Tips for making bat boxes safer for bats

Roost cluster for research study in Indiana, Photo by Joy O’Keefe

Text on this page by Reed Crawford, with contributions from Joy O’Keefe. Last updated in October 2021.

Our lab has been conducting research on artificial bat roosts (e.g., bat boxes) since 2015, with ongoing projects in Illinois, Indiana, Kentucky, Florida, and Minnesota. Increasingly, we are aware of bat mortality in bat boxes that is likely attributable to overheating. For instance, recent work in Australia documented the deaths of 30 juvenile large forest bats from hyperthermia in a dark-green bat box [1]. Other researchers are also noting the potential dangers of boxes that get too hot and can be dangerous to bats. Reed Crawford and Joy O’Keefe published a perspectives paper on the topic of heat stress in bat boxes in the journal Conservation Science and Practice.  Here, we provide some background and guidance to reduce the risk of heat stress for bats in artificial roosts.

Artificial roosts and heat stress

Bat boxes and other artificial roosts are commonly deployed to aid bats. However, these increasingly popular conservation tools can pose a risk to bat safety and health in certain environments [2]. Some bat box designs overheat (>104°F), leading to bats experiencing heat stress or dying [1,3,4]. When artificial roosts are poorly designed or placed, they may offer less suitable microclimates than the natural tree hollows and crevices to which bats are adapted [5,6]. Climate change adds another layer of risk to this problem and may compound the risk of heat stress to bats in artificial roosts [2,7,8]. Unfortunately, there is still much uncertainty about optimal artificial roost design and placement, which will vary by species and landscape [1]. Herein, we provide a list of suggestions and strategies to reduce overheating risk, backed by empirical research, with the goal of improving the safety of artificial bat roosts for bat species worldwide.

Decreasing overheating risk

Artificial roost placement

  • High solar exposure can increase artificial roost temperature and increase overheating risk [9–11]. Because maternal bats typically select solar-exposed artificial roosts over shaded alternatives [12,13], we recommend roosts receive some shade during the hottest part of the day. We have had good luck with bats using artificial roosts positioned near tree lines so that they are shaded for part of the day [11].

Color

  • Avoid dark-colored artificial roosts as dark colors substantially increase overheating [7,9] and bats may preferentially select dark artificial roosts [10,14–16], which increases the risk.
  • We recommend lighter colors like gray or tan, as these absorb less solar radiation and will be cooler, thus decreasing overheating risk [9,17]. In 2021, we initiated a paint color study and will update this page when those results are known.
Cluster of three light-colored bat box styles deployed in southern Florida. Photo by Joy O’Keefe

Roost type, material, and size

  • Small artificial roosts (both in volume and height) with low mass may overheat more readily and not offer bats safety from high temperatures [2,5]. We recommend tall artificial roosts with four sides (or multiple accessible roost chambers), which will provide large temperature gradients [11,17–20] and allow bats to move to avoid high temperatures.
  • Larger artificial roosts made of dense materials with low thermal conductance will have a higher thermal mass and thus take longer to heat and overheat [7,11,17,19,20].
  • Adding extra vents to an artificial roost will reduce overheating risk and make the roost cooler for bats [7,19].
  • Adding insulating layers (e.g., polystyrene) to the outside of an artificial roost will also reduce temperatures [8], and could enhance social thermoregulation.
  • Adding shading (e.g., large or elevated roof) to an artificial roost will also reduce overheating risk [4,19].

Considerations for different bat species

  • We recommend larger (in height and volume) artificial roosts for bat species that form large groups. If a larger group forms in a small artificial roost, there will be less room for individual movement and, thus, some bats may be trapped in hot parts of roosts.
  • Recent work has shown that thermal tolerance varies by bat species depending on the stability of their natural roost structures. For example, bats that typically roost in caves with stable temperatures may have lower heat tolerances than bats that roost in more variable temperature conditions like under exfoliating bark) [21–23]. Therefore, it is important to consider life history traits of target species to understand their potential susceptibility to heat stress.

Alternative ways to help bats

  • Carving hollows in live and dead trees is effective for creating suitable roosting habitat for some bat species [24]. Further, created tree hollows are more robust to temperature extremes than non-tree roosts [25,26].
  • Retaining and generating dead trees is a viable option to create bat roosting habitat [27,28], if the trees don’t pose a safety hazard.
A dead tree (snag) in full sun is natural roosting habitat for many bat species. Photo by Joy O’Keefe

References

1. Griffiths SR. Overheating turns a bat box into a death trap. Pacific Conserv Biol. 2021. doi:10.1071/PC20083

2. Crawford RD, O’Keefe JM. Avoiding a conservation pitfall : Considering the risks of unsuitably hot bat boxes. Conserv Sci Pract. 2021; e412. doi:10.1111/csp2.412

3. Flaquer C, Puig-Montserrat X, López-Baucells A, Torre I, Freixas L, Mas M, et al. Could overheating turn bat boxes into death traps? Barbastella. 2014;7: 39–46. doi:10.14709/BarbJ.7.1.2014.08

4.   Alcalde JT, Martínez I, Zaldua A, Antón I. Conservation of breeding colonies of cave-dwelling bats using man-made roosts. Barbastella. 2017;10. doi:10.14709/BarbJ.10.1.2017.02

5.  Rowland JA, Briscoe NJ, Handasyde KA. Comparing the thermal suitability of nest-boxes and tree-hollows for the conservation-management of arboreal marsupials. Biol Conserv. 2017;209: 341–348. doi:10.1016/j.biocon.2017.02.006

6.  Strain C, Jones CS, Clarke RH. Spout hollow nest boxes provide a drier and less stable microclimate than natural hollows. Conserv Sci Pract. 2021; e416. doi:10.1111/csp2.416

7.  Bideguren MG, López-Baucells A, Puig-Montserrat X, Mas M, Porres X, Flaquer C. Bat boxes and climate change: testing the risk of over-heating in the Mediterranean region. Biodivers Conserv. 2018. doi:10.1007/s10531-018-1634-7

8.  Larson ER, Eastwood JR, Buchanan KL, Bennett ATD, Berg ML. Nest box design for a changing climate: The value of improved insulation. Ecol Manag Restor. 2018;19: 39–48. doi:10.1111/emr.12292

9.  Griffiths SR, Rowland JA, Briscoe NJ, Lentini PE, Handasyde K, Lumsden LF, et al. Surface reflectance drives nest box temperature profiles and thermal suitability for target wildlife. PLoS One. 2017;12: 1–22. doi:10.1371/journal.pone.0176951

10. Kerth G, Weissmann K, König B. Day roost selection in female Bechstein’s bats (Myotis bechsteinii): A field experiment to determine the influence of roost temperature. Oecologia. 2001;126: 1–9. doi:10.1007/s004420000489

11. Crawford RD. Bat boxes as mitigation tools: Factors impacting microclimate and Myotis sodalis roost selection. Eastern Kentucky Universty. 2020.

12. Brittingham MC, Williams LM. Bat boxes as alternative roosts for displaced bat maternity colonies. Wildl Soc Bull. 2000;28: 197–207. doi:10.2307/4617303

13. Whitaker JO, Sparks DW, Brack V. Use of artificial roost structures by bats at the Indianapolis International Airport. Environ Manage. 2006;38: 28–36. doi:10.1007/s00267-005-0117-2

14. Doty AC, Stawski C, Currie SE, Geiser F. Black or white? Physiological implications of roost colour and choice in a microbat. J Therm Biol. 2016;60: 162–170. doi:10.1016/j.jtherbio.2016.07.015

15. Lourenço SI, Palmeirim JM. Influence of temperature in roost selection by Pipistrellus pygmaeus (Chiroptera): Relevance for the design of bat boxes. Biol Conserv. 2004;119: 237–243. doi:10.1016/j.biocon.2003.11.006

16. Rueegger N, Goldingay RL, Law B, Gonsalves L. Testing multichambered bat box designs in a habitat-offset area in eastern Australia: influence of material, colour, size and box host. Pacific Conserv Biol. 2020;26: 13–21. doi:10.1071/PC18092

17. Rueegger N. Variation in summer and winter microclimate in multi-chambered bat boxes in eastern Australia: potential eco-physiological implications for bats. Environments. 2019;6: 1–19. doi:10.3390/environments6020013

18. Hoeh JPS, Bakken GS, Mitchell WA, O’Keefe JM. In artificial roost comparison, bats show preference for rocket box style. PLoS One. 2018;13: 1–16. doi:10.1371/journal.pone.0205701

19.  Tillman FE. Bat box design affects microclimate and suitability as habitat. M.S. Thesis. Indiana State University. 2019.

20. Fontaine A, Simard A, Dubois B, Dutel J, Elliott KH. Using mounting, orientation, and design to improve bat box thermodynamics in a northern temperate environment. Sci Rep. 2021;11: 7728. doi:10.1038/s41598-021-87327-3

21.  Czenze ZJ, Naidoo S, Kotze A, McKechnie AE. Bat thermoregulation in the heat: Limits to evaporative cooling capacity in three southern African bats. J Therm Biol. 2020;89: 102542. doi:10.1016/j.jtherbio.2020.102542

22. Cory Toussaint D, McKechnie AE. Interspecific variation in thermoregulation among three sympatric bats inhabiting a hot, semi-arid environment. J Comp Physiol B Biochem Syst Environ Physiol. 2012;182: 1129–1140. doi:10.1007/s00360-012-0683-6

23.  Noakes MJ, McKechnie AE, Brigham RM. Interspecific variation in heat tolerance and evaporative cooling capacity among sympatric temperate-latitude bats. Can J Zool. 2021. doi:10.1139/cjz-2020-0276

24. Rueegger N. Artificial tree hollow creation for cavity-using wildlife – Trialling an alternative method to that of nest boxes. For Ecol Manage. 2017;405: 404–412.

25. Griffiths SR, Lentini PE, Semmens K, Watson SJ, Lumsden LF, Robert KA. Chainsaw-carved cavities better mimic the thermal properties of natural tree hollows than nest boxes and log hollows. Forests. 2018;9: 235. doi:https://doi.org/10.1016/j.biocon.2017.04.022

26. Terry W, Goldingay RL, van der Ree R. Can chainsaw carved hollows provide an effective solution to the loss of natural tree cavities for arboreal mammals? For Ecol Manage. 2021;490: 119122. doi:10.1016/j.foreco.2021.119122

27.  Timpone JC, Boyles JG, Murray KL, Aubrey DP, Robbins LW, Timpone JC, et al. Overlap in roosting habits of Indiana bats (Myotis sodalis) and Northern bats (Myotis septentrionalis). Am Midl Nat. 2010;163: 115–123.

28. Bergeson SM, O’Keefe JM, Haulton GS. Managed forests provide roosting opportunities for Indiana bats in south-central Indiana. For Ecol Manage. 2018;427: 305–316. doi:10.1016/j.foreco.2018.06.009

Clay roost in Catalonia, Photo by Joy O’Keefe