Hangars with open superstructures often attract birds to nest and roost in. Hangar doors are often left open to increase ventilation, especially in the evenings. Birds in hangars are in proximity to the airfield and their droppings are both a health and damage concern. Netting is often deployed across the superstructure of a hangar denying access to the rafters where the birds roost and nest while still allowing the hangar doors to remain open for ventilation and aircraft movements. Strip curtains and door netting may also be used but are subject to improper use (e.g. tying the strips to the side of the door) by those working in the hangar. (U.S. Air Force 2004, Commander, Naval Installations Command 2010).