Bat surveys

Bat survey methodology

Surveys to determine the presence or absence of bats are normally undertaken in two stages:

  • An initial building inspection (sometimes referred to as a Preliminary Roost Assessment or a Phase 1 Bat Survey) whereby the inside and outside of the building is thoroughly searched for bats and signs of bats.  This survey can be undertaken at any time of year.  If no signs of bats and few features such as cracks and crevices in which bats could roost then further surveys are not required, however if bats are found or the building has the potential to support roosting bats, further emergence and or dawn surveys will be required.
  • Emergence and or dawn surveys of potential summer roosts need to carried-out during the bat active season (i.e. between May and the end of August/ sub optimally till mid-October).  The Bat Conservation Trust’s bat survey guidelines recommend that up to 3 surveys may be required to determine the presence or absence of a bat roost.

If bats are found to be roosting in a building then a roost characterisation is normally required.  This ensure that there is sufficient information to determine the likely impacts of the proposals and to inform the mitigation plan.

On occasions it can be necessary to determine how bats are using areas away from their roosts, for example where a development will light up a linear feature the planning authority may want to establish what the impact of the proposals will be on bats using the site.

We carry-out bat surveys and can guide you through the planning and licensing framework for bats.

Bat Conservation

There are 17 species of Bat that are known to breed in the UK mainland with a further three   that are thought to occur as rare migrants or have small populations in the UK.

Bat populations have undergone a significant decline in the past sixty years.  For example, estimates from the National Bat Colony Survey suggest that the UK pipistrelle population (one of our commonest bat species), declined by approximately 70% between 1978 and 1993.  Factors contributing to this decline include:

  • Loss of, and damage to, roosting sites, including buildings, trees, and underground structures (mines, tunnels, ice-houses, cellars, etc).
  • Loss and fragmentation of suitable insect-rich feeding habitats such as wetlands and deciduous woodland.
  • Reduction in the abundance and diversity of insect prey due to intensive agriculture, particularly over-grazing and the use of pesticides.
  • Loss of linear features such as tree-lines and hedgerows, depriving bats of commuting routes between roosts and feeding areas.
  • Loss of winter roosting sites in buildings and old trees.
  • Disturbance and destruction of roosts, including the loss of maternity roosts due to the use of toxic timber treatment chemicals.

Bat Roosts

Bats use a variety of roosts of different types including trees, buildings, caves, mines and other structures.  Most species are colonial and roost in groups.  This can make populations particularly vulnerable to loss of roosts as the loss of a single roost may affect the whole population.  Some species hang in obvious locations, such as the timbers near to the apex of a roof, others roost in cracks and crevices, such as the gaps under tiles, and as such can be very difficult to locate.

During the winter (November – February), when there is a reduction in insect numbers, bats hibernate to conserve energy.   They prefer sites with a constant low temperature and a high relative humidity.  On mild winter’s nights, bats may wake up and feed.   However, bats are particularly vulnerable to disturbance at this time of year as, flying in winter uses up large quantities of energy that cannot easily be replaced.

In the spring, after emerging from hibernation, bats often move from site to site and may congregate in small groups.  Female bats gather together in the summer (approximately May to August dependant on species) in maternity roosts.  Once the young have stopped suckling, and the baby is independent, bats tend to disperse and use other roosts.  Maternity roosts are particularly vulnerable to disturbance, as bats may have come from a wide geographical area, and have a strong tradition of returning to the same roost year after year.

During the late summer and early autumn males occupy mating roosts which are visited by several females.  After mating some species gather together at swarming sites to fatten up prior to hibernation.

Bat Habitats

In addition to roosts, bats also need foraging habitats, to find suitable food resources, and, commuting routes to get to these areas.  As would be expected, the highest numbers of bats are found in areas with abundant invertebrates.  Some species specialise in catching small invertebrates in flight, whilst others specialise in catching larger invertebrates such as moths and beetles.  The distances that bats travel to foraging areas varies between species, records have shown some greater horseshoe bats travel up to 22km to forage, although many species will typically feed within 1km of a roost.

Bats tend to follow dark linear features (such as hedgerows and tree lines) to commute to their foraging habitats.  Lighting and gaps in these linear features can discourage commuting bats, particularly early in the evening, and it is important that gaps in linear features and lighting are minimised if these features are used by bats.