Large numbers of bats living in attics or walls are undesirable. Most bat colonies are small and go unnoticed for years, but when odor or noise increases or bat droppings accumulate, eviction and exclusion are the only safe and permanent remedies.
Entry points of large colonies are usually obvious. When bats start emerging at dusk to feed, watch the building to see where they exit. Daytime inspection should reveal the holes or cracks they are using. Often, these will be beneath eaves, around the chimney, around air and plumbing vents, near loose boards, beneath roof caps, or in openings that squirrels or other animals have made.
Another sign of a bat entry point is the staining caused by body oils (similar to rodent “rub marks”), bat urine, and bat droppings. Bat droppings, known as guano, crush easily, revealing shiny bits of undigested insects. Mouse droppings lack the shiny bits, and bat droppings never are white or chalky as are bird droppings. Seal entryways with caulking, putty, duct tape, self-expanding polyurethane foam, or hardware cloth (1/4-inch mesh) after excluding the bats.
Avoid excluding bat colonies when flightless young could be present, usually May through September in California. Excluding the parents will starve the young and create odor problems and an infestation of flies. Most bats leave in late fall in most areas, making winter an ideal time to implement exclusion techniques.
If exclusion needs to be done before bats have moved out of a structure, provide a way for them to leave at night that doesn’t allow them to re-enter (ONE-WAY SETUP) that allows the bats to drop down to leave.