Efficiency of Bat Houses
Last Update/Review: June 17, 2002
by M.J. Pybus
Alberta Environment
A
total of 420 bat house owners in 26 states plus Ontario were contacted.
Two hundred and twenty people had put up bat houses and then monitored
their use. These people were primarily in Ontario and northeast US,
southeast US, and the US west coast. The survey questionnaire gathered
information about house size and shape, treatment with paint or stain,
kind and age of wood used, mounting substrate, height above ground,
and location. Location criteria included urbanization, distance to
lakes and streams, hours of solar radiation, distance to nearest flight
obstacles, forest density, and presence of nearby agriculture.
Six basic house designs were in use. The primary features included vertical partitions and open bottoms. Space between partitions was, on average, 2 cm (3/4 in.). Three size groupings of houses were identified:
| Size | Height |
Width |
Depth |
|---|---|---|---|
| 1. Small | 28-36 cm (11-14") |
18-20 cm (7-8") |
15-18 cm (6-7") |
| 2. Medium | 56 cm (22") |
20 cm (8") |
20 cm (8") |
| or | 53 cm (21") |
24 cm (9.5") |
5 cm (2") |
| 3. Large—vertical | 65-91 cm (25-36") |
25-61 cm (10-24") |
14-29 cm (5.5-11.25") |
| or —horizontal | 30 cm (12") |
81 cm (32") |
14 cm (5.5") |
Occupancy
The overall average occupancy of 276 houses was 52%. However, this differed with different house sizes.
Small |
Medium |
Large |
|
|---|---|---|---|
| Occupancy | 32% of 95 |
46% of 69 |
71% of 51 |
Most houses used by bats were occupied in the first summer season after they were erected (70%). An additional 15% were occupied in the second season. Occupancy also is greatly affected by location (see below). Little Brown Bats and Big Brown Bats, the two most common species in Alberta, in addition to the less common Eastern Long-eared Bats, readily used houses in this survey. Our experience in Alberta has been less successful.
Colony Size
The mean number of bats per house did not differ with different sized houses.
Small |
Medium |
Large |
|
|---|---|---|---|
| Colony Size | 28 (1-150)* |
21 (1-50) |
29 (1-250) |
| * average number of bats (range of colony size) | |||
Most people looked inside the bat house and counted the bats. This method tends to underestimate the number of individuals present (bats like to crowd at the top of the house, making it difficult to see each and every one). The average number of bats in small houses does not reflect the fact that most houses had only 1 to 5 bats present. In general, bats prefer the medium and large designs with long narrow roosting crevices. Such houses provide excellent conditions for nursery colonies.
Note: the bat house plan provided herein and recommended for use in Alberta is similar to the 'medium' house used above.
Materials
Bats use houses made of a variety of types of wood, including cedar, pine, and exterior plywood. Bat houses made of old wood were more often used in the first season than those made of new wood. Age of wood affected only the timing of occupancy, not the probability of use. Application of a solution of water and bat faeces may reduce the time to first occupancy in houses made of new wood (samples sizes were too small to test this properly). Similarly, filling a house made of new wood with damp or rich soil for a few days may 'age' the wood more quickly.
Bats living in areas north of 40° 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.
Site Characteristics
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° 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.
Design
Some people modified the basic designs and provided new information which may increase the success of bat houses. A rough interior surface is important in providing bats with ample places to hang on. The addition of hardware cloth or insect screening as a lining on the vertical partitions may increase the number of bats using the house. One fellow in Oregon tilts his bat houses at 10° (the bottom farther away from the wall or pole than the top) and has documented that young bats are less likely to fall from tilted houses.
Volunteer Research Associates Needed
Bat Conservation International (BCI) is looking for interested people to help fill gaps in our knowledge about bat houses. Putting up bat houses and making careful observations offers an excellent opportunity to learn more about roosting preferences and requirements. You can provide a valuable contribution by participating as an active volunteer on this long-term project.
Even the most abundant bat species are losing roosting habitat at an increasing rate. To date we know that at least three species found in Alberta will use bat houses. Certainly the most common species, Little Brown Bats, are well known for taking advantage of such alternative housing. The study outlined above did not include any data from bat houses west of Ontario (in Canada) or north of Washington state. Thus we have the opportunity to provide data which can be compared to the more southern and eastern results and to help determine what conditions are most appropriate for bats in the northwestern portion of the continent. If you are interested in joining this fun and worthwhile project, check out Bat Conservation International's home page and follow the links to bat houses!
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