Bird nests can result in alot of problems even if your flue is unused. They can                                                        block a used flue effecting draw and allowing gases back in to the room. On                                                                    an unused flue it can cause damp patches on internal walls from the moisture                                                            of the nest.

        

                    

                                     Damp caused by birds nest in flue.

                   

              Fire smoking back into the room caused by a nest in the flue.