The Cliff Swallow breeds in North America, and in highest densities in the west of the continent. This is the famous swallow that returns en cue to the mission of Capistrano in California, although perhaps due to climatic changes the swallows return earlier and earlier each year! Cliff Swallows likely did breed on cliffs originally, and in some areas of the west they can still be found nesting in riverside cliffs. Many nesting sites now are man-made structures, most of them actually, often bridges, overpasses, dams and tall concrete buildings. The nests are made of mud, and are fully sphere shaped with a side entrance, unlike the open cup nests of its close relative the Cave Swallow. Cliff Swallows are highly colonial, you do not find single Cliff Swallow nests, but often they are present side by side in tens or even hundreds. This species is an extremely long distance migrant, the bulk of the population migrates south to central and eastern Argentina, and once there they appear to be nomadic following outbreaks of small insects. They are present for a few days at a specific area, and then move on, this can include roost concentrations of thousands upon thousands of birds. The wintering ecology of this common species still needs to be studied. As has happened recently with Barn Swallows, the Cliff Swallow has begun to nest during the Austral Summer in areas of Argentina, why this new colonization of the southern hemisphere is occurring is a mystery. One pair attempted to build a nest in central Chile as well! What these birds do in the non-breeding season is a mystery; do they “winter” in North America during the northern summer?