In the UK, Leach’s Storm Petrel Hydrobates leucorhous breeding colonies are few and geographically very remote. Single Leach’s Storm Petrels were heard calling from burrows on Gloup Holm, north Shetland, in July 2019, July 2020 and August 2020, in response to call-playback. The occupied burrow network found in August 2020 was examined by endoscope and a Leach’s Storm Petrel eggshell was seen, constituting the first proven breeding record at this site. A sample call was recorded from each bird heard, sonograms produced, and comparison with criteria for determining sex from calls indicated the bird heard in 2019 was female and those in 2020 were one or possibly two males. Currently, Leach’s Storm Petrel breeds in very low numbers in Shetland on just two small islands. Restoration projects to eradicate introduced predators and preserve natural breeding habitat on select other islands could result in increased breeding numbers.

Leach’s Storm Petrel Hydrobates leucorhous is a highly pelagic burrow-nesting storm petrel, active on land primarily at night (Cramp & Simmons 1977). Individuals can live for 30 years, normally start breeding at four to six years, and both sexes share incubation of a single white egg (Huntingdon et al. 1996; Brooke 2004). The species is very numerous and widespread across the northern hemisphere (estimated global population > 6 million pairs) but is Red-listed and classified as ‘Vulnerable’ by IUCN, due to recent declines of ≥ 30% (BirdLife International 2021). In Europe, breeding has been proven on only a few dozen islands, all in the northeast Atlantic, and the UK breeding sites are very few and extremely remote (Brooke 2004; Mitchell et al. 2004). The most recent complete UK census (Seabird 2000) recorded 15 active colonies and an estimated total population of 48,047 occupied burrows (‘apparently occupied sites’, AOS), with 99.9% on remote islands off the Western Isles — primarily St Kilda (94.5%), the Flannan Isles (3.0%) and North Rona (2.3%) — and 0.1% on two islands in Shetland: Foula and Gruney (Mitchell et al. 2004; Pennington et al. 2004).

Foula was the first Leach’s Storm Petrel colony discovered in Shetland (Pennington et al. 2004). Breeding had been suspected since the early 1900s but was confirmed in 1974 (Mainwood 1975). Fifteen AOS were reported during Seabird 2000, but the colony is now thought to be extinct due to predation by Feral Cats Felis catus and possibly Great Skuas Stercorarius skua (Mitchell et al. 2004; Miles et al. 2012). On Gruney, Leach’s Storm Petrels were first found in August 1980 (at least 10 occupied burrows) and breeding was proven in July 1981 (Fowler 1982; Fowler & Butler 1982). To date, full-scale total population surveys using call-playback methods have been logistically unfeasible on Gruney, due to the time required for full surveys, and also access restrictions imposed by severe sea and weather conditions frequent in Shetland even in summer; occupied burrows have been found using call-playback on just one or a few ad hoc site visits per year (see Ratcliffe et al. 1998 and Murray et al. 2016 for full survey methods). Numbers found have fluctuated but generally decreased since 1980, with five in 2020 being the most recent count (Figure 1). However, such ad hoc methods are likely to underestimate numbers because birds ashore do not always respond to call-playback, due to a variety of possible reasons such as age, sex, time of day, time of year, weather conditions and the acoustic properties of call-back tracks (Ellis et al. 1998).

For many years there was speculation that Leach’s Storm Petrels might breed on other remote islands in Shetland (Pennington et al. 2004). In 2010 and 2011, the Holm of Skaw, Muckle Flugga, Tipta Skerry, Cliff Skerry, the North and South Holms of Woodwick (all off north Unst), Gloup Holm (off north Yell) and Fair Isle were prospectively surveyed for new breeding colonies (Miles et al. 2010, 2012). Three occupied burrows were found on Gloup Holm in 2010, one containing two adults with a nest and the other two each a single adult, but no eggs or chicks were found (Figure 1; Miles et al. 2010).

Burrow occupancy by non-breeding petrels is a normal behaviour that occurs frequently, especially among immature birds that may ‘play house’ and practice nest building for several years before actually breeding (Brooke 2004). Studies of Leach’s Storm Petrels at St Kilda, North Rona and Gruney, for example, have shown that up to 30% of all burrows where a response to playback is heard are occupied by apparently non-breeding individuals with no eggs or chicks (Money et al. 2008; Bicknell et al. unpubl. data 2010; Miles et al. 2012). Therefore, to confirm breeding at any Leach’s Storm Petrel colony requires an egg or chick (or remains thereof) to be sighted or a chick to be heard. Confirmation of whether or not breeding occurs on Gloup Holm, by careful examination of occupied burrows by endoscope, has been one of the key aims of survey visits there since 2010.

Gloup Holm (60°45’N 1°07’W) is a small island of approximately 10 hectares, located 0.5 km off the northwest coast of Yell and approximately 15 km northeast of Gruney. It was resurveyed using call-playback in June and August 2011 and one occupied burrow was found (Figure 1). It was examined by endoscope but was too deep to check conclusively, and no egg or chick was seen (Miles et al. 2012). This short paper presents the results of subsequent Leach’s Storm Petrel surveys of Gloup Holm, in 2019 and 2020, the first proven breeding record, and sonograms of recorded chatter calls.

We are grateful to the Shetland Oil Terminal Environmental Advisory Group (SOTEAG) and the University of St Andrews for logistical support. Our thanks go to Tony Bicknell and Sheila Gear for information regarding Leach’s Storm Petrels on North Rona and Foula, respectively, and to two anonymous referees who provided useful feedback on the draft.

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