Influence of bat house design on hibernating bats - a case study in Herefordshire (UK)

Abstract

In England, bats and their roosts are protected by national legislation. To permit development actions that would otherwise result in an offence relating to bats, it is first necessary to obtain a protected species mitigation licence containing protective measures. Due to the complexity of the topic, combined with the fact that monitoring is often limited, it can be difficult for practitioners to be certain of real conservation benefits of these measures. To build a new access road near Hereford (UK), a former artillery magazine (confirmed bat roost) building was demolished. Therefore, a legally binding English Nature/Natural England European Protected Species (EPS) Development Licence was obtained (2005). This licence stipulated mitigation and compensation measures to ensure the works could be carried out without harming bats and ensuring their favourable conservation status was maintained. Roost compensation measures were applied to two identical retained buildings. These included blocking doorways, provision of bat access grilles/internal roosting crevices, diverting downpipes inside, and installing straw matting (approx. 5cm deep, within one building only). The latter two measures were designed to increase internal humidity levels. Pre-compensation monitoring recorded two hibernating common pipistrelles in addition to lesser horseshoe and brown long-eared bat droppings. Post-compensation monitoring (2006-2016) recorded a minimum of three brown long-eared bats, three lesser horseshoe bats, one common pipistrelle and one barbastelle, suggesting the compensation methods may have increased both the numbers of species, and individual bats. These increases were small, hence not conclusive. Notably, during the post-compensation hibernation monitoring, brown long-eared bats were found in areas with lower humidity levels (48.6-78.8%) than lesser horseshoe bats (67.8-93.5%). The magazine containing straw matting had winter humidity levels approximately 20% higher than the other and supported a higher number of hibernating lesser horseshoe bats, but a lower number of hibernating brown long-eared bats. Within both buildings, all hibernating brown long-eared bats were found behind chipboard (approx. 70cm x 150cm) attached to wooden battens approx. 2cm from the internal walls rather than wooden or sawdust/ cement composite bat boxes.