When Winter Bat Surveys are Appropriate: Constraints and Misconceptions

Winter is often regarded as a period when bat surveys cannot be undertaken. While this is generally correct for activity-based assessment, there are specific situations where winter survey methods are appropriate, proportionate, and informative. Misunderstanding the limitations of winter surveys can lead to flawed expectations, unnecessary delay, or inappropriate design assumptions. This article clarifies what winter surveys can and cannot achieve within the Scottish regulatory framework, and when they form a useful component of ecological due diligence.

1. What winter surveys cannot provide

The most common misconception is that winter surveys can determine likely absence or confirm that a structure is not used as a bat roost. Winter conditions do not allow for reliable activity-based assessment, as bats enter long periods of torpor and are unlikely to emerge predictably (NatureScot 2020). As a result:

• Winter surveys cannot be used to demonstrate absence.

• Winter surveys cannot replace the requirement for summer activity surveys where these are necessary under NatureScot guidance.

• Winter acoustic surveys do not provide reliable data on species diversity or patterns of use.

Winter findings must therefore be interpreted within clear methodological limitations.

2. When winter surveys are appropriate

Despite these constraints, winter surveys have legitimate applications. The following survey types can provide valuable supplementary data:

2.1 Internal building inspections

Where access is available, winter offers an opportunity to inspect roof spaces, wall heads, voids, and structural features for physical evidence of bat use. This may include droppings, staining, feeding remains, or structural features with roost potential (Bat Conservation Trust 2023). Winter inspections are particularly helpful where bat activity surveys cannot be completed before early design or budgeting decisions.

While they cannot confirm absence, they can identify positive indicators of roost presence or confirm that specific voids are unsafe or inaccessible, helping refine the scope of future surveys.

2.2 Tree inspections

Winter is often the most practical time for detailed tree assessment because foliage is absent and features such as cavities, splits, and lifted bark are easier to identify. Under NatureScot guidance, preliminary roost feature assessment can be undertaken year-round (NatureScot 2020).
Again, this does not confirm absence but supports proportionate survey planning.

2.3 Hibernation site checks (licensed personnel only)

Inspections of known or suspected hibernation sites may be permitted under licence, typically involving minimal disturbance and conducted only by experienced ecologists. These surveys are highly controlled and used mainly for conservation monitoring or to support risk assessment at structures such as bridges, tunnels, or underground sites. They are not a substitute for summer surveys.

2.4 Project programming and risk evaluation

Winter surveys can contribute to risk-based assessments where developers require clarity on programming, feasibility, or budget implications. Identifying high-potential roost features early in the planning cycle helps avoid unrealistic assumptions about survey windows and mitigation requirements.

3. Why timing misconceptions cause project delays

A frequent source of delay arises when clients expect winter surveys to fulfil evidential requirements for planning or licensing. Because licensable works near roosts require robust summer activity evidence, projects that rely solely on winter inspections often face avoidable seasonal hold-ups (NatureScot 2020; BCT 2023).

Similarly, winter acoustic data may be incorrectly interpreted as proof of absence. This can lead to inappropriate design decisions, underestimated mitigation requirements, or delays when regulators request additional information.

4. Integrating winter surveys into a proportionate strategy

When properly applied, winter surveys form an important part of a proportionate, risk-led ecological strategy:

• Feasibility and early design: winter inspections identify constraints early enough for design adaptation.

• Capital budgeting: awareness of potential roost features informs realistic programming and resourcing.

• Scoping: winter surveys refine the scale and complexity of planned summer surveys.

• Asset management: for infrastructure managers, winter inspections can inform maintenance schedules and identify features requiring specialist attention.

Winter surveys therefore support—rather than replace—other methods.

5. Lessons for developers and project managers

Key principles for avoiding misconception-driven delays include:

• Treat winter surveys as supplementary evidence, not determinative proof.

• Do not assume that a lack of winter signs indicates low risk.

• Recognise that licensing decisions rely heavily on summer evidence, particularly for roost characterisation.

• Engage ecological specialists early to interpret winter findings within an appropriate regulatory context.

When these principles are applied, winter surveys can contribute meaningful risk management without creating unrealistic expectations.

Conclusion

Winter bat surveys have clear limitations, but when used correctly they provide valuable early-stage information that supports proportionate survey design, risk management, and programming. Misunderstanding their purpose is a source of unnecessary project delay. Understanding when winter surveys are appropriate—and when they are not—is essential for legally compliant and well-sequenced development in Scotland.

References

Bat Conservation Trust 2023. Bat Surveys for Professional Ecologists: Good Practice Guidelines. Bat Conservation Trust, London.
NatureScot 2020. Bats: Guidance for Professional Ecologists. NatureScot, Inverness.
Scottish Government 2019. Wildlife Crime and Protected Species. Scottish Government, Edinburgh.