Identifying bat roosts in trees presents several challenges for ecological consultants. The traditional survey process of undertaking ground level tree assessments followed by emergence and return to roost surveys of potential roosting features has several notable limitations. In this article, we explore how tree climbing can be used alongside traditional survey techniques to identify the presence or likely absence of bat roosts in trees with greater confidence and efficiency.
Limitations of traditional survey techniques
The first step in the traditional method of surveying trees for bats comprises looking for potential roosting features from ground level using binoculars and a powerful torch. However, this assessment technique is only reliable where all potential roosting features are fully visible from ground level. This means that consultants are regularly left to make judgment calls, and consequently the level of further survey recommended is often higher than would otherwise be necessary. Features that appear suitable for bats from ground level frequently turn out to be unsuitable upon closer inspection. For example, woodpecker holes that turn out to be superficial or rot holes that only protrude downwards and therefore collect rainwater.
Dusk and dawn surveys present additional challenges; not least of which is identifying bats emerging or returning to roost when trees are in full leaf (which happens to neatly coincide with the active season for bats). Lastly, traditional survey methods present few options for identifying the presence of hibernation roosts within trees. This can leave ecological consultants and their clients in a tricky legal dilemma.
Tree climbing surveys in the existing guidelines
The use of tree climbing to identify bat roosts has grown in popularity over recent years. However, as a survey technique for identifying bat roosts it is not well documented in the guidance. There is little mention of tree climbing in the 2012 Bat Conservation Trust’s good practice guidelines (we hope this will be addressed in the third edition), and the draft British Standard 8596:2015 ‘surveying for bats and trees in woodland’ only currently addresses tree climbing as a survey tool to assess known roosts.
Guidelines for tree climbing surveys for bats
Lets start with the basics. Tree climbing surveys simply allow surveyors to access trees using a rope and harness to check potential roosting features using an endoscope and torch. As well as looking for bats or evidence of bats, surveyors may also assess the suitability of features to support roosting bats.
In order to conduct the survey, surveyors should be certified in tree climbing and aerial rescue and should be accompanied by a second climber at all times. As there is a reasonable likelihood that bats may be disturbed during the survey, the surveyor should also hold a level 2 bat survey licence (or equivalent).
Trees can be climbed at any stage of the survey process; however, we encourage tree climbing to be undertaken as early in the process as possible; either prior to any previous assessment or after the initial ground level tree assessment has been undertaken. Providing all potential roosting features can be fully assessed, we can see no reason why the tree climbing inspection may not be counted as the equivalent of either one emergence or one return to roost survey providing it is undertaken during the active season for bats. Lastly, trees with the potential to support hibernation roosts may be climbed between December and February to establish the presence or otherwise of hibernating bats. We have put together a simple flow chart to assist in the preparation of surveys.