Barn swallows and cliff swallows both build nests out of mud. While the barn swallow most typically builds on man-made buildings, such as barns, the cliff swallow tends to nest under bridges. Both birds build gourd-shaped nests about the size of a grapefruit.
Each mud nest has a small opening to allow for entry and exit. These birds build their nests by collecting mud in their bills and mixing it together with grass stems to create pellets. Starting with a small shelf for sitting, the birds build the nest's sides around themselves. Swallows often take materials to line their nests from neighboring nests.