WITCHES BROOM

The growth that can be found on hard woody plants -( trees,) especially the Silver Birch, is often mistaken for birds nests or a squirrel's drey, but it is a disease, a deformity, and is caused by parasites.
These parasites can be in the form of :- fungi, oomycetes (micro organism known as water mould,) insects, mites, nematodes, viruses, phytoplasmas (bacteria) and mistletoe.