Tagging and Tracking of Seabirds on the West Coast of Scotland

6. Conclusions

• This study represents the first GPS tracking of Leach’s Storm-petrels in UK, and only the second study to track European Storm-petrels – and first from the West coast of Scotland. The techniques for capture, handling and tag attachment proved successful. No injuries occurred and the evidence available suggests that (with a few possible exceptions) pattern of nest attendance was similar to that of untagged birds. There was some evidence that a few individuals may have been deterred from returning the nest by nocturnal fieldwork activity and it is suggested that future tracking of these species employs co-deployment of micro VHF tags (0.16g) with GPS tags, that will alert fieldworkers when tagged birds arrive back at the colony, to ensure that disturbance is kept to an absolute minimum.
• In contrast to the GPS tagging, the use of ring-mounted GLS tags in this study was not successful for either species and is likely to have caused welfare impacts. As a result, all GLS tagging of both storm-petrel species ceased. However, as a result of subsequent discussions with storm-petrel researchers elsewhere in Europe and Canada, (some of whom found similar issues and others who have been successful) modified procedures have been recommended, and may now be implemented in future studies in UK.
• The tracking data provide the first information on marine space use of Leach’s and European Storm-petrels known to be breeding at protected colonies on the West Coast of Scotland, unbiased by time of day, the birds’ activity or observation conditions.
• There was little inter-individual and inter-annual variation in marine space use among the three years of tracking for Leach’s Storm-petrel, whereas for European Storm-petrel we found greater variation in space use among both individuals and years. For both species, the sample of foraging trips obtained represented the expected colony-level distribution well (representativeness scores of 89.3% and 81.5% for Leach’s and European Storm petrels respectively).
• In broad terms, pattern of marine distribution align with boat-based survey data collected several decades ago (Webb et al. 1990, Bloor et al. 1996, Pollock et al. 2000), showing the use by Leach’s Storm-petrels of deep water offshore areas associated with underwater bathymetric features, and use by European Storm-petrels of neritic waters closer to the colony. Modelling of habitat use showed Leach’s Storm-petrel densities increased as water depths increased from 200m to 1000m and an association with higher Chl and higher sea surface temperatures. European Storm-petrel distribution was positively associated with seabed slope, and negatively associated with Net Primary Production and chlorophyll gradient.
• The night-time core usage areas of both species were centred closer to the breeding colonies compared with the respective daytime distributions, reflecting the restriction of colony attendance to the hours of darkness. In addition, European Storm-petrels were highly coastal at night, visiting many inshore areas in the Minches, Hebridean and Irish Seas, possibly associated with nocturnal intertidal foraging.
• Both species used extremely large marine areas. The 95% UD of Leach’s Storm-petrels covered an area of c. 164,000 km (twice the land area of Scotland) and that of European Storm-petrels extended c. 40,000 km. Despite usage of extremely large marine areas, we found no overlap in space use by Leach’s Storm-petrels breeding on St Kilda with areas leased for offshore windfarm development, due to the remote location of the colony, and highly directional foraging movements towards the northeast, to deep water areas. In contrast, we found considerable overlap between area use by European Storm-petrels and offshore windfarm lease areas, reflecting the less remote location of the breeding colony and expansive use of shallow shelf seas, including inshore areas at night. Nineteen of 38 tracked European Storm-petrels passed through at least one offshore windfarm area, and 18 of these birds accessed the Machair Wind site, which lies within the colony-level core foraging area (50% UD). A further four offshore windfarm areas were accessed, including the Talisk area to the North of the North Rona and sites off Northern Ireland and the Republic of Ireland.
• Having demonstrated considerable spatial overlap between areas used by European Storm-petrels and offshore windfarm development areas, and extensive nocturnal movements of storm-petrels, future research should focus on accurately determining flight height and speed, under a range of environmental conditions and across the diurnal cycle, as well as the extent of attraction to navigation lights required for the illumination of offshore structures and vessels associated with offshore windfarms.
• Several extensive offshore windfarm lease areas lie close to the SPA breeding colonies of Leach’s and European Storm-petrels of North Rona and the Flannan Islands, and European Storm-petrel colonies of Auskerry, Sule Skerry and Priest Island. Further work is required to track these protected species from these colonies. Tracking of Leach’s Storm-petrels will be greatly facilitated by the recent development of 1.5g GPS tags, with capability to remotely download data to a base station. It is anticipated that 1.0g remote download tags, (small enough for deployment on European Storm-petrels) will also become available.
• Further work is also required to characterise the marine distribution of these two storm-petrel species at earlier, and later, breeding stages i.e. pre-laying and early incubation, when birds are more sensitive to disturbance and in late chick-rearing, and outside the breeding season. The use of GLS tags to provide data on movements and marine distribution over longer time periods (many months, or years) could provide such information, and (due to the light-logging capability of such tags) could also reveal the extent to which storm-petrels are exposed to artificial light at night.
• Tracking and assessment of demographic rates (breeding success, survival etc) is greatly facilitated where storm petrels use nest boxes, which allows access to nesting birds with a minimum of disturbance. Better understanding of demographic parameters is important for Collision Risk Models in wind farm assessments. In the longer term it would be highly beneficial to instigate a programme of nest-box installation at key SPA colonies, to facilitate such data collection.