Although weavers are named for their elaborately woven nests, some are notable for their selective parasitic nesting habits instead. The nests vary in size, shape, the material used, and construction techniques from species to species. Materials used for building nests include fine leaf fibres, grass, and twigs. Many species weave very fine nests using thin strands of leaf fibre, though some, like the buffalo-weavers, form massive untidy stick nests in their colonies, which may have spherical woven nests within. The sociable weavers of Africa build apartment-house nests, in which 100 to 300 pairs have separate flask-shaped chambers entered by tubes at the bottom. The sparrow weavers live in family units that employ cooperative breeding. Most species weave nests that have narrow entrances, facing downward. Many weaver species are gregarious and breed colonially. The birds build their nests together for protection, often several to a branch. Usually, the male birds weave the nests and use them as a form of display to lure prospective females.