The Black garden ant (Lasius niger) is a very successful ant species. Part of their success lays in their adaptability. They are opportunistic in their choice of food: they feed on dead animals, nectar and pollen, seeds and honeydew. They are farmers, tending for aphids and scale insects and collecting the honeydew they secrete in exchange for protecting them from parasites. At the moment, ants are very active, feeding on plum and cherry nectar. They can actually enter the flower where the nectar is hidden. Although ants are known to pollinate a few plants species, they are mostly 'nectar thieves'. The reason the ants do not usually pollinate flowers are twofold. First, they are so small in relation to the size of the flower that they do not seem to touch the anthers or stigmas when entering the flower, and pollen does not adhere well to the ants surface. Second, individual ants do not travel between different flowers in a single bout of feeding, but just get enough to travel back to the nest from a single flower - notice the distended abdomen of the ants leaving the plum flower on the photo above. They seem to feed on nectar on any cup shaped flowers lacking specific mechanical obstructions to them entering the flower (such as labiatae, the pea family). In fact, aspects of flower shape might be adaptations by plants to exclude these generalistic nectar thieves.
Black garden ants find Paeony buds secretions irresistible, and it is hard to spot a paeony bud without its handful of feeding ants.
The ants are now also "milking" scale insects. They encourage the scale insects to excrete the honeydew by 'tickling' them with their antenna.
C.M. HERRERA, J. HERRERA and X. ESPADALER (1984) Nectar thievery by ants from Southern Spanish insect-pollinated flowers. Insectes Sociaux, 31: 142-154. Here.