Design modifications affect bat box temperatures and suitability as maternity habitat

Abstract

Artificial roosting structures (e.g. bat boxes) are widely used as conservation tools for many animals, including bats. Although it is relatively easy to monitor bat box temperatures, we know little about the effect of design on temperatures within a box. Box microclimate affects energy budgets and physiological processes and, thus, suitability as a roost. Optimal temperature varies during the period when reproductive females aggregate to rear pups; warm roosts enhance pup development during gestation and lactation, while cool roosts facilitate energy savings by torpor, which is often important during post‐lactation. To better understand the relation of design to internal temperature, we simultaneously compared 20 box designs (19 variations of a rocket box and one three‐chamber flat box) in an open site, May to September 2018. We measured temperatures at the top, middle and bottom of each box and tallied counts of daytime and nighttime cool (≤30°C; TCOOL), permissive (30.1–39.9°C; TPERM) and stressful (≥40°C; TSTRS) temperature observations. We also measured temperature, solar radiation and wind speed at the site. We used generalized linear models with negative binomial distributions to test the effects of design, environmental variables and their interactions. Adding an external jacket or decreasing ventilation increased daytime and nighttime counts of TPERM. Increasing box volume (i.e. lengthening box by 50%) also positively affected daytime counts of TPERM, whereas decreasing box volume (by 50%) had the opposite effect. Adding an external water jacket was the only modification we tested that decreased counts of TCOOL at night. Counts of TSTRS were elevated by warmer, sunnier and less windy conditions outside, but these effects were lessened by increasing roof shading or reflectivity, adding ventilation or external jackets, or decreasing box volume. These results inform the development and implementation of novel bat box designs as conservation and management tools for maternal colonies of bats, with consideration for the effects of weather on internal temperatures.

Evaluating bat boxes: design and placement alter bioenergetic costs and overheating risk

“…For facultatively heterothermic bats, forest placements resulted in the lowest DEE heterothermic and virtually eliminated overheating events. At the same time, our study affirms overheating risk is higher in solar-exposed locations and risk is further increased by box designs with low surface reflectance, low thermal mass or poor ventilation ( Griffiths et al, 2017 ; Martin Bideguren et al, 2019 ; Rowland et al , 2017 ; Tillman et al , 2021 ). Three box designs (CH, EJW and WTR) reduced overheating risk compared to the REF design, but only one (EJW) simultaneously decreased endothermic energy expenditure.…”

“…40 rocket boxes total; 20 per site). All design variants were modifications of the reference design described by Tillman et al (2021) . Our designs were reference (REF), vent removal (VR), chimney (CH), white tile roof (WTR) and external water jacket (EJW) ( Fig.…”

“…Design alterations were intended to promote microclimates different from the REF design while providing equal entrance area, roosting surface area and volume. Compared to REF, the VR design increases minimum temperatures by reducing ventilation, the CH design decreases maximum temperatures by venting rising hot air out a black chimney, the WTR design decreases maximum temperatures by reflecting solar radiation with a white ceramic tile roof and the EJW box buffers both high and low temperatures as a result of a sealed external chamber filled with three 750-ml water packets per box side, which increases thermal mass and, thus, time to heat and cool ( Tillman et al , 2021 ). We constructed boxes from 1.91-cm thick pine boards, offering a 1.91-cm chamber spacing, and coated exterior surfaces in two layers of medium brown paint.…”

“…Bat box microclimates are influenced by various abiotic factors, for example, by structural components like construction material ( Martin Bideguren et al, 2019 ; Rueegger, 2019 ), colour ( Doty et al , 2016 ; Griffiths et al, 2017 ) and volume ( Tillman et al , 2021 ). For example, black three-chamber boxes average 5°C warmer than white three-chamber boxes ( Lourenço and Palmeirim, 2004 ).…”

“…For example, black three-chamber boxes average 5°C warmer than white three-chamber boxes ( Lourenço and Palmeirim, 2004 ). Furthermore, environmental factors like cloud cover ( Hoeh et al , 2018 ), ambient temperature ( Bartonička and Řehák, 2007 ), wind ( Tillman et al , 2021 ), humidity ( Rueegger, 2019 ) and solar exposure ( Brittingham and Williams, 2000 ) all modulate microclimate. For instance, on days with clear skies, bat box temperature ranges can vary by as much as 10°C from top to bottom, whereas on cloudy days there is little variation in box temperature top to bottom ( Hoeh et al , 2018 ).…”