Tagging and Tracking of Seabirds on the West Coast of Scotland

1. Summary

A rapid expansion of offshore renewable energy infrastructure is underway in the Atlantic waters off the west coast of Scotland. This expansion is driven by Scottish Government and UK Government clean energy targets as part of the transition to Net Zero by 2045 in Scotland (Scottish Government 2020) and 2050 for the whole of the UK (BEIS 2021). To meet these targets while protecting biodiversity, it is vital to gather information on the marine distribution and behaviour of seabirds. Such information will help inform measures to avoid, mitigate and compensate for any negative impacts on seabirds from marine renewables and other human activities.

Little is currently known about the marine distribution and behaviour of two seabird species that occur in Scottish waters, Leach’s Storm-petrel Hydrobates leucorhous and European Storm-petrel H. pelagicus. Their small size, remote breeding locations and nocturnal colony attendance makes them challenging to study. However, recent advances in technology to reduce the size and weight of tracking tags now permit the tracking of even the smallest seabird species, such as Storm-petrels, to provide information on their marine distribution.

This report provides results from fieldwork completed in 2021, 2023 and 2024 by RSPB staff to track, during the breeding and non-breeding seasons, Leach’s Storm-petrels from the largest UK colony, St Kilda, and European Storm-petrels from the Treshnish Isles, Inner Hebrides. No tracking was possible during 2022 due to restrictions on the capture and handling of seabirds in Scotland due to the outbreak of Highly Pathogenic Avian Influenza (HPAI). We present GPS tracking data on breeding season foraging trip characteristics, utilisation distributions (UDs), overlap with proposed offshore wind farms (OWFs) and habitat associations. We also report attempts to deploy geolocator tags on both species and present the first information from Leach’s Storm-petrels tracked outside the breeding season from a UK colony.

Whilst not an original aim of the project, we report results of blood sampling a small number of birds to assay for antibodies to HPAI.

Leach’s Storm-petrel

Data were obtained from a total of 50 birds tracked using GPS tags across the three field seasons, resulting in 78 complete trips by birds actively engaged in breeding attempts. We excluded from all analyses six further short trips undertaken by a bird that was no longer attending an active nest, and five additional, atypical, shorter trips (of five different individuals) that were considered to have resulted from birds prevented from accessing their nests to resume incubation due to researchers’ activity at the nest on nights they retuned. These birds returned to the sea for a further day, before being recaptured. The sample of 78 trips considered to represent normal foraging behaviour of birds attending active nests averaged 52.2 ± 20.2 hr in duration, 281.2 ± 103.6 km maximum distance from the colony and 848.4 ± 332.7 km total distance. Leach’s Storm-petrels were very consistent between years in their direction of travel and foraging distribution, and habitat association modelling showed they selected deep water and areas of higher sea surface temperature. The tracking data indicated no overlap between Leach’s Storm-petrel foraging areas and currently proposed offshore wind energy development.

Only three birds of 20 birds fitted with ring-mounted geolocator tags in 2021 were recaptured (in 2023, due to prohibition of fieldwork in 2022). Two birds had lost their tags, associated with leg injuries likely caused by the tagging procedure. The third individual retained its tag and appeared healthy. The tracking data showed that it departed the colony in mid September (indicating successful breeding) and migrated southward off the West coast of Ireland, and wintered in the Benguela current and mid-Atlantic. In light of the low recapture rate of birds tagged with geolocators, no further tagging was carried out.

No antibodies to any form of avian influenza virus were found in blood samples from eight breeding Leach’s Storm-petrels sampled in 2023 on St Kilda, indicating lack of recent exposure to HPAI. Samples taken from a further four individuals in 2023 were insufficient for analysis.

European Storm-petrel

A total of 38 birds were successfully GPS tracked across the three field seasons resulting in 48 complete trips averaging 36.9 ± 17.3 hrs in duration, 116.6 ± 53.0 km maximum distance from the colony and 418.6 ± 178.0 km total distance (model estimates accounting for similarities of multiple trips by the same individual). Habitat association modelling showed reduced usage of areas more distant from the colony (reflecting central-place foraging constraints), but increasing usage with increasing seabed slope, which may increase mixing of the water column and accessibility of prey. The tracking data indicated overlap between European Storm-petrel foraging areas and currently proposed offshore wind energy development. Nineteen of 38 tracked individuals spent time in areas leased for windfarm development; across all tracked birds the proportion of distance at sea per trip within these areas averaged 3.5% ± 6.8 (0 – 28.8%).

None of 20 birds fitted with ring-mounted geolocator tags in 2021 were recaptured in 2023, whereas 7 of 22 control birds (that were only ringed) were re-encountered that year. It is likely that the difference on re-capture rates was due to the tagging procedure, and no further tagging was carried out.

No antibodies to any form of avian influenza virus were found in a blood sample from a single breeding European Storm-petrel caught in the Treshnish Isles in 2023, indicating lack of recent exposure to HPAI. Samples taken from a further three individuals in 2023 were insufficient for analysis.