Researchers at the University of Illinois have used meticulous temperature measurements to help optimise the design of artificial roosting boxes for bats.
According to the researchers, bat box designs vary widely and many commercial varieties are untested and risk cooking the animals that they’re designed to shelter. The boxes are often small and painted dark colours, meaning that internal temperatures may rise to dangerous levels on sunny days in summer.
‘Previous work made broad temperature comparisons among a hodgepodge of bat box designs, usually at only one spot inside the box. In those kinds of studies, it’s hard to know the full temperature gradient bats experience or which design elements are most important for maintaining optimal temperatures,’ said Joy O’Keefe, an assistant professor and wildlife extension specialist in the Department of Natural Resources and Environmental Sciences at the university.
O’Keefe’s team built a so-called ‘rocket box’ – a one-metre-long, four-sided box with two vertical chambers on each side – and then created 18 variations on the them, adding or taking away vents, changing the construction material, roof design and overall length, and adding various insulating jackets. They also built a standard three-chambered flat-front design as a 20th point of comparison.
The researchers then positioned 12 temperature sensors at the top, middle and bottom of the outer chamber on each side of each box. ‘There are some studies that put a sensor in each of the chambers in a three-chamber box, or they put sensors at the top and the bottom of one design. But ours is top to bottom in all four cardinal directions, so it is unique,’ O’Keefe says.
The boxes were set atop five-metre posts along a gravel road in a mixed woodland and agricultural landscape in Indiana, but were kept closed so that the researchers could understand the temperature gradients without the complication of bat body heat. ‘A rocket box can hold more than 200 Indiana bats; such a mass of warm bodies could increase roost temperatures from stressful to lethal on hot days,’ explained Frank Tillman, a former graduate researcher who worked with O’Keefe.
The results showed that conditions changed dramatically inside the boxes over the course of a day. ‘When a bat chooses a box near sunrise, they all pretty much look the same – they’re all cold. So the bats can’t really know what they’re getting into. Then, by the middle of the day, temperatures are starting to get comfortable. Everybody’s hunky dory. It’s the end of the day where you can get these deadly temperatures. We’ve seen temperatures in the bat boxes being 68 degrees Fahrenheit [36 degrees Celsius] higher than the outside air temperature,’ O’Keefe said.
Generally, the tops of the boxes were heating up more, and more rapidly, than the middle and bottom portions; however, because rocket boxes are long and four-sided, the bats could theoretically move vertically or away from the sun-blasted sides of the box. A longer box, such as the 1.5-metre-long design tested in the experiment, offers a greater range of temperatures.
In the end, many of the boxes either reduced the risk of overheating or would enable bats to move to areas in which they could avoid extreme temperatures. According to O’Keefe, there’s still a lot of experimentation to be done, but in the meantime, she maintains a website that provides tips for making bat boxes safer for bats.
The research has been published in Ecological Solutions and Evidence.