Best practices for the construction, installation, and monitoring of artificial roosts for bats

By Joy O’Keefe, University of Illinois Urbana Champaign

CONSTRUCTION

•      Use quality wood to build bat boxes and material that is safe for bats

•      Use untreated lumber because treated wood may negatively affect reproduction/health

•      Avoid plywood because it is not as durable as solid wood and glue is attractive to gnawing animals

•      Materials will be $150-200 for a large box

•      Construct a tall box (3’ or higher)

•      The taller the box, the larger the vertical gradient of temperatures. The bottom will be close to the outside air temperature, while the top can be much warmer on a hot day.

•       Include multiple ¾” chambers, which gives bats more space; provide corridors between chambers (drill 1” holes) to allow bats to move around. 

•      Optimal designs will include some element of heat storage and insulation

•      Water packets or silicone baggies filled with water inside the walls are a relatively lightweight form of heat storage, as tested by Tillman et al. (2021), Bakken et al. (2022), and Crawford et al. (2022). One way to do this would be to tack up some silicone baggies partly filled with water - say, staple to outer wall of box. Then add a layer of ¾" insulation, then add the outer piece of wood.

•      Insulation should be external to heat storage (3/4” thick foam board should work)

•      Building a box so that a bat chamber is inside a layer of insulation and heat storage will offer more moderate temperatures on warm, sunny days (less likely to overheat) and should retain heat into the night, providing better conditions for pups

•      Build roosts with small entrances at the bottom (3/4”) to keep predators out

•      Create footholds for bats inside roosts

•      Scuff up the inside walls with a drill or hammer or cut grooves with a saw or Dremel tool

•      Avoid using mesh inside because it will deteriorate and block up the space eventually

•      Paint color is an easily modified property of a bat box

•      Avoid dark paint colors unless boxes have both insulation and heat storage components.

•      A 40-50% dark (i.e., medium) color should be ideal for the Tennessee climate

INSTALLATION 

•       Best practice is to monitor temperatures in any artificial roost, ideally before offering it to bats

•       iButton sensors or other thermometers can be placed on a thin wood stick and inserted into the box to record temperatures (near the top is where most extreme temperatures occur). Revise design if suboptimal temperatures (>40°C or 104°F) are routinely detected on warm days.

•      An optimal design will stay above ambient temperature at night while not overheating during the day.

•      Note that the presence of bats can substantially increase box temperatures.

•      Position roosts so that the entrance is at least 12’ above ground (ideally 15’ to give pregnant bats space to drop into flight).

•      Avoid mounting roosts on trees but deploying them near trees is ideal to give bats a safe haven during emergence.

•       Avoid placing roosts near bright lights.

•       Deploy multiple roosts in an area to give bats options

•       Bats like to switch roosts every few days and more roosts accommodates a larger overall colony.

•       Place roosts where they’ll receive ample sun during the day

•       If roosts have a long/wide face, orient that face to the south or east

•       West-facing roosts that are painted dark are likely to reach lethally warm temperatures at the end of the day

MONITORING

•      Monitor boxes after installation

•       Can use a spotlight and binoculars to check boxes during the day but be aware bats are sensitive to disturbance on hot days. Be careful around guano piles, which may accumulate fungal spores dangerous to people.

•       Trail cameras can be useful for observing bats in the roost or people and predators around roosts

•       Exit counts at dusk will give better estimates of colony size, as many bats are obscured in roosts during the day

•       Use a standard roost counting protocol and employ multiple observers per roost if many bats are suspected

•       Changes in roost use over time can inform box deployment and management strategies

•       Maintain boxes over time

•       Note that wasp nests will sometimes build up inside roosts. While not a problem for the bats, the wasps may occupy valuable real estate.

•       Bat bugs and other bat parasites may accumulate in roosts over time, which might necessitate replacement.

•       Replace worn roosts to sustain a colony but also promote natural roost habitat in the area to give bats the types of habitat they are most adapted for.

•       If you observe evidence of overheating, such as pups falling out of boxes, bats roosting on the outside, or mortality, consult with TWRA as to whether you should remove or modify the box. Modifications could include adding a shading element or repainting the box a lighter color.