from venezuela southwards to northeast argentina .
this species is a very rare vagrant to western europe .
this bird averages 13 cm (5.1 in) long with a tiny bill .
the adult cliff swallow has an iridescent blue back and crown , brown wings and tail , and buff rump .
the nape and forehead are white .
the underparts are white except for a red face .
the tail is square-ended .
young birds are essentially brown above and whitish below , except for the buff rump and dark face .
the only confusion species is the closely related cave swallow , which is richer in colour and has a cinnamon rump and forehead .
american cliff swallows nest in colonies ranging from a few breeding pairs to thousands of birds , depending on the area and year .
the number of breeding pairs in an area per year generally increases with temperature , though larger colonies are more likely to suffer from parasites and intraspecific competition .
they build conical mud nests and lay three to six eggs .
the natural nest sites are on cliffs , preferably beneath overhangs , but as with the eurasian house martin , man-made structures are now the principal locations for breeding .
female american cliff swallows are known to lay eggs in and move previously laid eggs into the nests of other birds within the colony .
this species has always been plentiful in the west of north america , where there are many natural sites , but the abundance in the east has varied .
european settlement provided many new nest sites on buildings , but the population declined in the late nineteenth and early twentieth centuries as the supply of unpainted barns declined .
there has been a subsequent revival as dams and bridges have provided suitable sites .
these are the famous swallows whose return from villa ventana , argentina every year to the mission san juan capistrano in california on (or around) march 19 is celebrated with a festival .
in recent years , the swallows have failed to return to the mission .
the "capistrano swallows" are now nesting in the chino hills of southern california , north of san juan capistrano .
thousands of the small birds , up from "wintering" in argentina , have built their mud nests in the eaves of the vellano country club , with community buildings situated around a golf course in the chino hills .
