Excavator: Ground. Nests may occur as small assemblages, or in dense, compact aggregations. Archer (1984) describes a North Yorkshire nesting site which consisted of a level, stable, sandy surface which was moss-covered, with intervening bare patches. He estimated that the area contained a total of about nine hundred freshly-excavated burrows. I once dug out occupied cells from the soil still adhering to the roots of a prostrate, wind-blown tree.