Bats and Hibernation
Bats depend on invertebrates as their food source, with some species consuming thousands in one night. During winter invertebrates decrease in numbers and so food is less available. As a result, bats go into hibernation from mid-October to March, after building up a fat reserve to see them through the winter. This process allows them to save energy over the colder months.
Bats hibernate either on their own or in small groups, depending on the species, size and type of roost. Roost conditions should be cool and stable, with high humidity to minimise the risk of bats becoming dehydrated. Bats are vulnerable to disturbance and predation as they can take up to 30 minutes to become fully alert after torpor or hibernation. Examples of hibernation roosts include disused railway arches, caves, trees and buildings. The common pipistrelle is one of the most widespread species of bat in the UK and yet very few hibernation roosts for this species have been identified. The success or failure of a bat to survive winter depends on finding the correct conditions and some species have different requirements at different times.
The overall aim of surveying at a proposed development site is to collect robust data to allow an assessment of the potential impacts a proposed development will have on the bat populations on and around the site (Hundt L., 2012). If a building or structure is thought to have potential to support hibernating bats, then further surveys are required. These comprise two inspections carried out at least a month apart, within the period of December – February. Features that may be considered suitable include large trees, cavity walls with access points, basements or tunnels and bridges. Unlike during the summer, the roofs of buildings have very little potential to support hibernating bats.
When it is not possible to fully rule out the presence of a bat roost in a feature, or structure based on a visual inspection, or when a non-hibernation bat roost is identified, bat surveys are undertaken to establish the presence or likely absence of bats, or to establish their patterns of use of that roost. As bats are inactive during the hibernation season, to detect if bats are present the timing of surveys is dictated by when bats are most active, generally between May and August inclusive. Surveys are also possible in April and September but they are considered as sub-optimal.