Osmia bicolor (Red-tailed Mason-bee)
The black and red females of this medium-sized mason bee are very distinctive. Males are brownish or greyish and most likely to be confused with faded males of O. aurulenta in the field but have smaller heads and the margin of tergite 6 smoothly rounded. Males of O. spinulosa and Hoplitis claviventris are also rather similar but much smaller.

O. bicolor is predominantly a species of calcareous habitats, especially chalk and limestone grasslands, quarries and cuttings and calcareous coastal dunes. However it can occasionally turn up in non-calcareous brownfield sites and sandpits and also in the rides and clearings of calcareous woods.

Most records fall within southern England south of the Severn-Wash line, though there is clear indication of a current spread in the Midlands using brownfield sites as stepping stones. There are also records for the Gower Peninsula, South Wales.

Males are amongst the first bees to appear in spring (usually early March). Females appear a week or two later but persist until June or early July. A wide variety of flowers are visited, notably spring-flowering shrubs such as sallows, Blackthorn and Hawthorn, and low flowers such as dandelions, violets, Ground-ivy, Kidney Vetch and bird's-foot-trefoils.

Nesting occurs in empty snail shells, with a preference for those screened by dead grass stems. Females will collect grass stalks and other items to place on the shells, presumably to disguise them further and make them less prone to attack by parasites and predators such as Sapyga wasps.

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