The nesting habits of both cliff and barn swallows cause them to come into frequent contact with humans. In the wild, swallows construct their nests on cliff faces, mountainsides, caves, and in rock crevices near wide open expanses such as fields and bodies of water. Buildings, bridges, and overpasses create excellent substitutes for these natural habitats. Swallows are likely to build their nests, made of mud and crafted into either a cup or gourd shape, high up on the walls of buildings just beneath an overhanging ledge.
Barn swallows mate for life, and mated pairs work together to build their nests in which they raise their young. Large numbers of cliff swallows tend to build colonies of nests together, sometimes numbering in the hundreds, while barn swallows nest singly or in small groups.