Guano is another term for bat excrement. It is similar in shape and size of mouse droppings and can be easily confused. Guano is easy to determine with a flashlight or other lighting means. If you crush it with a gloved hand and shine a light on the material left and it sparkles it is most likely guano. This is due to the exoskeletons of the insects that bats feed on nightly. Guano also has a smell often described as pungent, acrid or musty. It is often referred to as a smell that one will not forget.