Renter: Existing cavities. Nests of this bee consist of clusters of naked cells attached to the undersides of rocks overlying shallow depressions in the ground. In sites where suitable stones are scarce, the species may nest in deep fissures in small rocky outcrops (as noted in Germany, P. Westrich, (pers. comm.)), and perhaps in niches in dry-stone walling or, in one case, to the underside of a discarded heat shield from a vehicle catalytic converter.
Nesting material: The cells are oval in shape and constructed from leaf mastic. They are a dull brown colour (even when freshly constructed), and in this discoloured state greatly resemble rabbit droppings. The number of cells per rock varies between 1 and about 230, but considerable numbers of cells may be the work of several generations of females, or more than one female in a single season. Nests illustrated by Priesner (1981), Müller (1990), Else & Edwards (1996), Müller, Krebs & Amiet (1997) and Edwards (1998).
In Austria, Priesner (1981) found up to 200 cells per rock and discovered that these cells were the result of up to twelve females working simultaneously. An example of one of these large, communal nests is illustrated by this author. In contrast, Schmiedeknecht (1885) states that in Germany the bee nests in rotten
conifer stumps, but it is probable that this report was based on a misidentification.
The nesting habits of the species in Scotland are described by Else & Edwards (1996) and M. Edwards (1998). Rearings from Scottish nests suggests that in Britain, this bee has an obligate two-year life cycle, the first winter being passed as a prepupa and the second as a diapausing adult within its cocoon (Else & Edwards, 1996). Some individuals may emerge from their nests three years after their cells were sealed (see F. Smith, 1851, 1855a [as Osmia parietina]).