This is witches' broom (or witch's broom) on a silver birch at Wallington Hall. I haven't yet found out why it is called that, although birch is used for making besoms - the type of broomstick associated with witches (or Quidditch). Perhaps anything abnormal, mysterious or unexplained was blamed on witchcraft.
It is caused not by witches but by a fungus (Taphrina betulina) which interferes with auxins or phytohormones that regulate normal branching. It produces a dense mass of abnormal shoots from a single point. In this tree they have been present for many years without appearing to do much harm. Quite what the fungus gets out of it is difficult to say.