In 2001, surveys of both Leach’s Oceanodroma leucorhoa and European Storm-petrel Hydrobates pelagicus were made on North Rona and Sula Sgeir, Western Isles, Scotland, using tape playback for the first time. North Rona held 1,133 apparently occupied sites (AOS) of Leach’s Storm-petrel (1,084 AOS after re-analysis of the 2001 data in 2009) and 371 AOS of European Storm-petrel; numbers on Sula Sgeir were five and eight AOS respectively. A repeat survey in 2009 using the same methods found 713 AOS of Leach’s Storm-petrel on North Rona, a decline of about 34% since 2001, and none on Sula Sgeir. A partial survey of European Storm-petrel on North Rona found 313 AOS, suggesting no significant population change since 2001; limited time on Sula Sgeir prevented a survey there for this species. Great Skuas Stercorarius skua and Great Black-backed Gulls Larus marinus breed on North Rona and take an unknown number of storm-petrels each year, but it has yet to demonstrated whether either or both together have caused the decline in Leach’s Storm-petrel. The timing of the decline in Leach’s Storm-petrels can only be dated from 2001, because earlier population estimates are not comparable with the tape playback surveys of 2001 and 2009.

Leach’s Storm-petrel Oceanodroma leucorhoa breeds on both sides of the North Atlantic, with the largest known colony, on Baccalieu Island, Newfoundland, holding c. 3.4 million pairs (Sklepkovych & Montevecchi 1989) out of a global population of 9–10.6 million pairs (Mitchell et al. 2004). The northeast Atlantic colonies are much smaller, the largest, on the Westmann Islands, Iceland, holding 80–150,000 pairs (Icelandic Institute of Natural History 2000). In Britain and Ireland, the largest Leach’s Storm-petrel colonies are in the Western Isles of Scotland, with an estimated 45,433 apparently occupied sites (AOS) on St Kilda (94% of the total population) in 1999–2000, followed by 1,425 on the Flannan Isles, and 1,133 on North Rona; the remaining 11 colonies held a total of only 367 AOS in 2000–02 (Mitchell et al. 2004).

European Storm-petrel Hydrobates pelagicus breeds widely in the northeast Atlantic and Mediterranean, the largest colonies being in the Faeroe Islands. In Scotland, it is found from the Western Isles, north through Orkney to Shetland, co-occurring with Leach’s Storm-petrel on St Kilda, the Flannan Isles, North Rona and Sula Sgeir, all of which are in the Western Isles and also Gruney in Shetland. All these populations are relatively small, with estimates for North Rona and Sula Sgeir of 379 AOS in 2001, or 1.4% of the Great Britain population; St Kilda held 1,121 AOS, the Flannans 7 AOS and Gruney 12 AOS (Mitchell et al. 2004).

Population trends of both species on these islands are poorly known. On St Kilda, numbers of Leach’s Storm-petrels declined by 54% between 1999 and 2006 on Dun, the island holding the largest subcolony in the archipelago, but trends on the other three breeding islands are unknown (Newson et al. 2008). The cause of this decline remains unclear, but predation by Great Skuas Stercorarius skua, which also breed in small numbers on North Rona and the Flannan Isles, is thought to be mainly responsible (Phillips et al. 1999a). Anecdotal evidence suggests the European Storm-petrel has declined on St Kilda since the last survey in 2000 (Mitchell et al. 2004) but the exact status of the population is unknown at present. The Flannan Isles were last surveyed for both species in 2001 (Mitchell et al. 2004), while a partial survey on Gruney in 2004, for Leach’s Storm-petrel only, found 10 AOS c.f. 20 in 2000 (Moncrieff 2005). The paucity of data from these remote colonies and the decline of Leach’s Storm-petrel on Dun since 1999 motivated a repeat survey of both storm-petrel species on North Rona and Sula Sgeir in 2009 for any evidence of population change since 2001.

The North Rona expedition was a partnership project, with funding provided by the Seabird Group, Scottish National Heritage, the Scottish Ornithologists’ Club, the Gibson Estate, the Royal Commission for Ancient and Historical Monuments of Scotland, Historic Scotland and the British Broadcasting Corporation. We thank Tim Dee for assistance with storm-petrel surveying, Dave Cowley for help with skuas, and Paddy Pomeroy for permission to use the SMRU hut. Thanks also to Bob Theakston, who did much more than we asked in delivering us to Rona and supporting us while we were there. Stuart Newson and Norman Ratcliffe improved the manuscript with their rigorous criticisms.

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