Guidance note - Assessing the significance of impacts on bird populations from onshore wind farms that do not affect protected areas

Published: March 2025

Scope and purpose

The purpose of this guidance is to assist with assessments of terrestrial wind farm proposals where potential effects do not affect notified interests or qualifying features of protected areas (SSSI, SPA or Ramsar sites). It is primarily directed at developers and consultants and provides a framework for assessing effects on bird populations for an environmental assessment report. It may also be useful for NatureScot staff and consenting authorities when they are consulted on proposed developments. The guidance describes the assessment process and the information required to assess the significance of effect on any species.

The guidance is not appropriate for assessment of offshore wind farm developments although some species may be exposed to impacts from both onshore and offshore developments. When assessing cumulative effects, the effects of offshore developments may need to be incorporated into onshore assessments for some species.

Legal context

The guidance takes account of the legal and policy obligations which apply where protected areas are not affected. These include the EU Birds Directive and Habitats Directive, the Scottish Biodiversity Strategy and Scottish Government’s National Planning Framework 4 and Scottish Planning Policy. 

The Conservation (Natural Habitats, &c.) Amendment (Scotland) Regulations 2019 updates earlier versions of the Habitats Regulations, ensures that we continue to apply the requirements of the Birds and Habitats Directive post-Brexit and encompasses devolved matters for Scotland’s land and inshore waters.  The amended Regulations require Competent Authorities to take the necessary steps to secure the preservation, maintenance or re-establishment of a sufficient diversity and area of habitats for wild birds in Scotland. 

Introduction and background

The number of onshore wind farm developments has increased markedly in recent years resulting in an increasing need to identify significant effects on habitats and species. They can impact on bird populations in four ways:

• Direct mortality or injury from collision with turbine blades or towers.
• Loss of habitat due to construction of wind farm infrastructure.
• Displacement through indirect loss of habitat if disturbance causes birds to avoid wind farm and surrounding area (in some cases, this may only be temporary e.g. some species may habituate to infrastructure once built and may return to use habitat close to/within wind farm. This may need considered for the assessment)
• Barrier effects if populations are prevented from reaching some destination.

The assessment process

Understanding the impact of a wind farm on bird species should follow a defined process.  The stages are:

1. scoping to identify the bird species that require assessment;
2. undertaking survey work (see NatureScot, 2017);
3. quantifying the nature, direction and magnitude of impacts on the relevant species; and
4. assessing the significance of likely impacts on these species and their populations. This will depend on: the conservation status and sensitivity of the species and its supporting habitats; its abundance in the area; any special ecological role fulfilled by the site in question; and the resilience of the population in the face of potential losses. It should also consider any mitigation, compensation or enhancement measures that may be intended to eliminate or reduce such adverse impacts.

The scale at which likely impacts are assessed is key to the assessment of significance.  Scale is addressed in detail later in this guidance.

The assessment process needs to demonstrate that the proposed wind farm will not affect the maintenance or recovery of a species’ Favourable Conservation Status (FCS). Where the species forms a qualifying interest of a Special Protection Area (SPA), it may also be necessary to carry out an appropriate assessment of the effects of the proposed development on the integrity of the site. Wind farm developments that have an adverse effect on a species’ conservation status may require amendment or mitigation included in the proposal or, in some exceptional cases, may be the basis of an outright objection.  We will only object to a proposal that does not have an impact on a species within a protected area if we consider the consequences of an approval raise issues of national interest. More information about national interest is available on our website in the guidance on Identifying Natural Heritage Issues of National Interest in Development proposals.

Bird species that require assessment 

All wild bird species are afforded a level of protection through the Wildlife & Countryside Act 1981 (as amended) and the EU Birds Directive. There are specific obligations within the Birds Directive relating both to protection of species and maintenance of habitats however only some species are of concern, either because they are rare or vulnerable or they are dependent on habitats which are limited or subject to land use change. Birds on Annex 1 of the Birds Directive, regularly occurring migratory species and birds on Schedule 1 of the Wildlife & Countryside Act are recognised in statute as requiring special conservation measures. 

There are currently 26 species that are widespread across Scotland which utilise habitats or have flight behaviours that may be adversely affected by a wind farm.  A further 13 ‘restricted range’ species may be encountered in specific parts of the country. Annex 1 provides lists of these species.  The assessment within most wind farm EIA reports will be limited to these species where they occur within or around a proposed development site.

Birds of Conservation Concern (BoCC) presents a more comprehensive picture of birds whose populations are of conservation concern either across or in parts of their natural range.  The majority of BoCC species also have the legal protection afforded by the Birds Directive or the Wildlife & Countryside Act. Under Scottish Government’s biodiversity duty, where BoCC species are not covered by statutory protection they may still require assessment if their conservation status is likely to be adversely affected. Assessment of the development does not normally need to consider species not covered by the above categories.

Favourable Conservation Status

A species’ conservation status is defined by the sum of the influences acting on it which may affect its long-term distribution and abundance, within the geographical area of interest (which for the purposes of this guidance is Scotland), and that a species’ conservation status is favourable when:

• population dynamics indicate that the species is maintaining itself on a long-term basis and is therefore likely to persist in the habitat it occupies; and
• the natural range of the species is not being reduced, nor is likely to be reduced for the foreseeable future; and
• there is (and will probably continue to be) a sufficiently large habitat to maintain its populations on a long-term basis.

Our response to any development proposal is guided by maintaining a viable population across the species’ natural range along with a sufficient area of suitable habitat.  An effect should therefore be judged as of concern where it would adversely affect the existing favourable conservation status of a species or prevent a species recovering or maintaining its favourable conservation status in Scotland.

Information required to determine significance of an impact

Any assessment of the magnitude and potential significance of effects arising from wind farm developments will depend on:

• the size and direction (whether positive or negative) of the impact and its resulting effect;
• the geographical extent of the impact, such as the habitat area lost from the effect of the proposed wind farm;
• the duration and reversibility of the impact;
• the timing of the impact (which may only occur over part of the year or at particular stages in a species’ life history); and
• whether the impact can be reduced or eliminated through reasonable mitigation or compensation.

It is necessary for such information to be included within the EIA report, or other supporting material to assess the significance of any effects on the species at risk from the proposed wind farm development.  Guidance on this is set out in section 5 of our Bird Survey Guidance. 

In order to assess significance, information is also needed on the number, trends and distribution of each species at the relevant regional and national geographical scales, as well as natural mortality and productivity where available. Data on demographic parameters can be accessed from the BTO Bird Facts web page. Where full information is not available, consideration should be given to what reasonable judgements can be made on available information, taking a precautionary approach where levels of uncertainty are high. National population estimates may be obtained from the latest Avian Population Estimates Panel (APEP) report (Woodward et al, 2020), and Scottish estimates from 'The Birds of Scotland' (Forrester et al. 2007), although this is now >18 years old and population estimates should only be used if there is no other alternative.

Scale

While a species’ conservation status can be assessed at several scales, our primary concern is on impacts on its Scottish population. We will not normally object to a wind farm proposal on account of purely local or regional (within Scotland) effects provided these do not affect populations within a protected area.

Regional populations may be of particular importance to a species conservation status at a national or international population because:

• they are core or ‘stronghold’ areas and the overall viability of the population is dependent on the maintenance of such areas; or
• they are ‘edge of range’ populations, which may (over time) be important in maintaining range as well as providing the potential for expansion or range shift under climate change. 

For example, the Scottish golden eagle population encompasses areas that can be considered to be core and edge populations.  The ‘Golden eagle framework’ indicates the variation in vulnerability of the golden eagle in both core and edge of range areas to additional impacts, such as those from wind farms, across Scotland.

Developments should typically be assessed both alone and (where data is available) in combination at Natural Heritage Zones (NHZ) level for their effect on the conservation status of a species, however the position we will adopt on the proposed development will depend on the significance of those effects at the Scottish scale as set out in our National Interest guidance.  NHZ-level population estimates for a number of breeding bird populations and a number of estimates for key wintering waterfowl populations are available (Wilson et al., 2015). NHZ assessments are best applied where species have relatively stable distributions (such as during the breeding season) or where species occupy a habitat in the non-breeding season that is consistent and predictable.

In some cases, such as wintering goose and swan populations that are highly mobile, it may be necessary to undertake assessment at a much broader scale such as that of the entire Scottish population.  Passage migrants and some wintering populations may show high levels of movement within the non-breeding season and it is therefore difficult to define coherent regional populations with any confidence. This will be especially true where there is substantial site-based turnover in species’ populations.

Alternative geographical areas to NHZs may be acceptable as the basis for assessment where there are definable regional or biogeographical populations that do not conform to NHZ boundaries. E.g. where a core population straddles boundaries and a combined NHZ assessment may be appropriate or where a larger regional or national population such as with reintroduced raptors may be necessary. 

For some migratory species patterns of migration may determine the spatial scale at which effects should be considered, e.g. corncrakes migrate up the west coast of Ireland and Scotland and any impacts during migration would be likely to affect the population as a whole. In considering a species’ distribution, it is important to consider its distribution across its range.

Where a non-NHZ approach to assessment is proposed it should be pre-agreed with NatureScot.

Assessing whether effects on bird species are significant

The EIA report should set out the consequences for the integrity of the species population in terms of its size, trend, distribution (where known) and the area of suitable habitat. The assessment should identify whether the impact is likely to adversely affect the conservation status of the species, by:

• preventing a recovering or reintroduced species from reaching favourable conservation status, at a national or international level; or
• changing a species’ status from favourable to unfavourable; or
• for a species that is already in decline, the assessment should focus on whether the proposal would undermine the potential for halting its decline and allowing it to recover to favourable conservation status. 

In some cases, population models could be used to determine the potential future impact a windfarm will have on a population.  This will only be possible where suitable demographic data exists for the species concerned.   In the majority of cases simple deterministic population models may be sufficient to quantify the significance of population impacts however more complex models do exist for some species. For example, a number of Population Viability Analyses (PVAs) are available for wintering populations of geese. These have been published as NatureScot commissioned reports giving levels of mortality above which populations will be at significant risk of declining.

PVAs generally require good long-term data on numbers, as well as birth and death rates, i.e. information generally only available for a few species.  Most PVA models developed thus far operate at the national level only, though in theory they can be extended to regional scales as long as good information exists on emigration and immigration rates. 

Our decision on whether to object or not will be dependent on the conclusions that arise from the above decisions.  We will provide advice where effects may be mitigated or other measures that will limit adverse effects. 

Cumulative impacts and their effects

An Environmental Statement should include a cumulative impact assessment. The resulting cumulative effects may be additive, synergistic or compensatory (see IEEM EIA Guidelines). The purpose of a cumulative impact assessment is to examine if the impact of a number of developments in combination results in a significant effect on a species’ conservation status, regardless of whether individual proposals have a significant effect or not. Cumulative impacts should be assessed at the relevant biogeographical scale, such as the NHZ, so that the assessment of the impact of the development can be made alone and in combination with other developments. We have published guidance on assessing cumulative effects of wind farms in “Assessing the Cumulative Impact of Onshore Wind Energy Developments”. 

Mitigation, compensation and enhancement 

Mitigation, compensation and/or enhancement are all measures that may limit or offset adverse impacts that might arise from a wind farm development.  Minimising or eliminating adverse impacts will reduce their significance so should be part of the assessment process. Further information is available in Guidelines for Ecological Impact Assessment (EcIA). CIEEM. 

Monitoring after consent has been granted

The outcome of certain forms of mitigation may be uncertain. It will therefore be important to consider how and when mitigation will be monitored and what actions will be taken to modify mitigation measures to ensure its aims are met. It is for the Competent Authority to ensure that mitigation is delivered.  We will normally advise the Competent Authority that an independent ecological clerk of works should be appointed to oversee and report on the mitigation schemes.

An increasing proportion of wind farm developments are consented and constructed with mitigation schemes and monitoring requirements as consent conditions.  In order to inform future developments, it is very important for monitoring outcomes to be made publicly available through the Consenting Authority within agreed timescales.  Such information will be the key, both for NatureScot and the industry, to understand the residual impacts of wind farms on the natural heritage, and to understand the effectiveness of mitigation measures.

It has often proven difficult to encourage developers and their consultants to provide and share data from constructed wind farm sites, especially where mitigation measures have been adopted.  Mindful of the need to protect commercial confidentiality in some circumstances, we strongly encourage developers to share data and experience with other developers, statutory agencies, NGOs and researchers undertaking independent research on the impacts of wind farms and their effect on birds. 

References

Forrester, R.W., Andrews, I.J., McInerny, C.J.et al. (eds). 2007. The Birds of Scotland. The Scottish Ornithologists Club, Aberlady.

NatureScot, 2017. Recommended bird survey methods to inform impact assessment of onshore windfarms. NatureScot Guidance Note. 

Wilson, M. W., Austin, G. E., Gillings, S. and Wernham, C. V. (2015). Natural Heritage Zone Bird Population Estimates. SWBSG Commissioned report.

Woodward, I., Aebischer, N., Burnell, D., Eaton, M., Frost, T., Hall, C., Stroud, S. & Noble, D. 2020. Population estimates of birds in Great Britain and the United Kingdom. British Birds. 113: 69 – 104. 

NatureScot Contact
Sophie Christman
Email: sophie.christman@nature.scot

Annex 1. Priority bird species for assessment when considering the development of onshore wind farms in Scotland.

Table A1: Widespread species potentially at risk of impacts from onshore wind farms. These species are likely to be found at many wind farm sites and should be considered at scoping as possible species that may be present. 

* Key issues are fostering range recovery and maintenance of pioneering pairs.

** check updated connectivity distances for all gull species. Gulls can forage large distances and may be connected to an SPA and need assessed as such.

Table A2:  Species with ‘restricted ranges’ potentially at risk of impacts from onshore wind farms. These species have a very restricted distribution and are unlikely to be found on many proposed wind farm sites. Their presence should be established through discussion with relevant staff in SNH, at scoping.