We contend with Cliff Swallows every year. Perhaps you do as well? They like to return to previous nesting sites. If the nest was well-constructed and is not heavily infested with parasites they will use it again. Otherwise, if the nests are gone, they re-build in the same spots. When they arrive, they fly for hours making repeated swooping flights from mud puddle up to the chosen nest site which is on a vertical surface and frequently under the eaves of a building.
The nests are constructed of mud pellets which the swallows collect in their beaks. They start with an outline and add row after row until the shape of the nest is like a gourd with a small opening. They line the insides of the nests with grass, hair, and feathers. You can see in this picture where a previous nest had been built. More about that below.
As mentioned, we go through this every year. On one hand it is a fantastic spectacle of nature I enjoy watching, especially when the kids were small. On the other hand, the nests are infested with fleas and ticks and other yucky parasites, not to mention the white splash of droppings which would accumulate once they began flying in to feed their young. So - we knock the nests down every year before they lay their eggs. Usually if we keep knocking away their first rows of mud they give up and find a different place. They are beautiful birds though, and I enjoy seeing them sitting together like little lovebirds.