Guidance - Assessing the cumulative impacts of onshore wind farms on birds

Purpose

Cumulative impacts are “Impacts that result from incremental changes caused by other past, present or reasonably foreseeable actions together with the project” (Walker & Johnston 1999). The individual and combined (cumulative) impacts of developments on the natural heritage need to be carefully considered in the planning process. In this guidance we specifically refer to the contribution of the proposed wind farm, through its potential impact on habitat availability, bird behaviour and mortality rates, and the combined impacts on notable species of all relevant developments for the appropriate area (see section 4 below).

This guidance sets out our advice for developers, and their ecological consultants, on assessing the cumulative impacts associated with onshore wind farms on birds.

Although cumulative impacts for wind farms are likely to accrue from other wind farms (including their associated infrastructure), impact assessments should not be restricted to these but should include all plans or projects in the area which could result in significant habitat loss, increased mortality or displacement/disturbance.  This includes, but is not limited to, mineral extraction, built development, power lines, telecommunications masts, forestry or recreational pressures, indeed anything that could potentially cause a change in bird abundance in the area. The methods presented in this guidance may be helpful when considering these also. However, we recognise that the availability of data on all such potential impacts may not be available or easy to come by, so we encourage you to liaise with NatureScot with regards the developments for inclusion. 

This guidance is restricted to onshore wind farms and complements our other advice on the ornithological assessment of onshore wind farms which can be found on our website via this link. 

Background to wind farm impacts on birds 

Operational wind farms are known to have a number of impacts on birds and bird populations. These impacts have been documented at wind farms both onshore and offshore and can apply to one or more bird species and can be species-specific (e.g. Marques et al. 2021). These are well described in the scientific literature and include: 

• collision with turbine towers, blades (moving or stationary) and/or associated infrastructure;
• displacement of birds due to loss of suitable feeding, roosting, breeding, or wintering habitat;
• disturbance within and around the turbine envelope; and
• creating a barrier to dispersal, regular movements or migration. 

The impacts on all sensitive bird species that are present on, or adjacent to, the proposed wind farm site are usually addressed in an Environmental Impact Assessment (EIA). Guidance on this process is available on the NatureScot website via this link. Further guidance, for example ‘Recommended survey methods to inform impact assessment of onshore wind farms’, accessible on the NatureScot website via this link, identifies which species should be prioritised for assessment. This is mainly based on species’ legal and conservation status, both nationally and internationally. 

Collision risk for sensitive species is usually calculated for onshore wind farm applications using the Band Model (Band et al. 2007). Collision Risk Modelling (CRM) produces indicative figures for annual losses (individuals per annum) or a total sum over the lifetime of the wind farm. To estimate cumulative impacts, CRM values are summed for each species across all the relevant developments in an area. To adjust for the different timescales over which wind farms will be operational it is important that comparison is made on annual rates of collision mortality and not total estimated mortality. Even small increases in annual mortality (1-5%) can significantly increase the likelihood of population decline (Schippers et al. 2020).

Displacement occurs when bird density on and perhaps around a wind farm is reduced due to changes in range use, territory use or flight pattern between roosting areas and feeding areas over time, so that the range use (or flight paths) no longer bring birds into the vicinity of an operational wind farm (Marques et al. 2021, Tolvanen et al. 2023). Displacement effects result in a loss of habitat for a species, and this is likely to be long term unless birds habituate to the development. 

We differentiate between displacement and disturbance, the latter being short term and may occur primarily during construction (e.g. Pearce-Higgins et al. 2012). The level of disturbance caused to birds is more difficult to assess because it relies on predictions of how birds will respond behaviourally and is often temporary. Since the potential for habituation is rarely taken into account, predicted losses from the development area may exceed reality. Birds may also tolerate levels of disturbance up to a critical disturbance threshold above which they will avoid the development area. Qualitative assessments may be all that is possible in these situations. 

Wind farms may act as a barrier to species that commute between a roost site or breeding area and a feeding locality. Under this scenario birds may be forced to move round the wind farm (e.g. Masden et al. 2010) or gain altitude and fly well above turbine height. Regularly undertaking such movements has an energetic cost. Wind farms may also cause migrating birds to divert from their usual routes (Masden et al. 2009).

Assessments often focus on collision risk, which is easier to quantify, but often displacement/habitat loss has the greater effect on population viability (May et al. 2020), so should be incorporated as far as possible.

The nature of cumulative impacts 

Cumulative impacts arise when there are effects from two or more developments. These may be additive, antagonistic or synergistic (Foden et al. 2010; Raiter et al. 2014; Humphreys et al. 2016).

• Additive: where combined effects are equivalent to the simple summation of the individual effects.
• Antagonistic: the cumulative impact is less than the sum of the individual effects.
• Synergistic: the cumulative impact is greater than the sum of the individual effects. 

While antagonistic or synergistic effects may occur, the approach adopted in this guidance is the simple additive model which sums impacts from different developments. 

Summing impacts can, however, lead to individual errors being compounded and in some cases correction may be needed when bird populations are small. Where a cumulative impact of national or regional significance is possible we are likely to require population viability analyses to be undertaken. 

It is important that impacts on birds are quantified in order to provide comparable data that can be combined to investigate cumulative impacts. In practice, however, some effects such as disturbance or barrier effects may need considerable additional research work to assess impacts quantitatively. A more qualitative process may have to be applied until quantitative information becomes available for developments in the area, e.g. from post-construction monitoring or research. NatureScot may hold some helpful information in this regard. 

The following assumptions are made when calculating cumulative impacts: 

• impacts remain constant over the lifetime of the development;
• impacts are independent of changes in population size or species distribution (N.B. this can be partially addressed through population modelling); and
• for wind farms, collision risk estimates for separate, nearby developments are independent of each other. 

The scale at which impacts should be assessed 

Our prime aim is to maintain the conservation status of the species’ population at the national scale. We assess impacts on a species' population size, its population trend and its natural range within Scotland nationally, and regionally where regional impacts have national implications (for example where a specific region holds the majority of the national population). 

The impacts of wind farm (and other) developments on any species’ population can be assessed at a number of scales, ranging from the very local (e.g. on the wind farm site, designated site, etc.), to a regional scale, such as a Natural Heritage Zone (NHZ), or at a national (i.e. Scottish) scale. Population estimates of key bird species at the NHZ level are available in a Scottish Wind Farm Bird Steering Group report at this link. The issue of scale at which impacts are assessed has been dealt with in other SNH guidance, outlined below, and will not be discussed in detail here. 

Developments that are likely to have a significant effect (LSE) on Special Protection Areas (SPA) or Ramsar sites (this link connects with guidance on Natura sites), either alone or in combination with other plans or projects, require a Habitats Regulations Appraisal (HRA) in line with the requirements of the ‘Habitats Regulations’ (this link connects to guidance on Habitats Regulations). If there is a LSE, the HRA must include an appropriate assessment of the impacts of the project (this link will provide further information on making such an assessment). This should be carried out by the competent authority, with information provided by the developer, and be conducted in relation to the conservation objectives of the SPA. 

For wind farms which do not have an impact on designated sites, our guidance on ‘Assessing significance of impacts from onshore windfarms on birds outwith designated sites’, available via this link, highlights the relevance of the NHZ as the basis for the geographical range selection. 

Recording cumulative impacts 

The predicted impacts of the development in question, along with those from any relevant developments nearby, should be listed in a table or spreadsheet so that the cumulative impact(s) can be calculated (see section 10 below). Each table should include a brief overview or heading stating the primary development under discussion, the species to which it relates (with a separate table or worksheet completed for each species), the NHZ that encompasses the development site and the names of any designated sites of relevance. The body of the table or spreadsheet should comprise information about any relevant developments to be included in the cumulative assessment, with sites listed on separate rows of the table. 

For each site, record the date that consent was given, the number of turbines present for wind farm developments, the area covered by the turbines and the associated buffer zone, plus any collision mortality, displacement effect, barrier effect or habitat loss recorded during that development’s assessment. Using this format, new developments can be added over time, if needed. Remember, developments are not restricted to wind farms, but should include, for example, forest planting, power lines and any major structural development or land use change. Any such developments thought to be significant should be included even where there are little or no quantitative data on impacts available.

NatureScot hold some of the required data for inclusion in the tables, but it will be for the person undertaking the assessment to source and verify all data required. 

Cumulative assessment is an ongoing process. These tables or spreadsheets must take account of new information or changes in important parameters (such as avoidance rates). As post-construction studies are completed and published, conclusions should also be factored in where these have a material effect on earlier cumulative assessments 

The order in which developments should be factored in when considering cumulative impacts is set out below: 

• Developments that are already operational, and those that are consented, and likely to be built should be considered first as the impacts arising from these (once mitigation has been factored in) are unavoidable.
• Applications that have been formally submitted to a planning authority or Scottish Government but have yet to be determined, and consented and built development applications should then be factored in. Confidential data (e.g. on Schedule I species) from such assessments will not necessarily be in the public domain. 

These two types of development should be identified in your table.

The same principles apply to other developments though they may not have the same range of impacts identified for wind farms. For example, a new power line may increase collision risk, unless birds avoid the power line altogether, but may present little additional disturbance or habitat loss. 

Judgements on cumulative impacts should also take into account mitigation or enhancement measures which are provided to offset some of the resulting adverse impacts arising from wind farm construction but only when the benefits are demonstrable or there is a high degree of confidence that they will lead to such benefits. 

Data Needs 

Under normal circumstances, we will expect the developer to undertake the cumulative impact assessment as part of the EIA process. However, it is recognised that developers will need access to data for such assessments, and that access to such data will not always be possible. NatureScot may hold some data of relevance and might be in a position to help.

Data for cumulative impact assessments will generally be derived from Environmental Statements. Unless there is good reason not to do so, figures will be accepted as presented in the various source Environmental Statements. Developers should also refer to our response letters to ensure they have the agreed figures, as there are occasions where we may disagree with the information presented in Environmental Statements. 

We can also help to identify, at scoping, those developments that need to be incorporated in to the cumulative assessment. 

In some cases it may be necessary to consider offshore developments where these may have an impact on terrestrial species’ populations (e.g. some gulls that use inland and coastal habitats). 

Contact

Dave Parish, dave.parish@nature.scot

Versions

• First issued August 2003
• Revised March 2005
• Revised March 2012
• Revised August 2018 – Summary of key changes: separated from cumulative landscape impacts guidance, no substantive changes, key references and guidance documents updated, revised format.
• Revised August 2024 – Summary of key changes: emphasised the need to consider non-wind farm developments in all cumulative assessments; improved text accessibility (expanded the description of information to be recorded in the table); contact details updated, key references and guidance documents updated.

References

Band, W., Madders, M., Whitfield, D.P. (2007). Developing field and analytical methods to assess avian collision risk at wind farms. In “Birds and Wind farms: Risk Assessment and Mitigation” Eds. Manuela de Lucas, Guyonne F. E. Janss and Miguel Ferrer. pp. 259–275. Quercus: Madrid. Available from https://www.natural-research.org/application/files/4114/9182/2839/Band_et_al_2007.pdf 

Foden, J., Rogers, S. & Jones, A. (2010). Recovery of UK seabed habitats from benthic fishing and aggregate extraction – towards a cumulative impact assessment. Marine Ecology Progress Series 411: 259-270. Available from https://www.int-res.com/articles/meps2010/411/m411p259.pdf 

Humphreys, E.M., Masden, E.A., Cook, A.S.C.P. & Pearce-Higgins, J.W. (2016). Review of Cumulative Impact Assessments in the context of the onshore wind farm industry. Scottish Windfarm Bird Steering Group Commissioned Report number 1505. 75pp. Available from https://www.nature.scot/professional-advice/planning-and-development/planning-and-development-advice/renewable-energy/onshore-wind-energy/scottish-wind-farm-bird-steering.

Marques, A.T., Batalha, H., Bernardino, J. (2021). Bird displacement by wind turbines: assessing current knowledge and recommendations for future studies. Birds 2: 460-475. https://doi.org/10.3390/birds2040034 

Masden, E.A., Haydon, D.T., Fox, A.D., Furness, R.W. (2010). Barriers to movement: Modelling energetic costs of avoiding marine wind farms amongst breeding seabirds. Marine Pollution Bulletin 60: 1085-1091. https://www.sciencedirect.com/science/article/pii/S0025326X10000342 

Masden, E.A., Haydon, D.T., Fox, A.D., Furness, R.W., Bullman, R. & Desholm, M. (2009). Barriers to movement: impacts of wind farms on migrating birds. ICES Journal of Marine Science 66: 746–753. https://doi.org/10.1093/icesjms/fsp031 

May, R., Middel, H., Stokke, B.G., Jackson, C. & Verones, F. (2020). Global life-cycle impacts of onshore wind-power plants on bird richness. Environmental and Sustainability Indicators 8: 100080. https://doi.org/10.1016/j.indic.2020.100080 

Pearce-Higgins, J.W., Stephen, L., Douse, A. and Langston, R.H.W. (2012). Greater impacts of wind farms on bird populations during construction than subsequent operation: results of a multi-site and multi-species analysis. Journal of Applied Ecology 49: 386-394. https://doi.org/10.1111/j.1365-2664.2012.02110.x

Raiter, K.G., Possingham, H.P., Prober, S.M., Hobbs, R.J., (2014). Under the radar: mitigating enigmatic ecological impacts. Trends in Ecology & Evolution 29: 635-644. Available from https://api.research-repository.uwa.edu.au/ws/portalfiles/portal/35148232/Raiter_et_al_2014_Under_the_radar.pdf 

Schippers, P., Buij, R., Schotman, A., Verboom, J., van der Jeugd, H., Jongejans, E. (2020). Mortality limits used in wind energy impact assessment underestimate impacts of wind farms on bird populations. Ecology & Evolution 10: 6274–6287. https://doi.org/10.1002/ece3.6360

Tolvanen, A., Routavaara, H., Jokikokko, M. & Rana, P. (2023). How far are birds, bats, and terrestrial mammals displaced from onshore wind power development? – A systematic review. Biological Conservation. 288: 110382. https://doi.org/10.1016/j.biocon.2023.110382. 

Walker, L.J. & Johnston, J. (1999). Consulting guidelines for the assessment of indirect and cumulative impacts as well as impact interactions. Brussels: EC DGX1 Environment. Nuclear Safety and Civil Protection. Available from https://wayback.archive-it.org/12090/20151221014945/http://ec.europa.eu/environment/archives/eia/eia-studies-and-reports/pdf/guidel.pdf

Example table showing relevant information to be recorded.