Study on great skua predation on St Kilda announced

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.

Late start to 2006 breeding season and lack of sandeels both take their toll

2006 has been a year of mixed fortunes for the breeding success of Scotland’s internationally important seabird colonies, according to monitoring by two of the country’s leading conservation organisations. 

Experts on coastal reserves run by RSPB and The National Trust for Scotland have reported that guillemots appear to have suffered worst, with parents unable to find enough of the right fish to feed their young. As a result, hundreds of chicks died on nest ledges and in the sea beneath the colonies. 

Breeding failures for guillemots were noted in particular at Fowlsheugh and the Isle of May off the east coast, and for most species off the west coast at widely spaced sites including Handa, St Kilda, Tiree and Ailsa Craig. 

Kittiwakes have fared little better. On St Kilda only 1,516 kittiwake nests were found overall – a decline of 61% since 1999. The species also suffered on Mingulay with a 23% reduction in numbers since 2003 and a 30% reduction in guillemots over the same period. Even on Canna, where the removal of nest marauding, non-native rats has caused an overall improvement in the seabirds’ fortunes, productivity was only 0.5 chicks per kittiwake nest, significantly less than the long term average. 

Both guillemots and kittiwakes rear their young to fledging almost exclusively on a diet of energy-rich sandeels. However, this year they were observed bringing in pipefish instead, a stiff, bony fish that lacks the nutritious value of the sandeel and often becomes lodged in the throats of young birds. 

The failures are thought to be the result of many factors combining to have a greater influence than they would individually. The late start to the breeding season was caused by prolonged, inclement weather throughout most of May. Sandeel stocks in the North Sea may also have been given insufficient time to recover properly as the fishery was reopened this year following only a 9 month ban in 2005. The large Danish fishery is believed to have closed prematurely this year due to a scarcity of fish. Field reports have indicated that the size of the individual sandeels was very small this year.  

Norman Ratcliffe, a senior research biologist at RSPB, who specialises in seabirds, said: “The season started off well for most species, but then deteriorated late in the year at a time when the chicks were well grown and in a typical year would have survived. It is plausible that the failures are explained by a timing mismatch: sandeels tend to burrow into the substrate after mid July and so become unavailable to most seabirds. Usually this does not matter since most seabirds will have fledged their chicks by then and can disperse to find alternative prey, but with the delay to the breeding season this year, seabirds still had chicks in July and so the loss of key prey locally resulted in starvation.”  

He added: “Guillemots may have suffered in particular because they are only able to transport fish back to their young one at a time. Other species can catch and transport a load of small fish at one go in their beak or crop, and so prey size is less of an issue for them. With guillemots, it’s like taking chips back to your kids at home one at a time rather than buying them in a bag.”  

The Centre for Ecology and Hydrology, along with RSPB, conducted detailed diet studies of guillemots at a large number of colonies around the UK this summer, and will be analysing the data to investigate whether fish size or quality was a contributor to the observed failures. Unfortunately, the future work of the CEH seabird team, who carried out this work, is in doubt owing to the closure of the Banchory research station. This type of research is vital if we are to understand better what is going on in the North Sea. 

Other species, including puffins, Arctic terns and Arctic Skuas fared rather better. Success was patchy on Orkney, but productivity of most species in Shetland was similar to or slightly improved on 2005. Colonies along the Irish Sea coast again remained high, indicating that this region seems to be consistently avoiding the problems evident further north on the west coast and in the North Sea. 

Richard Luxmoore, Head of nature conservation with the National Trust for Scotland, said: “Here in Scotland we have a global responsibility, with over 40 per cent of the seabirds breeding in the European Union, and yet we know little of what impacts their populations. While some studies have been carried out in the North Sea, on the west coast, where the vast colonies of St Kilda, Mingulay, Rum and the Shiants are located, we know virtually nothing of the relationships between seabirds and the factors at sea driving their vital food supplies. Until we devote more resources to this crucial research we will simply be documenting their inexorable demise.”  

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