Are they in the belfry?

Mexican free tailed bat is a migratory native of Washington.

What can be done?

Due to the protection on the different species of bats, the best method found is full house exclusion. Bats don't have the teeth or body design to chew through wood or tear through screen like a raccoon or a squirrel, but they are very good at finding tiny spots around a home to roost in. Once the home is secure, remediation of the attic can be done to remove the animals scents and fecal material.

When are they a problem?

Bats getting into a home are a very real problem. Given enough time, bat droppings (known as guano) can present a  potent health hazard. Different diseases and bacteria can be very dangerous to families, especially young children. Bats can carry rabies, as can all warm blooded mammals. Rabies can be treated before symptoms develop, but is incurable once they do.

A bit about bats.

Bats are severely misunderstood animals, one that actually is a large benefit to the environment. Bats are a truly nocturnal animal, which actually are not blind. All bats are under some level of protection, most species are under state protection with a handful under federal protection.