1. DESIGN
The most successful bat houses are approximately
two feet tall, at least 14 or more inches wide, and
have 3- to 6-inch landing areas extending below the
entrances. Houses with one to six chambers can be
successful, but three or four chambers seem to be
optimum, especially for nursery colonies. Roost partitions
should be spaced between 1/2-inch and 3/4-inch apart.
Partitions and landing areas must be roughened. Wood
surfaces can simply be scratched, but are best covered
with durable plastic screening (1/8-inch or 1/4-inch
mesh). Do not cover ventilation slots and be sure
screening is attached to one side of each roost partition
as well. Ventilation slots are critically important
in all houses to be used where average high temperatures
in July are 85°F or above. They should be 1/2-inch
wide to reduce entry of light and unwanted guests,
such as birds. The front vent should extend from side
to side about six inches above the bottom. A vertical
vent, 1/2-inch wide by six inches long, should be
included at each end of the rear chamber of multiple-chamber
houses. The vents greatly reduce the odds of overheating
on extra hot days and especially contribute to success
in moderate or hot climates. They may be unnecessary
in exceptionally cool areas.
2. CONSTRUCTION
Half-inch plywood is ideal for fronts, backs, and
roofs, while 1-inch board lumber is best for the sides.
Use of 1/4-inch plywood for roosting partitions substantially
reduces overall bat house weight and enables more
roosting space for a given house size. Staples used
to attach plastic mesh should not protrude from the
far sides of panels and will last longer if they are
exterior grade and galvanized. All seams must be caulked,
especially around the roof, prior to painting. Latex
caulk is easiest to use. Providing sufficient warmth
without overheating is a key element in attracting
bats. All outer surfaces and entry areas of bat houses
should be painted with at least two coats of exterior
latex paint to ensure against moisture, air leaks,
and wood
deterioration.
3. SUN EXPOSURE
When choosing a bat house location, both sun exposure
and heat absorption (according to house color) must
be carefully considered. Bat houses in cool climates
need to absorb much more solar heat than those in
hot climates. They should be black where average high
temperatures in July are 80-85°F or less; dark
(such as dark brown, gray, or green) where they are
85-95°F; medium or light where they are 95-100°F;
and white where they exceed 100°F. Bats in nursery
colonies like warm houses, ideally where temperatures
are between 80° and 100°F, meaning that their
roosts require solar heating in all but the hottest
climates.
4. MOUNTING
Bats find houses mounted on poles or buildings more
than twice as fast as those on trees. Trees may be
less preferable, in part, because tree mounted houses
tend to receive less sun and may be more vulnerable
to predators. Houses mounted under the eaves on wood
or stone buildings, but still exposed to the sun,
tend to be better protected from rain and predators
and have been especially successful. Generally, bat
houses should be mounted 15-20 feet above ground,
though 10-12 feet may suffice. The best locations
are along the borders of streams, rivers, or lakes
or along a forest edge, because these are natural
bat flyways.
5. PROTECTION
Safety from predators appears to be a key factor
in bat choice, and houses mounted on sides of buildings
or high up on poles provide the best protection. Locations
at least 20-25 feet from trees reduce obstructions
and predation and may receive more necessary sunlight.
In areas where climbing snakes occur, it may be necessary
to purchase predator guards from a supplier of purple
martin house products. You may make your own guards
by tightly covering the upper end of a 2-foot section
of 10-inch-diameter galvanized stove pipe with 1/2-inch
hardware cloth, allowing a hole in the middle for
a bat house pole. Place one such predator guard around
each pole about four feet above the ground.
