Leach’s Storm-petrel Oceanodroma leucorhoa was first recorded breeding on North Rona in 1883 and on Sula Sgeir in 1939. European Storm-petrel Hydrobates pelagicus was first recorded on North Rona in 1885 and on Sula Sgeir in 1958. Since then, there have been attempts to estimate the population size of both species on North Rona but there is little information about their current status on Sula Sgeir. In 2001, systematic surveys of both species using tape playback were conducted for the first time on both islands. North Rona held 1,133 Apparently Occupied Sites (AOS) of Leach’s Storm- petrel but only 371 AOS of European Storm-petrel; numbers on Sula Sgeir were five and eight AOS respectively. The combined population of both North Rona and Sula Sgeir of Leach’s Storm-petrel and European Storm-petrel, comprise 2.3% and 1.4% respectively, of the total number of each species breeding in Great Britain.

North Rona (area = 128 ha, highest point = 108 m), uninhabited since 1844, lies about 70 km north of the Butt of Lewis in the Western Isles at 59°08’N 5°50’W. Leach’s Storm- petrel Oceanodroma leucorhoa and European Storm-petrel Hydrobates pelagicus were first discovered there in the 1880s (Swinburne 1885; Harvie-Brown 1888), but there has never been an accurate census of either species. Both are difficult to survey, since they nest in burrows or in rock crevices and are nocturnal. However, the recently developed tape playback technique (Ratcliffe et al. 1998) enabled this study to obtain an accurate count of Apparently Occupied Sites (AOS) of both species for the first time.

Sula Sgeir (20 ha, 70 m) lies about 17 km west of North Rona, at 59°06’N 6°09’W. It is no more than 1 km in length and just 200 m across at its widest point. It is sparsely vegetated due to lack of soil, and is subject to heavy erosive pressure from breeding seabirds and sea-spray. It has rarely been visited by ornithologists and ‘stormy petrels’, species uncertain, were first confirmed present in 1930 (Dougal 1937). North Rona holds extensive evidence of past human occupation, most prominently a ruined village, graveyard and chapel enclosed by pronounced cultivation ridges. Sula Sgeir, although never permanently occupied has been visited annually for centuries by the men of Ness in Lewis, who have built dry stone bothies on the rock. The structures on both islands are an important component of the available breeding habitat for both petrel species. North Rona and Sula Sgeir are designated as a Special Protection Area (SPA) under Article 4.1 of the EC Birds Directive (79/409/EEC) for supporting more than 1% of the Great Britain breeding populations of Leach’s and European Storm- petrel, which are both listed in Annex 1 of the Directive (Stroud et al. 2001). The islands also qualify for SPA designation under Article 4.2 of the Directive by supporting more than 1% of the relevant bio-geographic breeding populations of Northern Gannet Morus bassanus and Common Guillemot Uria aalge (Stroud et al. 2001).

Leach’s and European Storm-petrel were cited as qualifying species for the North Rona and Sula Sgeir SPA based on estimates of colony size and Great Britain population size given in Lloyd et al. (1991). These estimates were derived before tape playback was developed and most were expressed as orders of magnitude of the number of birds present at colonies during the night. The aim of the present study was to use tape playback to accurately census both species of storm-petrel and, combined with the latest Great Britain population estimates (also derived using tape playback) (Mitchell 2004; Mitchell & Newton 2004), assess their conservation status and validate the islands’ SPA designation with respect to these species.

The work outlined in this report was funded by the European Research and Development fund under the Atlantic Area Programme of the Interreg II C Initiative, as part of Project No. 414, awarded to the Joint Nature Conservation Committee in partnership with BirdWatch Ireland. We are grateful to Norman Ratcliffe and Adrian Plant for their comments on an earlier draft; to Paddy Pomeroy for the use of the Sea Mammal Research Unit hut and equipment; and especially to our boat skippers, Murdo MacDonald and Andy Tibbits.

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