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Population decline of Leach’s Storm-petrel Oceanodroma leucorhoa within the largest colony in Britain and Ireland

Newson, S. E.1*, Mitchell, P. I.2, Parsons, M.2, O’Brien, S. H.2, Austin, G. E.1, Benn S.3, Black J.4, Blackburn, J.1, Brodie, B.3, Humphreys, E.5 ORCID logo , Leech, D.1, Prior, M.6 & Webster, M.7

https://doi.org/10.61350/sbj.21.77

1 British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK

2 Joint Nature Conservation Committee, Dunnet House, 7 Thistle Place, Aberdeen AB10 1UZ, UK

3 19 Culloden Court, Inverness IV2 7DX, UK

4 3 Newgate Street, Bingham, Nottingham NG13 8FD, UK

5 BTO Scotland, School of Biological and Environmental Sciences, Cottrell Building, University of Stirling FK9 4LA, UK

6 1 Rother Close, Greenmeadow, Swindon, Wiltshire SN25 3PZ, UK;

7 16 Grice Close, Sheringham, Norfolk NR26 8UG, UK

Full paper

Abstract

This study used diurnal playback of vocalisations to examine the abundance of breeding Leach’s Storm-petrel Oceanodroma leucorhoa on Dun, St Kilda in 2003 and 2006 in relation to the only previous survey conducted using similar methodology in 1999. The number of Apparently Occupied Sites in 2006 was 12,770, not significantly different to the 14,490 found in 2003, but significantly lower than the 27,811 found in 1999, by 54%. The magnitude and rate of the decline are of major conservation concern. Great Skua Stercorarius skua predation is thought the most likely cause but other factors such as poor food supply cannot be ruled out. The importance of continued monitoring of Leach’s Storm-petrel and Great Skua is discussed.

Introduction

The Leach’s Storm-petrel Oceanodroma leucorhoa has a highly localised distribution in the east Atlantic, with breeding confirmed on only about 13 remote islands and archipelagos off the coasts of the Republic of Ireland, Scotland, Faeroes, Iceland and Norway (Mitchell 2004). One of the largest of these is on the St Kilda archipelago off northwest Scotland, probably second only in size to the colony on the Westmann Islands off southern Iceland that is estimated to hold 80,000–150,000 pairs (Icelandic Institute of Natural History 2000). During 1999, the Seabird 2000 survey (Mitchell 2004) estimated that St Kilda held about 94% of the British and Irish population (45,433 Apparently Occupied Sites, AOS) of which 27,811 were in a single subcolony on the island of Dun. The lack of accurate population estimates for this species in Britain and Ireland before this time makes it impossible to assess whether there has been a significant long-term change in the status of this species.

On Dun and other islands of St Kilda, predation by Great Skuas Stercorarius skua is thought to pose a serious threat to Leach’s Storm-petrel (Phillips et al. 1997, 1999a; Votier et al. 2006). Great Skuas have increased dramatically on St Kilda, with the population rising from about 42 pairs in 1986 to a peak of 240 pairs in 2000 (Furness & Ratcliffe 2004). Based on their numbers in 1996, it has been estimated that Great Skuas predate approximately 14,850 Leach’s Storm-petrel on St Kilda each year (Phillips et al. 1999b). Although there has been a small decline in the proportion of Leach’s Storm- petrel in the diet of Great Skua since 1996, the level of predation appears to have remained high (Votier et al. 2006). With declines in the availability of offal and discards from commercial fishing boats predicted due to changes in fisheries management, Great Skuas may increase their reliance on seabird prey (Votier et al. 2004). Because of the importance of Dun for Leach’s Storm-petrel in Britain and Ireland, there was a need to provide reliable monitoring data for this population. Here we report on two subsequent surveys of Leach’s Storm-petrel on Dun conducted in 2003 and 2006, and examine whether the size of the population has changed since the first survey in 1999.

Acknowledgements

Thanks to Maggie Robinson for help with survey work in 2003; to QinetiQ for use of their boat in 2003; to Neil Mitchell, Sarah Money and Susan Bain of the National Trust for Scotland and the QinetiQ staff for their support on Hirta. The survey in 2006 was part funded through a grant from the Seabird Group. Thanks also to Norman Ratcliffe and Steve Newton who provided extremely useful comments on an earlier version of this manuscript.

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