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Chalicodoma siculum (Rossi) is a mason bee that usually nests in dispersed sites, constructing spherical mud nests on twigs or hemispherical nests on hard surfaces. Under favourable conditions it may nest in aggregations of several hundred bees. At the nesting aggregation, nest usurpation often occurs, either by the laying of eggs in finished but unsealed cells, by opening a sealed cell and replacing its egg, or by taking over an unfinished cell. Mutual construction and provisioning of a single cell by several females was also often observed. Because participant bees display extreme aggression in prolonged disputes over cell ownership, these interactions can be regarded as competitive rather than cooperative. Moreover, since the total investment in such cells is not different from that in cells located in dispersed sites, the female that actually manages to lay an egg gains by participating in a cell under construction.

We discuss the advantages of nesting in aggregations vs. the risks of being usurped, as selective forces underlying nesting behaviour of bees.