I have a very tall specimen Leylandii in the corner of my country garden. It has provided a nest site and shelter to a pair of wood pigeons for several years, but two of the topmost limbs blew down in the wind of last winter. I am now worried that more may come down next winter. Is it possible to "top" the tree which may prevent this, or will this kill the tree? I would rather not cut it down completey in case I lose my pigeons!!