Glasgow University and the National Trust for Scotland have been awarded a PhD studentship to study the impact of great skua predation on Leach’s storm-petrels. A candidate has been selected and the student will work for three years, starting in January 2007. Numbers of great skuas breeding on St Kilda have increased very rapidly from 10 pairs in 1971 to 240 pairs in 2000, making this the fastest-growing large great skua colony in Scotland. Great skuas can feed on sandeels or on fishery discards, but when these are in short supply may switch to killing seabirds in large numbers. However, on St Kilda, great skuas feed predominantly on seabirds, killing large numbers of kittiwakes, auks and storm-petrels, as sandeels or fishery discards are much less available there than in Orkney and Shetland. Leach’s storm-petrels are a particularly large component of great skua diet at St Kilda.
In 2000, St Kilda was estimated to hold 45,433 pairs, representing 94% of the entire EU population of this species. St Kilda is designated an SPA for Leach’s storm-petrel. In 2003, a JNCC survey reported a 48% decline in Leach’s storm-petrel breeding numbers since 1999, at the largest colony on St Kilda, a figure that is entirely consistent with the increasing and very high predation rate on this species by great skuas. It was estimated that great skuas killed 14,850 Leach’s storm-petrels in 1996 alone. In 2004, a short investigation by radio tracking and direct observation of great skuas at St Kilda by night-vision equipment demonstrated that these birds kill Leach’s storm-petrels at night at the petrel colonies. Further study of pellets in skua territories indicated that some great skuas feed predominantly on storm-petrels while others do not kill any. A repeat survey of the Leach’s storm-petrel colony is planned in 2006.
For any management to be developed to conserve Leach’s storm-petrel as a European breeding species, an improved understanding of skua-storm-petrel relationships is essential. The aims of this project are: to quantify the predation by great skuas on Leach’s storm-petrels; to investigate factors that influence the rate of predation on storm petrels by individual great skuas; to determine when and how great skuas catch storm petrels; to determine how predation rate by skuas varies between different Leach’s storm-petrel colonies on St Kilda; and to develop a scientific basis for management
The studentship is funded by a NERC CASE studentship and the National Trust for Scotland. Project supervisors are Professor Bob Furness and Dr Richard Luxmoore.