For the last week or so there have been a few small, dark ants running about on the rim of the window box and scrambling about in the sallow coppice.
Through many iterations in different books and web sites they persistently ran down to the very common Lasius niger, variously known as the black ant, the black garden ant, the small black ant etc.
The trouble was all the examples I saw were half to three quarters the size of L. niger, with which I am very familiar from elsewhere in the garden.
An enquiry on the Bees Wasps and Ants Recording Society discussion group quickly produced an undoubtedly correct explanation from Mike Fox. He wrote "What you are probably looking at here are 'minims'. These are first, or at least early, generation workers from a new colony. It is likely that a new queen of Lasius niger, after mating during last years nuptial flight, has spent the winter in your window box and has, this spring, started a new colony. Her first batch of workers will be much smaller than normal and once the colony becomes properly established normal size workers will be produced."
Apparently it helps a colony to increase more rapidly in size and become firmly established if there are a larger numbers of small workers initially, rather than a smaller number of large ones.
I found these minims enjoyed a drop of golden syrup painted on one of the sallow stalks - makes them easier to photograph too.