Dunnocks enjoy (if thats the appropriate word) an interesting social lifestyle.
"menage a trois" is hardly the description for it.
Without going into all the detail here it can be gleaned from many birding sites on the Net.
Suffice to say that the male and female meeting in the images below that I shot recently was unlikely to be the only encounter the Lady had that day or the male for that matter.
An unsuitable male it would appear here as she made off.