Populations of burrow-nesting seabirds were surveyed on the St Kilda archipelago, off the northwest coast of Scotland, in June and July 2019 as a contribution to the national Seabirds Count census. This paper details the results for Atlantic Puffin Fratercula arctica, European Storm-petrel Hydrobates pelagicus and Manx Shearwater Puffinus puffinus. Puffins were surveyed via counts of Apparently Occupied Burrows, while the other two species population sizes were assessed by call-playback. The surveys of Boreray and Soay were the first to have been carried out since the Seabird 2000 survey (1999–2000), with surveys of Dùn and Hirta building on more recent surveys. Comparison with the Seabird 2000 survey showed that Puffin numbers have remained relatively stable on all islands. There appears to have been a decline in European Storm-petrel populations on Hirta, confirming the results shown by the long-term monitoring programme on St Kilda. Manx Shearwater numbers have remained more stable, though we have found evidence of redistribution of the largest colony on the archipelago from the Carn Mòr boulder field on Hirta to the higher slopes. St Kilda is the largest colony of seabirds in the British Isles and it is therefore of critical importance that it continues to be monitored at regular intervals. While population trends can be deduced from monitoring easily accessible plots, whole-colony counts are needed to detect changes in the extent of sub-colonies.

St Kilda, a small archipelago to the northwest of Scotland, 64 km west of North Uist (Figure 1), is the site of the largest seabird colony in the UK (Mitchell et al. 2004). It is internationally important for a range of burrow-nesting seabirds, notably Atlantic Puffin Fratercula arctica (hereafter 'Puffin'), Leach's Storm-petrel Hydrobates leucorhous, European Storm-petrel H. pelagicus, and Manx Shearwater Puffinus puffinus. Difficulty of access, particularly to the uninhabited islands, has restricted the ability of researchers to carry out comprehensive seabird surveys of the archipelago. The last full survey of the archipelago's burrow-nesting seabirds was carried out in 1999–2000 as part of the Seabird 2000 census of Britain and Ireland's breeding seabirds (Mitchell et al. 2000), but some sub-colonies have been surveyed more recently. Surveys of Leach's Stormpetrel were carried out on the island of Dùn in 2003 and 2006 (Newson et al. 2008), and there is regular monitoring of sub-colonies of Manx Shearwaters and European Storm-petrels on Hirta (Lawrence 2019). A survey of Puffins on the islands of Dùn and Hirta was carried out in 2018 (Luxmoore et al. 2019), but weather conditions prevented access to the islands of Boreray and Soay. Here, we describe a follow-up survey conducted in 2019, designed to complete the survey of the Puffin colonies on Boreray and Soay, and to survey the storm-petrels and shearwaters on all the islands as part of the national Seabirds Count census (Burnell et al. 2023). Additional observations were made of Puffins on Dùn and Hirta to augment the results of the 2018 survey. The survey of Leach's Stormpetrels is described separately in Deakin et al. (2021). Here, we report on the surveys of the remaining three species.

Funding for the survey was provided through a partnership agreement between the National Trust for Scotland (NTS), the Royal Society for the Protection of Birds (RSPB), Cardiff University and University of Gloucestershire. We would like to thank Angus and Alexander Campbell of Kilda Cruises whose skill in landing personnel were invaluable in achieving access to the remote islands of St Kilda. We thank the St Kilda Rangers, Sue Loughran, Sarah Lawrence and Craig Stanford for their assistance and support, and Craig Nisbet, Derren Fox and Hebe Denny for assisting with the 2022 survey of Manx Shearwaters on Hirta. Ian Stevenson of the Soay Sheep Project provided weather data for St Kilda. Stuart Murray provided invaluable advice on survey methods and access. LIDAR data were provided by Historic Environment Scotland. ZD was supported by a NERC GW4+ Doctoral Training Partnership studentship from the Natural Environment Research Council [NE/L002434/1]. The survey of Atlantic Puffin, European Storm-petrel and Manx Shearwater in the St Kilda archipelago is part of 'Seabirds Count' (2015–21), the fourth national census of the UK's breeding seabird populations. It was made possible using funding received by RSPB and JNCC from EDF Renewables, Moray Offshore Windfarm (West) Ltd, Red Rock Power Limited, and SSE Renewables.

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