Witch's brooms on birch

Have you noticed these tufts of twigs on birch trees?  They are near post 10 at Filnore.  

They are called 'witches' brooms and are caused by a fungus, Taphrina betulina.
 

The fungus stimulates the tree to produce huge numbers of buds in one place.  These grow into short twigs, which then die, resulting in an ever increasing mass of dead and living twigs.