Success of Brandenbark™, an Artificial Roost Structure Designed for Use by Indiana Bats (Myotis Sodalis)

Abstract

Abstract. The federally endangered Indiana bat (Myotis sodalis) is a concern for development projects in nearly half of the United States. The species roosts and rears young under exfoliating bark of trees, which has put it at risk for incurring adverse impacts from most projects that require tree clearing throughout its summer range. Project proponents generally incorporate avoidance and minimization strategies into the planning process. These strategies, however, are not always compatible with project goals and objectives, and mitigation is often required to offset adverse impacts to the Indiana bat. BrandenBark™ is an artificial roost structure that mimics the natural roosting habitat of Indiana bats. To date, 69 BrandenBark™ structures have been installed in 7 states (IL, KY, LA, OH, PA, TN and WV). Of these, 59 (86%) structures have been used by 6 species of bats, including northern long-eared bats (proposed for federal listing) and little brown bats (under status review); however, the majority of use (85%) has been by maternity colonies of Indiana bats confirmed by radio telemetry, capture, or genetic analysis of guano. Of the structures used by Indiana bats at Fort Knox Military Installation (n=21) in north-central Kentucky, 120 emergence counts have been conducted with an average of 81.3±7.1 bats per roost. Although the roost area under BrandenBark™ is slightly warmer ( X = 24.6±7.2°C [SD]) than that of natural bark ( X = 23.1±6.5°C), the temperature difference between BrandenBark™ and ambient ( X = 2.1±2.7°C) is less variable than the temperature difference between natural bark and ambient ( X = 3.9±4.0°C), possibly indicating a more stable thermal environment. However, both roost types are warmer than corresponding ambient temperatures. BrandenBark™ provides instant long-lasting habitat commensurate with natural roosts, is easy to install and monitor, and does not require the purchase of additional land for placement when used as a mitigation option.

In artificial roost comparison, bats show preference for rocket box style

“…The bark-mimic provided less usable space than the rocket box and, often, large proportions of the roost exceeded T CR . While all of these roost styles have documented use by Indiana bats [ 22 , 28 , 29 ] and are often used in mitigation, this is the first study to group these structurally-different roosts to assess microhabitat and selection. We cannot distinguish which of the characteristics of rocket boxes were the primary driver of roost selection, or if a combination of factors was at play because these common artificial roost designs differ with respect to multiple physical factors.…”

“…We mounted all roosts on posts with the roost top at 6 m. Tree roosts for Indiana bats average 8.6 m in height [ 36 ], though height relative to neighboring trees may be more important than absolute height [ 18 ]. To ensure the three styles were comparable with respect to mounting post dimensions, we cut a bark sheet (130 × 100 cm, full size) in half lengthwise (130 × 50 cm, half size) to fit the circumference of a 12-cm diameter post rather than wrapping a full sheet around a 30-cm pole as specified by the distributor [ 22 ].…”

“… Rocket box provided the greatest surface area, entrance area, and volume. a [ 28 ] b [ 29 ] c [ 22 ] …”

“…We can reduce some of the uncertainty regarding roost habitat preference by comparing bat use and microhabitat characteristics of replicated, structurally-specified artificial roosts of different designs. Various temperate-zone bat species use a wide array of artificial roost styles, likely presenting varied microhabitats—e.g., large, freestanding structures that accommodate thousands of bats [ 19 ], small boxes designed to mimic cavity roosts [ 20 ], and bark- and crevice-roost mimics (e.g., rocket box, [ 21 ]; bark-mimics, [ 22 – 24 ]). When assessing microhabitats, it is important to exclude bats for microclimate measurements, unless specifically considering the structure’s metabolic heat retention, as number of bats in a roost affects the internal temperature [ 25 , 26 ].…”

“…Although we can control bat presence and other factors in artificial roosts, there are relatively few studies comparing microhabitats in various artificial roost styles (see review, [ 27 ]). Despite our limited knowledge, resource managers are deploying a variety of artificial structures to provide bat habitat and evaluating success based solely on use by bats (e.g., [ 22 , 28 , 29 ]). Artificial roosts are often deployed without comparative evaluations of features that may contribute to selection (but see, [ 23 , 30 , 31 ]).…”