This little bird is a Dunnock or Hedge Sparrow or Hedge Warbler or Hedge Accentor. It goes by many names!!! I took these yesterday near where I live.
What is so fascinating about this bird is that the Females are rather promiscuous. This was only found out in the 1990's. The females will mate with several males and those chaps will help in the rearing of the young, ensuring a plentiful supply of food for the young chicks whose father may never be really known. No DNA testing in the world of birds.
In the South of England Dunnocks are often targeted by Cuckoos even though Cuckoo eggs are so different from there own. Just as well there are several Dads about to help, a Cuckoo chick takes some feeding.
I will not go into the sexual behaviour and copulation of the Dunnock but suggest you watch this TV except from Life of Birds it is truly extraordinary. Follow Link (ONLY FOR PEOPLE IN UK)