Tagging and Tracking of Seabirds on the West Coast of Scotland

2. Introduction

Scotland’s coasts and seas support internationally important marine biodiversity, and the Scottish Government has made strong, high-level commitments to the protection of the nation’s rich natural heritage. A key responsibility of Scottish Government's Marine Directorate is to manage Scotland’s seas for prosperity and environmental sustainability, and to achieve good environmental status to help ensure a healthy and sustainable environment. The economic pressures on the marine environment have substantially increased in recent years, and are set to increase still further, in the face of the growing concern over climate change. The Scottish Government’s Climate Change Plan (2018) has set out the need to achieve a net-zero carbon target for 2045, enshrined in the Climate Change Act (2009, amended 2019), which will require the development of extensive areas for marine renewable energy extraction in the near future (Figure 1). Ensuring that future developments do not adversely impact Scotland’s internationally important marine ecosystems, and especially the designated features of protected sites, is of utmost importance.

Information on the marine distribution and behaviour of seabirds is essential to assess whether, and if so where, seabirds may be exposed to high levels of threat from marine activities, which could jeopardise the conservation status of protected species and sites. Information on seabird marine distribution and behaviour also informs understanding of drivers of population change. The earliest information on the distribution of seabirds at sea around the UK was obtained from boat-based transect surveys (Blake et al. 1984), and more recently from aerial surveys (Buckland et al. 2012), which have been used to determine potential overlap of seabird distributions with those of marine threats to which they may be vulnerable, such as oil pollution (Williams et al. 1994, Webb et al. 2016), fisheries bycatch (Bradbury et al. 2017) and offshore wind farms (Marine Scotland Science 2018). Whilst transect based at-sea surveys provide an effective means of simultaneously surveying multiple species in any given area, such surveys may be economically unfeasible for species that travel very considerable distances from the breeding colony, and disperse over extremely large areas, in search of food during the breeding season. There is also uncertainty about the detectability of smaller species like storm-petrels, especially when close to the sea surface. Furthermore, at-sea surveys rarely provide information on the breeding status, or colony provenance, of the individual seabirds counted and are limited by weather and daylight conditions. Hence, it is difficult to determine which breeding colonies may be impacted, and to what extent, by a particular, localised, marine threat.

Recent advances in GPS tracking technology have resulted in reductions in the size, weight and cost of electronic tracking devices, enabling researchers to track large numbers of seabirds from breeding colonies, and to estimate colony-level (and in some cases regional-scale) marine distribution (e.g., Wakefield et al. 2017). GPS tags and other biologging devices can also provide information on seabird behaviour at sea, such as the location, and depth, of diving behaviour, locations of surface feeding, roosting, flight height and flight speed, and even ship-following behaviour. Such information may be of crucial importance in determining the extent to which seabirds may be vulnerable to threats including fishery bycatch, turbine collision, or surface pollution, when the distributions of seabirds and threats overlap.

Leach’s Storm-petrel is classified by IUCN as “Vulnerable” to global extinction (BirdLife International 2018) and was recently red-listed in a recent update to the Birds of Conservation Concern BoCC (Stanbury et al. 2024) following steep population declines at breeding colonies in the northwest Atlantic. The entire UK population breeds within Scotland, with six sites designated as Special Protection Areas (SPAs) for the species. Whilst population trends in the UK are not completely known, a large decline has been recorded within the largest UK colony on St Kilda (Newson et al. 2008, Deakin et al. 2021). The extent to which the marine distribution of birds from protected colonies may overlap with marine pressures is currently unknown. Published information on the species’ distribution at sea in UK waters currently derives entirely from at-sea transect surveys, many of which are now somewhat dated (e.g. Bloor et al. 1996, Pollock et al. 2000) resulting in significant knowledge gaps (Deakin et al. 2022). This study represents the first tracking of this species in the UK.

The UK holds internationally important numbers of breeding European Storm-petrels, with more than 88% of the UK population located within Scotland (Burnell et al. 2023), and eight Scottish colonies designated as SPAs for the species. The extent to which the marine distribution of birds from these colonies may overlap with marine pressures is currently poorly known. Since the European Storm-petrel species is Amber listed in the UK due to having a restricted number of breeding sites (Stanbury et al. 2024), understanding its distribution at sea is a priority to underpin sensitive marine planning and to inform the species’ conservation. As with Leach’s Storm-petrel, published information on the species’ distribution at sea currently derives almost entirely from at-sea transect surveys (e.g. Waggitt et al. 2019), although recent work has demonstrated that European Storm-petrels can be successfully tracked using GPS tags during the breeding season (Critchley et al. 2019, Bolton 2021).

This report summarises the results of fieldwork undertaken in 2021, 2023 and 2024 under two related projects to track Leach’s Storm-petrels from St Kilda and European Storm-petrels from the Treshnish Isles. The first project, entitled “Tagging and Tracking of Seabirds in West Coast of Scotland” (Lots 2 and 3) was awarded to RSPB in March 2020 as a results of a competitive tender submitted in February 2020 (tender reference CORR/5536). The objectives of that project were:

1. To track a minimum of 10 individuals of each storm-petrel species using GPS tags during the breeding season, to characterise their foraging distribution during the breeding season, annually from 2020 - 2022.

2. To track up to 20 individuals of each species using geolocator (GLS) tags to characterise their migration routes and wintering areas in each of the non-breeding seasons 2020-21 and 2021-22.

This onset of this project was delayed due to restrictions imposed by the UK government in response to the Covid pandemic in March 2020. Fieldwork commenced in 2021 when restrictions were lifted, resulting in GPS tracking of both species, and deployment of 20 GLS tags on each species, for retrieval in 2022. However, fieldwork was again prevented in 2022, when NatureScot imposed a ban on all seabird fieldwork in response to the outbreak of Highly Pathogenic Avian Influenza (HPAI). The Scottish Government Marine Directorate awarded a grant to RSPB (“Petrel tagging project extension”) in January 2023 to enable a further two years of data collection, 2023 and 2024, in order to complete the project. The grant for the project extension had the following specific aims:

1. Retrieve previously deployed GLS tags and undertake an additional year of GLS tagging of European Storm-petrel (Treshnish Isles) and Leach’s Storm-petrel (St Kilda).

2. Acquire an additional year of GPS tracking data on European Storm-petrel (Treshnish Isles) and Leach’s Storm-petrel (St Kilda).

3. Produce colony specific foraging distribution maps (including associated GIS files), including behaviours, habitat associations, temporal variation and uncertainty, to better understand potential for the species to encounter or interact with offshore wind farms during the breeding season

4. Where possible, undertake an assessment of HPAI impacts on population sizes and breeding success, potentially including blood antibody testing, ensuring that data used to inform impact assessments are robust.

Here we report on the work undertaken in 2021 under the original contract, and on the work undertaken in 2023 and 2024 under the grant awarded for project extension.