Mason. This is the only species of British Osmia which often builds exposed mud nests (Else & Edwards, 2018). These consist of a cluster of cells more or less joined together side by side with the lower part usually inserted in the soil, and with the upper part exposed (Waterhouse, 1884 [as O. atricapilla]. Gardner (1903) found nests in the roots of grasses. In other nests, a series of about six cells are arranged consecutively in a burrow in the soil (Mortimer, 1905; Osten, 1982). A nest containing five cells constructed in such a burrow is illustrated by Osten (1982). In one of the Welsh sites, some females excavated their nests in the sides of rabbit burrows (C. Clee, pers. comm.).
Nesting material: The cells are constructed from mud mixed with small pebbles and are pitcher-shaped, rounded at the bottom and sealed at the top with a flat lid (Waterhouse, 1844; Nielsen, 1902; Enslin, 1921b). A nest from roots of grass at Clacton-on-Sea, Essex, and now in the NHML, resembles those described by Waterhouse and Nielsen. Cells, including an opened, provisioned one, are illustrated by Westrich (1989) and Müller, Krebs & Amiet (1997).
On the Isle of Wight females probably nest at the roots of vegetation on the less steep gradients of cliff faces.Specimens of this sex have been seen visiting ground-water seepages at the base of these cliffs to collect mud for nest construction (the females were observed to burrow into the soil to reach mud of the most suitable consistency for nest-building) (pers. obs.).