There are many species of critters that can nest in your building's walls. Having them there doing this can cause structural damage. You can tell if they are there by the physical signs they leave, or noise you may hear. As a building owner you can get rid of the critters your self or call pest control, but the situation must get resolved.
The most common critters are rodents. Deer and house mice are two species that will invade your walls. The Norway rat will invade your roof. The critters enter via holes or cracks in your homes floors, walls and even your foundation. After building their nest they can give birth to up to 84 offspring just in one year alone. The critters are invading not only to nest but they are looking for food.
The carpenter bee is another invader of your walls. Their name comes from their proneness to develop nests in trees and wood structures. You can tell you have them by the circular holes or brown staining they create. Sealing any existing holes in your walls with a oil based paint will ward off the carpenter bee.
Those birds that sound so pretty in the morning are another wall invader. The common starlings and sparrows are known for this. These birds will nest even in your dryer vent. They build their nests with leaves and dry grass or straw, so look for these materials.
The worst of the lost is termites. Small insects that nest in walls with a wooden infrastructure. Termites cause billions of dollars annually to structures. They love warm temps and are most active in the spring months. You can identify their structures by looking for mud tubes.