There are 25 species of bats that can be found in California. Most of these are insectivores. Bats are mammals that traverse through the air to catch their prey, which are usually nocturnal insects. In areas populated by humans, bats usually make their homes within attics or barns. They have a relatively long lifespan, most living between 5 and 30 years.
Although bats can help keep insect populations down, and thus provide a service to their environment, they can cause concern to the people living around them. Dung can serve as a breeding ground for microorganisms. The most serious danger bats pose is that they can carry Rabies, a potentially deadly disease.
Migratory bats can find shelter in human structures and large quantities of droppings can accumulate. These can foster the growth of a fungus whose spores can become airborne and cause lung disease in humans. In severe enough cases, this disease can be fatal. Experts using the proper protective gear should be the ones to deal with these creatures. Small colonies can go unnoticed for years, but they can be spotted by such things as their excrement, or dark "staining" that can appear in places that bats have left their bodily oils as residue.