The mating rituals of the Dunnock are pretty bizarre; the male pecks insistently at the female's vent (the cloaca) to eject any previous attempts at mating. This is done to try and increase a chance at fatherhood - the male Dunnocks may have lots of girls to attend to, but they are still keen to father yet more!
If this sounds unpleasant, try not to feel too sympathetic for the female Dunnock. She will often shake off her alpha male in the hope of having a cheeky fling with one of the lower males in the territory!
So if you see a pair of Dunnocks, and the female seems to be having her rear (below her tail) pecked, don't be alarmed. This is in fact a common ritual for these birds, who like to experience several partners.