Offshore wind - birds on migration in Scottish waters: strategic review

Introduction

Background

The UK government aims to achieve net zero greenhouse gas emissions by 2050 with a 78% reduction by 2035^[1], while the Scottish Government aims to achieve net zero by 2045 with a 75% reduction by 2030^[2]. To support this, the offshore wind industry is likely to play a key role in the economy in coming decades, particularly in relation to a green economic recovery as we emerge from the COVID-19 pandemic.

At present, there are 40 operational offshore wind farms or demonstration projects in UK waters (Figure 1), including the world's first floating offshore wind project, and a further 11 which are either under construction or for which consent has been granted^[3]. Of these, six operational wind farms and nine which are under construction or for which planning consent has been granted are in Scottish waters^[4]. Building on this success, the Crown Estate recently announced six further projects in English and Welsh waters which will deliver almost 8 GW of additional energy, while in Scotland, the ScotWind Leasing Round aims to deliver up to 25 GW of generating capacity^[5].

Whilst offshore wind energy offers the potential to mitigate the negative impacts of climate change by reducing carbon emissions from fossil fuels, concerns remain over the potential for negative environmental impacts, particularly in relation to birds (Bradbury et al. 2014; Furness et al. 2013; Huppop et al. 2006). The key impacts associated with offshore wind on birds are believed to be collision with turbines, displacement and barrier effects (Cook et al. 2018; Dierschke et al. 2016; Masden et al. 2009, 2012; Mendel et al. 2019; Thaxter et al. 2019). Prior to consent for a development being granted, the potential for these impacts to negatively affect populations, particularly those of designated features of protected sites, must be considered as part of the Environmental Impact Assessment (EIA) and Habitats Regulations Appraisal (HRA) processes.

During the breeding season, the UK hosts internationally important populations of seabirds, mostly within a network of protected sites (Mitchell et al. 2004). As a result, much of the development in impact assessment methodology has focussed on tools to assess the potential impact of offshore wind farms on breeding seabirds (Masden, 2015; McGregor et al. 2018; Searle et al. 2018). Of particular note is the development of a stochastic Collision Risk Model (sCRM) which attempts to quantify the uncertainty associated with collision risk estimates (Masden, 2015; McGregor et al. 2018). The model is based on the Band (2012) CRM with input parameters sampled from a range of plausible values, rather than being considered as a single, fixed value.

In addition to hosting internationally important populations of breeding seabirds, UK waters are an important migratory flyway and the UK hosts internationally important populations of waterbirds (swans, geese, ducks, waders and other waterbirds) during the winter (Frost et al. 2021; Wernham et al. 2002). Such species may interact with offshore wind farms during spring and autumn migrations, but also during moult migration (e.g. Green et al. 2021) or in response to cold weather movements. In addition to waterbirds, significant numbers of raptors and passerines may pass through UK waters during migration (Wernham et al. 2002). Consequently, the expansion of the offshore wind industry raises the potential for significant cumulative impacts of offshore wind farms on migratory populations. This may become particularly important given the development of floating turbines which can be placed further offshore, in deeper water areas. In such circumstances, whilst the exposure to breeding seabirds may be substantially reduced, migrating species passing through the area will still be exposed to the risk of collision in relation to these projects, which may lead to concerns at a cumulative scale.

Of particular concern in relation to migrating birds is the potential for individuals to collide with turbines or, for the wind farms to act as a barrier, increasing the distance birds must travel on their migrations (Huppop et al. 2006; Masden et al. 2009, 2012). Evidence from onshore wind farms highlights the response of migrating birds to this risk (Johnston et al. 2014; Villegas-Patraca et al. 2014). There is also emerging evidence of the response of migrating waterbirds to offshore wind farms (Masden et al. 2009; Plonczkier & Simms, 2012) and, evidence of substantial migration across the North Sea at altitudes that would place birds at risk of collision (Fijn et al. 2015). Given that a substantial proportion of migration may take place at night, this may lead to an increased level of collision risk given the potential for migrating birds to be attracted to turbine lighting (Rebke et al. 2019).

The assessment of migrant collision risk differs from that for seabirds as a result of how the flux rate, the number of birds estimated to pass through the turbine rotor swept areas, is calculated. For seabirds, the existing sCRM derives a flux rate based on the density of birds (derived from at-sea surveys) within a wind farm at any given time assuming each bird flies through the wind farm in a straight line at a constant height and speed (McGregor et al. 2018). These assumptions enable the scaling up of the number of birds present in a wind farm at any given point in time to cover the time period over which collision risk is being considered. Following this approach, any given bird may be present within the wind farm multiple times (Band, 2012). In contrast, when assessing migrant collision risk, each individual bird is assumed to pass through any given wind farm only once during each migration season (Band, 2012). The total number of birds passing through the wind farm is estimated by considering the width of the wind farm relative to the width of the migratory corridor for the species concerned, and then multiplying this by the size of the population concerned (Wright et al. 2012; WWT Consulting, 2014).

As part of the National Marine Plan^[7], it is Scottish Government policy to ensure that decisions are informed by the best available evidence and make reasonable effort to address any gaps in knowledge. Reflecting this, the Marine Sectoral Plan process and Sustainability Appraisal identified a need to update guidance in relation to migrant collision risk, a need echoed in a NatureScot workshop on marine bird impact assessment guidance^[8]. To achieve this, the process for assessing migrant collision risk should be brought into line with that for seabirds. In particular, this should include the potential to incorporate uncertainty into estimates of migrant collision risk. This project will address these gaps in guidance through three work packages:

1. Strategic review of birds on migration in UK waters.

2. Develop stochastic CRM tool for migratory species.

3. Strategic study of collision risk for ScotWind leasing sites for birds on migration in Scottish waters

This report will focus on work package one, originally a strategic review of bird migration in Scottish waters but, subsequently expanded in scope to cover UK waters as a whole.

Scope of Strategic Review of Bird Migration in UK waters

As highlighted above, to support the development of a stochastic collision risk model for migrants, a strategic review of bird migration is required to develop an evidence base with which to underpin this model. Guidance on the use of the model will be available in documentation released alongside it.

Previous reviews of migrant collision risk in relation to offshore wind farms have included both seabird and non-seabird species. For the purposes of this review, we consider that seabird exposure to offshore wind farms will be captured in the density estimates entered into the existing sCRM. Distinguishing between migrant and resident seabirds is not possible. To avoid double-counting of collisions in seabirds, assessment of collision risk of seabirds is best undertaken using the existing model. Consequently, reflecting current policy requirements, the focus of this review is on non-seabird features of Special Protection Areas (SPAs) including swans, geese, ducks, waders, raptors and other non-passerines.

Our review includes species or species populations (hereafter 'species') which are designated features of UK SPAs. For a full list of species considered, see Table 1. Collectively, these species are features of 248 SPAs in the UK (Figure 2; Table 2).

To build the evidence base with which to assess collision risk for each of the 70 target species, we conducted literature searches, data extraction and analyses to acquire information on:

• Population estimates;
• Migratory routes;
• Timing of migration;
• Migratory flight heights;
• Migratory flight speeds;
• Avoidance behaviour and rates.

Finally, we consider the overall potential for climate change to alter migrant species' exposure to collision risk.