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Bat Emergence Survey (Phase 2)

This survey is conducted at dusk and dawn and is seasonally restricted to between the months of May and September inclusive. It is sometimes required where bats, evidence of bat activity or habitat value is present at your site.

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Bat emergence surveys

The decline in bat populations across the 18 native species in the last 50-100 years has been linked to the advance in the footprint of our towns and cities and subsequent loss of high quality bat roosting and feeding habitat, throughout the UK.

In response to this, government has transposed the EC Habitats Directive in the UK, through the Habitat Regulations 2010, effectively criminalising the disturbance of bats and their habitat.

This doesn’t mean that the presence of bats or their habitats can stop you from obtaining a planning consent. It simply means your local planning authority will need to see evidence that you can provide mitigation appropriate to the species of bats, population and roost type that is present at your site of proposed development.

This ‘Phase 2’ level of our professional bat survey is appropriate in the circumstances only if you have already obtained a preliminary roost assessment (from us or any other consultant) that identifies triggers for and makes a clear recommendation for (two examples of triggers might be; very high quality bat habitat, or bat droppings).

The way this is achieved is by undertaking scientifically rigourous emergence and/or dawn re-entry surveys, in accordance with the provisions of the Bat Conservation Trust publication; Bat Surveys – Good Practice Guidelines (BCT Guidelines), which generally limits the survey season to May through September inclusive.

How We Work

Bat emergence surveys are not especially complicated, though we appreciate it must certainly seem that way if this is your first experience of bats and planning!

We use sophisticated detectors (that convert the bats’ echolocation calls, beyond the range of human hearing) to produce a frequency readout – a sonogram – that in turn identifies the species of bat.

In addition to this, where practicable, we set up night-vision cameras to record activity within e.g. a roof void. You can watch a YouTube video of this below, where during the 2013 bat emergence survey season, we recorded a brown long-eared bat (Plecotus auritus) in a roost in Surrey, southeast England.

So Where to Begin?

Let’s start with why there is an emergence survey season. Bats are mammals that predate on insects, which in the vast majority are not prevalent in the winter months. During the warmer, drier months, bats emerge in the evening, generally just before dusk, to use their highly specialised echolocation calls to literally catch insects on the wing.

They will gorge themselves all night and return to roost, just before or after dawn. Different species have slightly different preferences in the timing of emergence and re-entry from and to their roosts, so the best practice is to arrive and set up your survey well before dusk.

During the course of the emergence survey, if bats are indeed using your building as a roost, which hopefully they aren’t, the focus is on collecting and indexing three types of data:

  1. What species of bat are present?
  2. What is the population (or best approximation with large numbers) for those species?
  3. What are they using your site for?

This process and the results will inform your bat survey report as to what level and type of mitigation is appropriate. Clearly a single male soprano pipistrelle (Pipistrellus pygmaeus) using your barn as a transitionary summer roost, will require very different mitigation to e.g. a maternity colony of the ultra-rare Barbastrelles (Barbastella barbastellus).

Bat survey mitigation

What is Bat Mitigation?

As bats hibernate in temperate climates (bat surveys can only be conducted between May and September, and even then are weather dependent), to avoid being active at times where the insects they predate on are much less abundant, so they have to undergo much biological change in order to survive this period – called hibernation.

During hibernation a bat’s metabolism drops significantly; it lives entirely off fat stores built up during the summer; its heart will beat only a few times a minute; and it will allow it’s temperature to drop to match it’s surroundings (within reason) thereby minimising heat loss.

This ‘suspended-animation’ state is known as torpor. In consequence, bats are very sensitive to being disturbed while in a state of torpor and indeed, to do so could kill a bat.

For this reason, often your mitigation needs to be executed outside of the hibernation season, which is typically though not always November through April inclusive.

That being the case, delays and costs associated with this are implicit. The earlier you engage us, the better informed you will be and the faster we can resolve any issues on site.

Bat mitigation itself, as a rule, is simple. Provide for bats so there is no net loss of habitat and you can’t really mess it up.

Oftentimes, say in the case of a single common pip (Pipistrellus pipistrellus), this could be as simple as installing some bat boxes at your site, or making use of innovative building products, like ‘bat access tiles’, that can be finished and installed to ensure they don’t stand out or make your property look unsightly.

Of course, it can get more complex than this. Larger species of bats, like the brown long-eared bat (Plecotus auritus) are not crevice dwellers, like their pipistrelle cousins. They require a void in which to make short flights that prepare their body for the strenuous nature of hunting flying insects – like an athlete warming up if you will – so need to elevate their heart rate and pump some blood into their wings before emerging from their roosts.

This void can quite easily be a section of your roof void or loft space, partitioned off, but if that presents an issue – say if you are planning a loft conversion for instance – then other solutions that fit into your development plans need to be developed. Like this for instance.

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