They have access to food, water and shelter. They are able to perch high above the ground out of the way of predators. Their droppings rain down on people, equipment and product below.
Bird Control Solutions
The most permanent exclusion comes from StealthNet, it creates an impenetrable barrier. Bird-Flite will work for pigeons, but won't work in nesting sites. Optical Gel is a great way to treat beams and other surfaces.
Considerations when Selecting Bird Deterrent Products
Find out where the facility needs access for lights, doors, heaters, etc. Zippers can be installed to allow for access.
If the pressure is heavy (see any nests?), it should only be protected with StealthNet.