How to spot a bad bat survey

How to Spot a Bad Bat Survey

Daniel Bennett, Glossop Bat Group.

Bat surveys are a legal requirement in many planning applications and local authorities have a duty to protect bats and ensure that any threats from development are properly assessed. Unfortunately bat surveyors are not subject to any form of formal verification and the quality of bat surveys varies enormously. This document has been produced to help people involved in planning to spot bad bat surveys and respond accordingly.

The standard for bat surveys is Hundt (2012) which sets out the minimum requirements for bat surveys in different types of development and in different habitats, provides a standard format for bat surveys produced in support of planning applications and explains how developments that will affect bats should proceed. However, without some knowledge of bat biology  and relevant legislation it can be very difficult to spot bat surveys that have been conducted incorrectly or reach spurious conclusions. This document is based on bat surveys published on local authority websites in the vicinity of Glossop.  It identifies the main weaknesses of some surveys and makes recommendations for improving them. It is produced in good faith and in the hope that it will save both developers and planning department staff time and money, and result in better protection of bats.

Bat surveys differ from other surveys produced for planning purposes in that they are not reproducible, and therefore not “proper science”. The results of surveys for trees, contaminants, archaeology, flood risk etc. can be verified by additional site visits, but a follow up visit to a bat survey site will rarely produce the same results as the original survey. Bats are very inconspicuous and their behaviours change regularly according to climate and season. Activity surveys on one evening can produce entirely different results from a survey the following evening.  Most species of bats shift roosts frequently and so at best a bat survey can only represent a snap-shot of how bats use a site. It is the duty of the bat surveyor to ensure that the survey is conducted in a way that will maximise the likelihood that any bat activity on the site is recorded. Bat surveys often fail to do this, and they regularly include recommendations that are illegal under the legislation protecting bats and their roosts. The following points address some of the most serious and common faults with bat surveys that I have encountered.

  1. Bat roosts are protected irrespective of whether bats are currently occupying the roost. Many surveys attempt to evade licensing and mitigation by identifying potential roosts and recommending that roost destruction is done at a time of year when bats are unlikely to be present. This is illegal.  The destruction of any bat roost requires exemption from the law through licensing and mitigation from Natural England. There are no circumstances under which bat roosts can be destroyed without a licence.
  2. Except in exceptional circumstances, bat surveys for development must include activity surveys. Even the most talented and best-equipped bat scientists in the world cannot expect to learn much about how bats use a site without visiting it at night.
  3. Surveyors must use equipment that will allow them to detect all species of bats that might be present, and all behaviours that bats might be expected to exhibit at the site. Most bat surveys rely on bat detectors to find bats, because bats produce ultrasound to navigate in the dark, find food and communicate with other bats. Bat detectors are devices that make sounds at frequencies beyond human hearing audible. At their simplest they are microphones capable  of recording all sound up to at least 100khz; at their most complex they scale the highest amplitude frequency by a certain factor and then use an “amplitude restore principle” to restore changes in volume. The prices of bat detectors range from around £40 to over £3000 and they vary greatly in their ability to pick up the noises that bats make. Most of the bats in Glossop are very noisy and even the cheapest detector will pick up their foraging calls. However, one species, the long-eared bat Plecotus auritus, is very quiet, sometimes hunting silently, and many bat detectors will fail to detect their presence. Long-eared bats are almost always found close to  woodlands and are rarely found in well lit areas.  However they are one of the most common bats in the U.K. In order to ensure that long-eared bats are detected by surveys, they must rely on methods other than detection of ultrasound.  The usual method is to use night vision equipment.

Because bat detectors vary widely in their ability to detect and interpret bat calls, at least two of the following types of bat detector should be used in all bat surveys; heterodyne, frequency division, time expansion. This is basically because heterodyne recorders can only monitor one frequency at any time, frequency division detectors can only record one sound element at a time (and frequently miss social calls) and time expansion detectors can only sample 6 minutes of every hour. The use of real-time full-spectrum bat detectors eliminates the deficiencies of other methods, but their use is very rare in local bat surveys. The “Anabat” system relies on frequency division principles and is not a full spectrum method.

The make and models of the bat detectors and night vision equipment used should be stated in the survey to ensure that the equipment used is suitable for the purpose.

  1. Surveys have to be conducted at times when bats are likely to be detected. Some species of bats in Glossop (Pipistrellus and Nyctalus) often emerge around sunset and return to their roosts around sunrise. But others do not emerge until light levels are low, often an hour or more after sunset. Plecotus and Myotis bats in particular are unlikely to be detected in surveys that end less than two hours after sunset or start less than 90 minutes before sunrise. Many surveys conducted in the area end when light levels become too low for bats to be easily visible, but unfortunately this is the time when many species begin to emerge. The dates and times of surveys should be stated explicitly in reports along with sufficient climatic data to demonstrate suitable weather.
  2. The identity of all surveyors must be made clear, along with details of their relevant experience. Bat surveyors vary in their ability and none are able to be in more than one place at a time. It is clear from many surveys that the manpower and equipment used was insufficient to adequately monitor bat activity at the site. Even the smallest house requires at least two people to watch for bats emerging from the roof. The problems are reduced to some extent in surveys conducted before dawn because although bats tend to leave their roosts quickly and fly away immediately, they spend much more time flying around the roost before they return in the mornings and are much more conspicuous. Suitable nightvision equipment is invaluable for properly surveying potential roosts and in many cases can reduce the manpower required to adequately survey a site.

Bat surveyors must be suitably experienced to conduct surveys. Some bat surveys that I have seen show ignorance of very basic bat biology and surveying methodology. Bat surveying is a multi-disciplinary task that requires field skills only acquired through experience.  Many of the UK’s best bat surveyors have no relevant qualifications, and some highly qualified bat surveyors may have virtually no experience of bat surveys. Without a synopsis of surveyors’ experience it is not possible to determine if they are likely to be capable of assessing threats to bats in a competent manner. Membership of trusts, groups and clubs, or professional bodies such as IEEM, does not indicate a competency to conduct bat surveys properly.


  1. The number of bat surveys conducted at a site and their spacing through the year should be in line with best practice recommendations. Table 7.2 of Hundt (2012) specifies minimum recommended site visit frequency and timing for bat activity surveys, and Table 8.5 gives the minimum number of visits required to provide confidence in negative preliminary roost assessments for buildings and trees for surveys conducted for planning purposes. For sites of less than 1ha that are deemed medium quality bat habitat, three visits between March and September are required, one of which should include dusk and predawn sampling within a 24 hour period. For low quality habitats, two activity surveys are required. Similar numbers of visits are required to assess roost potential. These surveys should be approximately spaced through the period of sampling. Other parts of Hundt (2012), specifically Box 2.1 (planning and development trigger list for bat surveys), Table 4.2 (guidance for assessing development sites for bats), Table 8.2 (features of buildings that increase likelihood of bats being present), and Box 8.4 (protocol for visual inspection of trees) give explicit details of when bat surveys are required. In many published surveys where night time visits are made at all, they are limited to a single visit or several visits within the space of one week. In most situations this is insufficient to assess bat activity.
  2. Bat surveys must include a search for records of bats in the area beyond those held by Derbyshire Wildlife Trust.  Many recent surveys report the same three records of bats in Glossop held by DWT. Better sets of records exist (e.g. National Biodiversity Network's Gateway at and the records held by the Glossop, Derbyshire and South Lancashire Bat Groups. Desk surveys should attempt to collect as much information as possible about bats in the area.

Glossop Bat Group are very happy to work with all parties to ensure that bats in Glossop receive the protection they are entitled to.



Unfortunately there is no statutory national standard for bat surveyors, and best practise is enshrined in Hundt L (2012) Bat Surveys: Good Practice Guidelines, 2nd edition,. Bat Conservation Trust. ISBN-13: 9781872745985

The following is based on a review of bat surveys on the HPBC planning portal in 2008 and 2011. It identifies the main weaknesses in bat surveys conducted in the area.