Comparable surveys have been made of the Leach’s Storm-petrel Oceanodroma leucorhoa colony on North Rona, Western Isles, Scotland in 2001, 2009 and 2015 using responses to tape recordings played at burrow entrances. Response rates estimated from a calibration plot were used to correct the total number of responses to tape playback from an island-wide survey. A ‘simple arithmetic’ method of calculating the population estimate suggested 606 apparently occupied burrows (AOB) for 2015, which is the lowest number to date and represents a c.15% decline since the last survey in 2009, when 713 AOB were found. In 2001 there were 1,084 AOB, giving an overall decline of 44% since this first survey. The ‘first response’ method, which only makes use of the first response from each burrow in the calibration plot, is designed to avoid potential bias, caused either by habituation or by very vocal individuals. As this suggests a population of 805 AOB there is a need to be cautious about inferring a further island wide decline since 2009.

Leach’s Storm-petrel Oceanodroma leucorhoa (hereafter Leach’s petrel) breeds on both sides of the North Atlantic, with some the world’s largest colonies in Newfoundland, Canada (Robertson et al. 2006). Here, Baccalieu Island alone holds 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, in 1999–2002, the largest colonies were 45,433 apparently occupied burrows (AOB) on St Kilda (94% of the total population), 1,425 on the Flannan Isles and 1,084 on North Rona; the remaining five confirmed colonies held a total of 367 AOB (Mitchell et al. 2004). Population trends on these islands are poorly known. On Dun, St Kilda, numbers declined by 54% between 1999 and 2006, while on North Rona they decreased by 34% between 2001 and 2009. Predation by Great Skuas Stercorarius skua is thought to be responsible for the decline on St Kilda, and perhaps on North Rona, although this is still unproven (Votier et al. 2006; Newson et al. 2008; Murray et al. 2010). This paper reports on a further assessment of the numbers breeding on North Rona and compares the results with those from similar surveys made in 2001 and 2009.

North Rona (59°08’N 5°50’W) lies 70 km north of the Butt of Lewis in the Western Isles of Scotland. It is uninhabited, about 122 ha in extent and reaches 108 m at the highest point. There are no secure landing sites or anchorages and the island is exposed to the full force of Atlantic weather systems, which can make landings difficult to impossible, even in summer conditions. The island was declared a National Nature Reserve in 1956; classified as a Special Protection Area (SPA) under Article 4 of the European Birds Directive in 2001 and in 2005 as a Special Area of Conservation (SAC) under the European Habitats and Species Directive. Leach’s petrel is one of the designated features under both designations.

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 and the Royal Society for the Protection of Birds. Thanks are due to Mike Harris for helpful insights, Paddy Pomeroy for permission to use the SMRU hut and especially to Angus Campbell of Kilda Cruises who delivered and collected us from the island in testing sea conditions.

Efron, B. 1979. Bootstrap methods: another look at the jacknife. Annals of Statistics 7: 1–26. [Crossref]

Ellis, P., Ratcliffe, N. & Suddaby, D. 1998. Seasonal variation in diurnal attendance and response to playback by Leach’s Petrels Oceanodroma leucorhoa on Gruney, Shetland. Ibis 140: 336–339.
 [Crossref]

Gilbert, G., Gibbons, D. W. & Evans, J. 1999. Bird monitoring methods, a manual of techniques for key UK species. RSPB, Sandy.

Icelandic Institute of Natural History 2000. Red List of Threatened Species in Iceland. Vol. 2. Náttúrufræδistofnun Islands, Reykjavik.

Mayhew, P., Chisholm, K., Insley, H. & Ratcliffe, N. 2000. A survey of Storm Petrels on Priest Island in 1999. Scottish Birds 21: 78–84.

Miles, W. T. S., Tallack, R. M., Harvey, P. V., Ellis, P. M., Riddington, R., Tyler, G., Gear, S. C., Okill, J. D., Brown, J. G. & Harper, N. 2012. A survey of Leach’s Petrels on Shetland in 2011. Scottish Birds 32: 22–29.

Mitchell, P. I., Newton, S. F., Ratcliffe, N. & Dunn, T. E. (eds.) 2004. Seabird populations of Britain and Ireland. Poyser, London.

Money, S., Söhle, I. & Parsons, M. 2008. A pilot study of the phenology and breeding success of Leach’s Storm-petrel Oceanodroma leucorhoa on St Kilda, Western Isles. Seabird 21: 98–101. [Crossref]

Murray, S., Money, S., Griffin, A. & Mitchell, P. I. 2008. A survey of Leach’s Storm-petrel Oceanodroma leucorhoa and European Storm-petrel Hydrobates pelagicus populations on North Rona and Sula Sgeir, Western Isles, Scotland. Seabird 21: 32–43. [Crossref]

Murray, S., Shewry, M. C., Harden, J., Jamie, K. & Parsons, M. 2010. A survey of Leach’s and European Storm-petrel populations on North Rona and Sula Sgeir, Western Isles, Scotland, in 2009. Seabird 23: 25–40. [Crossref]

Newson, S. E., Mitchell, P. I., Parsons, M., O’Brien, S. H., Austin, G. E., Benn, S., Black, J., Blackburn, J., Brodie, B., Humphreys, E., Leech, D. I., Prior, M. & Webster, M. 2008. Population decline of Leach’s Storm-petrel Oceanodroma leucorhoa within the largest colony in Britain and Ireland. Seabird 21: 77–84. [Crossref]

Phillips, R. A., Bearhop, S., Hamer, K. C. & Thompson, D. R. 1999a. Rapid population growth of Great Skuas Catharacta skua at St Kilda: implications for management and conservation. Bird Study 46: 174–183. [Crossref]

Phillips, R. A., Thompson, D. R. & Hamer, K. C. 1999b. The impact of great skua predation on seabird populations at St Kilda: a bioenergetics model. Journal of Applied Ecology 36: 218–232. [Crossref]

Ratcliffe, N., Vaughan, D., Whyte, C. & Shepherd, M. 1998. Development of playback census methods for Storm-petrels Hydrobates pelagicus. Bird Study 45: 302–312. [Crossref]

Robertson, G. J., Russell, J., Bryant, R., Fifield, D. A. & Stenhouse, I. J. 2006. Size and trends of Leach’s Storm-petrel Oceanodroma leucorhoa breeding populations in Newfoundland. Atlantic Seabirds 8: 41–50.

Sklepkovych, B. O. & Montevecchi, W. A. 1989. The world’s largest known nesting colony of Leach’s Storm-Petrel, on Baccalieu Island, Newfoundland. American Birds 43: 38–42.

Taoka, M., Sato T., Kamada, T. & Okumura, H. 1989. Sexual dimorphism of chatter calls and vocal sex recognition in Leach’s Storm-petrels. Auk 106: 498–501.

Votier, S. C., Crane, J. E., Bearhop, S., de León, A., McSorley, C. A., Mínguez, E., Mitchell, P. I., Parsons, M., Phillips, R. A. & Furness, R. W. 2006. Nocturnal foraging by Great Skuas Stercorarius skua: implications for conservation of storm-petrel populations. Journal of Ornithology 147: 405–413. [Crossref]

Watanuki, Y. 1985. Breeding biology of Leach’s Storm Petrels Oceanodroma leucorhoa on Daikoku Island, Hokkaido, Japan. Journal of Yamashina Institute of Ornithology 17: 9–22. [Crossref]