A group led by the Joint Nature Conservation Committee (JNCC) is currently studying the Leach’s Storm Petrel, one of the most enigmatic of St Kilda’s inhabitants. You will not see one during daylight, yet once darkness falls this seabird swarms in its thousands over the steep grassy slopes. There they nest in burrows excavated from the turf and vegetation, but they’ve also adapted to nesting underneath the base stones of cleits. St Kilda holds the largest population in the east Atlantic of this most pelagic of seabirds, though it is thought that the number on St Kilda has decreased rapidly in recent years. The reasons for this decline are currently being studied.
One focus of the work has been to catch and ring breeding Leach’s Storm Petrels among the giant boulder field of Carn Mor, on the western face of Hirta. Given repeated visits, it is intended to monitor the rate of mortality of the petrels, and see if fewer birds are indeed returning to breed each year. One suspect in the detective work of identifying a cause for the decline is the Great Skua - or ‘Bonxie’. Other studies have calculated that many thousands of petrels may be eaten by the Bonxies each year, and the latter have dramatically increased in number on St Kilda in recent years. It is too early in the study to know if the rate of mortality of petrels is increasing, but the evidence for predation by Great Skua is clear to anyone who wanders over the skuas’ breeding territories: the regurgitated remains of petrels are strewn over the island.
This year’s ringing has been restricted by poor weather, but even so there have been interesting discoveries made. For example, one individual we ringed in Village Bay was re-caught at Carn Mor two days later, confirming that some birds wander around Hirta -and probably the whole archipelago or even more widely - prospecting for nest sites for next season. Several birds ringed on St Kilda last year have returned to breed in 2005, but a few birds caught this year bear rings from elsewhere - it will be fascinating to discover the details of these. What is clear is that we still know relatively little about the biology or movements of this elusive seabird.
Matt Parsons, JNCC
Petrel boys - (Back) Mark Bolton, Dave Okill, Ken Bruce, Grahame Thompson, (Front) Pete Moore, Matt Parsons