Mitigation

Bats have very specific requirements and their choice of roosts is governed by factors such as temperature, proximity to suitable feeding habitat and, for a building, its age and construction. As a consequence, the creation of a new replacement roost that will be equally attractive to the bats as their original home can be difficult to achieve and the outcome often uncertain. Mitigation encompasses a range of measures to minimise and offset the impacts of a development on the affected bat population. These are to:

• minimise fragmentation/isolation of bat colonies
• minimise direct impacts on bat roosts
• minimise direct loss of foraging habitat
• incorporate measures to mitigate against the loss of continuity in key commuting corridors e.g. green bridges
• create new habitats or roost sites of real value to bats

If a bat roost will be unavoidably lost to development, knowledge of how the bats use the surrounding habitat, notably the identification of key flyways, will help in determining where a replacement roost should be located. The mitigation scheme should be informed by detailed survey undertaken in advance of an application for planning permission. The more information that can be collected on how the bats use their existing roost and the nearby habitat, the more likely the mitigation measures are to be successful.

Any proposed mitigation plan must be tailored to the circumstances involved. For the proposal to be acceptable, it must have a reasonable chance of success. There is a need to encourage novel techniques and support experimental studies to test the value of these. The two links (right) to the BCT Bat Mitigation Conference in 2007, provide examples of attempted mitigation in a range of circumstances, covering a range of species (including some that do not occur in Scotland), but note that the outcome is not always known.

Timing of works

Where proposed works will unavoidably affect bats or their roosts, they should be programmed to take place when the bats are not present. In general, works affecting a maternity roost, should normally only take place between the months of October to March inclusive. However, where major roof repairs are required, work may only be practical during the spring or late summer/early autumn to avoid the worst weather. In such cases, the months of April and September can be included in the timing of works. However, bats may be present for part of this time so special provisions may apply which would need to be specified on the licence. No work should take place between mid May and mid August in a maternity roost. Conversely, a hibernation site should not be disturbed during the period November to March inclusive and work may only take place in the remaining months of the year. Work in mating/swarming roosts should not take place during August to October inclusive. Note that weather conditions in the spring and autumn affect the above general timings - a cold wet spring will delay when female bats return to a maternity roost and this will have an influence on the date when work in the roost must cease. Similarly if the autumn is particularly cold, bats will enter hibernation earlier.

Artificial bat roosts

Currently, the most effective form of mitigation for the loss of maternity roosts of pipistrelle species is the heated bat house. This was originally based on an American (unheated) design available from Bat Conservation International  . The heated version was developed in Britain for our cooler, less predictable climate and various models are now commercially available. A wide range of other (unheated) bat boxes are also commercially available including traditional wooden boxes to the much more durable modern woodcrete boxes.