The North American bat species most frequently found in our homes or bat houses are the big brown bat and the little brown bat (Eptesicus fuscus and Myotis lucifugus).
Bats represent important members of the wildlife community and should be valued for the role they play as predators of insects. A single brown bat can easily capture 600 mosquitoes in an hour while a colony of 500 bats can eat a million insects nightly.
Out of the 900 bat species known in the world, about 40 species, mostly insectivorous, are found in Canada and the United States. Bats emit high-frequency sounds inaudible to humans. These sounds bounce off objects in their paths to enable them to avoid obstacles and detect flying insects. Sometimes bats can be seen around outdoor lights, which attract insects.
Contrary to popular belief, bats are not blind. In fact, some species have excellent vision. To save energy, bats have the ability to enter a dormant state which makes them appear to be sick, asleep or dead. They may also bare their teeth and squeak loudly, leading many people to believe they are vicious. In reality, the helpless creature is only trying to ward off a possible attack. Bats must also avoid the watchful eyes of their many predators, which include hawks, falcons, owls, cats, snakes, and humans.
Bats living in areas north of 40 degrees north latitude (i.e., anywhere in Alberta) prefer houses which retain as much heat as possible. Dark stain or even flat black paint on the outside surfaces of the bat house increases the heat retention and, thus, the occupancy rate. A layer of tar paper on the outside also increased the occupancy in northern areas.
Two features appear to be critical in increasing the probability that bat houses are used - distance from water and amount of solar radiation.
Bat houses located within a quarter mile of streams or rivers had an occupancy rate of 78%. The size of the river did not seem to matter. Houses located near lakes covering at least three acres had a higher occupancy rate than those near smaller lakes (62% versus 28%).
An arbitrary value of four hours of sun exposure was used to compare occupancy among different sites. Houses north of 40 degrees latitude, with or without a dark exterior, were more likely to be used if they received
four or more hours of sun each day (83% versus 26%). In Alberta, it is likely that no amount of sun would be too much, particularly for
Little Brown Bats, by far our most common species.
Houses should be placed south-facing in full sun.
Bat houses mounted on poles, buildings, or trees had different occupancy rates (81%, 73%, and 34%, respectively). The preference for poles likely maximizes the opportunity for exposure to sun and increased distance from obstacles on the flight path. The latter is a significant problem with houses placed in dense forests or on trees with branches below the bat house. However, in northern regions, houses mounted on trees which received at least four hours of sun were far more likely to be used than those that did not. In fact, such trees were more likely to be used than poles or buildings. Fortunately, in Alberta large poplar trees often have no lower branches and thus make excellent places to attach a bat house. A band of sheet metal or smooth plastic 45-60 cm (18-24 in.) wide wrapped around poles or trees will prevent climbing predators (i.e. cats) from disturbing the colony; however, unless there are unusually high numbers of cats, this is not necessary.
Houses raised to 4.5 to 6 metres (15-20 ft.) above ground were more likely to be occupied than those lower down (this may also reflect greater exposure to the sun). Arranging bat houses in groups of three or more may improve the number of bats present. Similarly, erecting bat houses before excluding bats from an attic may increase the chances of having the bats move into the bat house.
