These growths on this Silver Birch tree are caused by the fungus Witches Broom, Taphrina betulina. It gets under the bark where it has been damaged, perhaps by insects, and causes this multiple sprouting of small twigs. From a distance, one could imagine this tree was supporting a rather successful rookery! An infestation as dense as this is thought to reduce the vigour of the tree and lead to up to a 25% reduction in size over its lifetime. The specific name of the fungus is taken from the generic name, Betula, of the host trees, i.e. the Silver and Downy Birch.