In August 2010, six small islands in northern Shetland were prospectively surveyed for breeding colonies of Leach’s Storm-petrels Oceanodroma leucorhoa using vocalisation playback methods. Four adult birds, two with a nest, were located on 10 August in burrows at a new site for the species in the UK: Gloup Holm, off north Yell. Under license, three birds were temporarily extracted and examined; all had brood patches entirely bare of feathering and highly vascularised. Eggs and chicks were not located but burrows were all too deep to check conclusively. Future surveys of Gloup Holm and other small islands in the north of Shetland, carried out in June, may locate further adults, eggs and chicks.

In Britain and Ireland, the vast majority of breeding Leach’s Storm-petrels Oceanodroma leucorhoa nest on islands and sea stacks in the Western Isles, Scotland, in particular on St Kilda, the Flannan Isles and North Rona, where a total of c. 48,000 pairs were found between 1999 and 2001 (Mitchell 2004). In the Northern Isles, breeding pairs and colonies are far fewer, with less than five pairs recorded historically at one site (Sule Skerry) in Orkney (Robinson 1934) and a total of c. 35 pairs recorded breeding at two sites in Shetland (Fowler 1982; Mitchell 2004).

The first Leach’s Storm-petrel colony recorded in Shetland was on Foula, where breeding had been suspected since the early 1900s and was confirmed in 1974 (Mainwood 1975; Pennington et al. 2004). An estimated total of 15 apparently occupied nest sites were recorded in 2001–02 (Mitchell 2004); however, individuals have been found predated by feral cats Felis catus and the Foula colony could possibly be extinct (Shetland Ringing Group). In August 1980, using tape playback of Leach’s Storm-petrel calls, Fowler (1982) found birds in burrows on the island of Gruney (Figure 1). One adult was temporarily extracted and found to have a brood patch, but no eggs were located. In July 1981, Gruney was resurveyed and seven burrows were found containing adults, including two with eggs (Fowler & Butler 1982). Since then, the colony has been monitored by RSPB and between 17 and 23 occupied burrows recorded (1992–99: Pennington et al. 2004). It seems probable that Leach’s Storm-petrels may also have bred in Shetland on Fair Isle, where a downy juvenile was found at the South Lighthouse in October 1975, three were heard calling from the north cliffs in July 1981 and at least two were heard singing from the cliffs and scree in the Kirn o’ Scroo in July 2006 and July 2007 (Fair Isle Bird Observatory Reports 1970–2007).

It has been speculated that the species might breed on other islands in Shetland (Pennington et al. 2004). This, combined with unusually frequent captures of individuals during ringing sessions on Unst and Fetlar (see Discussion), led us to visit six small islands (each < 1 km2) off Unst and Yell in August 2010 to prospect for breeding Leach’s Storm-petrels.

Our special thanks go to Peter Hunter for taking us to the islands in his boat (and for the fried mackerel). We are also very grateful to the Shetland Biological Records Centre for logistical support, to Matt Parsons for advice on tape-luring equipment for storm- petrel ringing, and to Ian Mitchell for helpful comments on an earlier draft.

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