Cliff Swallows are social birds. All you have to do is look at how they nest to see how true that statement is. Cliff Swallows usually nest in large colonies, up to 3,500 active nests in the bigger colonies. An interesting fact about Cliff Swallows is that they have only in the last 100-150 years expanded their range to include the eastern US. The were originally restricted to the western US where they still use rock ledges for their nests. Their expansion came at the same time as the construction of bridges, culverts and other buildings conducive to nest building was really taking off.
This range expansion provides a possible preview for the expansion of Cave Swallows, a swallow species that nests in caves in New Mexico, Texas and Florida. The Cave Swallow is also expanding its range, although it seems they have only been using bridges and culverts for nesting in large numbers since the 1980′s. Each fall seems to bring an increase in reports of Cave Swallows to the northeast, a result of population increases and juvenile dispersal.