The nest, built by both adults over several weeks, is made from several hundred mud pellets and lined with feathers and soft, dry grass, hair, sheep's wool or plant down. It may be a half-cup when constructed under an overhang on a vertical wall or cliff, or shaped as a bowl like that of the barn swallow when placed on a sheltered ledge. The nest may be built on a rock cliff face, in a crevice or on a man-made structure, and is re-used for the second brood and in subsequent years. Caves are found in limestone formations and in the lava flows which cover much of western Saudi Arabia, and their ceilings are a favoured location for nesting pale crag martins, red-rumped swallows, and the little swifts which may appropriate the hirundines' nests. In buildings, nests are usually constructed against concrete, which provides adhesion similar to that of rock, but metal walls are sometimes used, and nests may be supported on beams or other horizontal supports. Birds sometimes breed in occupied buildings, and there is a record of a pair nesting in a busy restaurant kitchen. Artificial nests are readily used, and halved coconut shells have been successfully occupied in Abu Dhabi.