Barn swallows build their nests of mud pellets. Both parents deliver the pellets in their beaks to the nest site, which can be in barns, under bridges, on buildings and on cliffs. They construct a cone of mud and top it with a cup. Females then line the nests with softer materials, including feathers. Swallows commonly nest in colonies.
Chimney swifts construct an even more unusual variation of the cup nest.
Swifts favor enclosed vertical spaces, such as chimneys, silos, old wells and attics for their nests. Both males and females will break twigs from the ends of dead branches to use for their nests. They stick the twigs to the wall with their own saliva, which soon hardens into glue. Eventually they accumulate a half-saucer nest for the eggs. The cup is not the only design birds employ.