Scottish Marine and Freshwater Science Volume 5 Number 12: Strategic assessment of collision risk of Scottish offshore wind farms to migrating birds

Report to inform large scale indicative strategic assessments on the impact of offshore wind energy developments on birds.

2. Introduction


2.1. The Scottish Government has a target for 100% of Scottish demand for electricity to be met from renewables by 2020 by creating a balanced portfolio of both onshore and offshore technologies. 'Blue Seas - Green Energy: A Sectoral Marine Plan for Offshore Wind Energy in Scottish territorial waters' was published along with its associated Strategic Environmental Assessment ( SEA) Post Adoption Statement on the Scottish Government website on 18 March 2011. The Plan sets outs the Scottish Government's spatial policy for developing offshore wind energy up to and beyond 2020. The Plan was developed using SEA, HRA and consultation and informed by economic impact at the strategic level using Scotland's Marine Atlas as the main data source.

2.2. The Plan identified six options for development in the short term (by 2020) and 25 areas of search in the medium term (beyond 2020). Built into the Plan process is an Iterative Plan Review ( IPR) which allows Marine Scotland to take forward additional research and address knowledge gaps, identified through the SEA, Habitat Regulation Assessment ( HRA) and socio-economic processes. The IPR is undertaken throughout the lifecycle of the Plan which is programmed at a review every two years.

2.3. The six options for development in the short term are also important areas for a wide range of bird species, both in the breeding season and at other times of year.

2.4. Offshore wind energy developments have the potential to impact on birds by displacing individuals from feeding areas etc., and also through collision of birds in flight with the rotating turbine blades and through barrier effects increasing the energetic costs associated with transit between locations. These interactions are most often discussed in the context of breeding seabird populations. However, there is also the potential to interact with a wide range of species that migrate to and from the UK including seabirds, wildfowl, waders, passerines and raptors. The primary concern in relation to birds on migration is the risk of collision with turbine blades.

2.5. The established modelling approach to estimating collision risk for onshore wind development (the Band model, 2000, 2007) has since been updated for use in the marine environment in respect of seabird species (Band 2012). It retains a spreadsheet for modelling the risk of collision to other bird species on migration. Overall, the model predicts whether a bird entering the swept area of a wind turbine is struck by the rotating blades or not, and requires turbine and species specific input parameters ( e.g. size/speed of turbine, bird biometrics). Mortality is estimated within the model using data on the density of birds in flight, within the rotor swept area (at the development site prior to construction), coupled with information on day length, nocturnal activity etc. Finally, an 'avoidance rate' is applied to the output to account for the level of avoidance behaviour birds will exhibit at or around constructed turbines.

Use of this report

2.6. This project has, for the first time, attempted to quantify the combined effects of existing Scottish wind farms and those included in the Scottish Government's Blue Seas- Green Energy Marine Plan on migrant bird populations. Due to the very limited data available on migration patterns and how birds on migration may interact with offshore wind farms it has been necessary to make a number of assumptions. As improvements to our understanding of migration, including (but not limited to) factors such as flight heights, migration front widths and passage population sizes become available these should be taken into account in the interpretation of the results presented here.

2.7. For non-seabird species e.g. wildfowl, waders and raptors, the information on migration corridors (updating Wright et al. 2012, report SOSS-05) will be of relevance to strategic assessments for future leasing rounds of offshore wind development in Scotland. The estimates of collisions presented for each non-seabird species will allow a strategic overview to be taken of the risk presented to these species by all current offshore wind development proposed, consented and built (Robin Rigg) in Scottish waters. The estimated collision mortalities are considered in the context of the numbers of birds of each species that migrate to or through Scotland in autumn and spring.

2.8. For seabird species in the absence of empirical data on migration patterns a number of assumptions have been made, largely on the basis of expert opinion, which directly influence the predicted impacts. These include: (1) overall migration strategy; (2) migration bands ( i.e. distance from shore); and (3) geographic apportioning of populations. Furthermore, the migratory populations have largely been calculated from Forrester et al. (2007) and are based on coastal and/or near-shore surveys that are likely to underestimate the number of birds on migration. The proportion of the entire migrating population which is estimated to pass through Scottish waters has been assumed to migrate within a defined species specific coastal band. The consequences of this approach are difficult to predict since, wider migration bands will both reduce the proportion of the population passing through a given wind farm ( i.e. reducing collisions), but may also take in additional wind farms ( i.e. increasing collisions) collisions for any populations that migrate across a broader front. Narrow migration bands could result in the majority of birds being predicted to miss the majority or even all wind farms ( i.e. reducing collisions), or alternatively result in the majority of birds being predicted to encounter the majority or all wind farms ( i.e. increasing collisions). In addition, if individuals pass through a wind farm more than once on migration, which may be expected for a pelagic species ( i.e. whilst foraging or staging), collisions will be under-estimated.

2.9. In the absence of sufficient detailed empirical data on seabird migration behaviour with which to improve the models, this report's primary value will be informing large-scale indicative strategic assessments (the caveats and risks above notwithstanding).

2.10. Relevant caveats for use of this report are that the reference populations may not be appropriate to all situations and may not reflect more recent trends, appropriate thresholds to conclude no adverse effect need to be identified for the situation being considered, and the migration bands used may not be appropriate in other situations.


2.11. The aim of this project is to develop a series of strategic assessments of the collision risks to migrating bird species, and of the components of that risk arising from Scottish offshore wind farms. Specifically, the study will:

  • Identify the contributions of Scottish offshore wind farms currently proposed or operational at the strategic level to assess the collision risk to migrating bird species; and
  • Identify which populations of migratory birds could be at significant risk from potential collisions with turbines while on migration.

2.12. The project does not consider wind farms out-with Scottish waters, or the impacts of displacement and barrier effects, nor in-combination effects with 'wet' renewables and other activities, or impacts on populations outside of migration periods.

2.13. The work builds on the outputs from the SOSS-05 project commissioned by The Crown Estate through the SOSS group, and available from:


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