As this is a cleptoparasitic species, all flower visits will be for nectar only.
Flowers visited in GB (Else & Edwards, 2018): Prunus (Rosaceae); Ribes uva-crispa (Grossulariaceae); Salix repens (Salicaceae); and Taraxacum (Asteraceae).
Pawlikowski & Kruszynski (1997) list Spergula vernalis (Caryophyllaceae); Fragaria viridis, Potentilla arenaria, Prunus spinosa (Rosaceae) as nectar sources in Poland.
Smit (2004) , citing others, lists the following nectar sources: Salix sp. (Salicaceae); Taraxacum sp. (Asteraceae); Anthriscus sylvestris (Apiaceae); Prunus spinosa (Rosaceae).