Birds use a variety of materials to make their nests. Some use twigs and grasses to weave nests, while others nest in hollow trees or logs. Then there are birds that use mud. Mud is not the most commonly used material for nesting, but it's used to create a wide variety of nests.
Swallows and Martins
The nests constructed by swallows and martins (of the family Hirundinidae) are made entirely out of mud and are lined with dried grass and feathers. The birds gather mud in small mouthfuls, moistening it with saliva until it forms a pellet. They use those pellets to build their nests. Each nest is made up of about 1,000 pellets. They like wide open spaces, and their nests are usually built in the eaves of barns or houses, under bridges or on the sides of cliffs.
Flamingos build simple nests out of mud on the ground. Mud and faeces are piled up above water level and allowed to dry so that more mud can be added. When finished, the nest is essentially a large mound with a depression in the centre. Flamingos leave their nests bare, not lined. The nests can be up to 17 or 18 inches in height.
Magpie-lark and Willie Wagtail
Magpie-larks and willie wagtails (both of the Dicruridae family) build cup-shaped nests out of mud, usually on horizontal tree branches. Grasses and leaves are incorporated into the nests to strengthen the mud once it dries. The nests are lined with grasses, leaves, feathers and sometimes fur.
Apostlebird and White-winged Chough
Apostlebirds and white-winged choughs (of the family Corcoracidae) build nests similar to those of magpie-larks and willie wagtails, but their mud nests are bowl-shaped and larger, and are built by groups of birds. Apostlebirds live in a breeding group consisting of a male, more than one female and juveniles. Females lay their eggs in the same nest. White-winged choughs live in groups consisting of a breeding pair and their offspring, which remain with the parents until they reach maturity at four years. The offspring assist in building the nest, incubating the eggs and feeding the young. If mud isn't available, choughs use emu dung.
Other Birds That Use Mud for Nesting
Spotted morning warblers build cup-shaped nests constructed mostly of mud, like the magpie-lark and willie wagtail. Rock nuthatches build large nests out of mud on the sides of cliffs. The nests have been described as looking like rounded volcanoes protruding from the sides of cliffs. Rufous ovenbirds and crested horneros build complex nests, roughly the size of a football, entirely out of mud. Each nest has a domed entrance and a hallway that wraps around the inside of the nest before opening into the central chamber.