nest sites are varied, though cavities are preferred. nests are most frequently built in the eaves and other crevices of houses. holes in cliffs and banks, or tree hollows, are also used.p52–57 a sparrow sometimes digs out its own nests in sandy banks or rotten branches. more often it uses the nests of other birds, such swallows' nests in banks and cliffs, and old tree nest holes. it often uses deserted nests. sometimes it takes over active nests by driving away or killing the occupants. tree hollows are more commonly used in north america than in europe. this puts the sparrows in competition with bluebirds and other north american cavity nesters so the native bird population declines.