Though excluding birds from the entire airport environment is virtually impossible, it is possible to exclude deer and other mammals that constitute a small percentage of wildlife strikes. Three-meter high fences made of chain link or woven wire, with barbed wire outriggers, are the most effective. When used as a perimeter fence, these fences also serve to keep unauthorized people off of the airport (Seamans 2001). Realistically every fence must have gates. Gates that are left open allow deer and other mammals onto the airport. 4.6 meter long cattle guards have been shown to be effective at deterring deer up to 98% of the time (Belant et al. 1998).