We describe a bee mine in a park in Alkmaar where Osmia bicornis females collect building material. Through use of the concept 'cue' we give a possible explanation for the massive appearance of a species of solitary bee near such a mine. Only two foraging females were found, both on oak trees. These findings, together with data in the literatuur lead us to doubt such concepts as poly- and monolectic. We suggest using generalist and specialist, respectively, instead. The question how the bees find the catkins of oaks remains unanswered.