Seabird Group Seabird Group

Sule Skerry – an overspill gannetry from Sule Stack

Mike P. Harris1*, Jez Blackburn2, Dave Budworth3 and Adrian C. Blackburn4

* Correspondence author. Email: mph@ceh.ac.uk

1UK Centre for Ecology & Hydrology, Bush Estate, Penicuik, Midlothian EH26 0QB, UK;

2British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK;

3121 Wood Lane, Newhall, Swadlincote, Derbyshire DE11 0LX, UK;

4Willows End, 27 Palmer Road, Retford, Notts DN22 6SS, UK.

Full paper

Abstract

Northern Gannets Morus bassanus first bred on Sule Skerry, Orkney, Scotland in 2003. A count made from photographs taken using a drone in July 2018 showed that the population had increased to 4,515 apparently occupied sites with a mean annual rate of increase of 19% between 2009 and 2018. As Gannet numbers increased, they displaced several hundred pairs of Common Guillemots Uria aalge and about 500 pairs of Atlantic Puffin Fratercula arctica. Ringing of Gannet chicks showed that some of the increase on Sule Skerry was due to immigration from Sule Stack 8 km away. Northern Gannets on Sule Stack appear to occupy all suitable nesting habitat, and the breeding population has been relatively stable at around 4,500 pairs for the last 100 years. The colony on Sule Skerry is now as large as that on Sule Stack. Given that there appears to be plenty of suitable nesting habitat for more Gannets on Sule Skerry, it seems likely that the colony will continue to increase to the detriment of the nationally important population of Atlantic Puffins.

Introduction

Most species of seabirds breed colonially on isolated islands, high mainland cliffs or other situations where feeding conditions are favourable and safe from ground predators. Colonies can be large and persist for hundreds, even thousands, of years. The Northern Gannet Morus bassanus (hereafter Gannet) is a common seabird in the East and West Atlantic. The species was heavily exploited by humans during the nineteenth century, and at the start of the twentieth century there were only 16 colonies and the total breeding population was estimated at about 53,000 pairs (Gurney 1913). Subsequently, both the number of Gannets and the number of gannetries increased with the most recent census in 2013–14 estimating that the 54 colonies held 526,000 apparently occupied sites (Murray et al. 2015).

Traditionally, the Gannet’s stronghold has been in northwest Scotland (Fisher & Vevers 1943; Nelson 2002). One of the longest established colonies is on Sule Stack, sometimes called Stack Skerry (59°1’N 4°30’W), an isolated rock 68 km west of mainland Orkney, Scotland which has had a large colony of Gannets since at least 1710 (Fisher & Vevers 1943). Thirteen counts of the Sule Stack gannetry between 1904 and 2013 suggest that numbers have been relatively stable with the most recent count putting the population at 4,550 apparently occupied sites (Fisher & Vevers 1944; Murray et al. 2015). Many accounts have noted that Gannets have colonised the entire suitable nesting habitat, the remaining areas being regularly wave-swept and occupied by large numbers of adult-plumaged but non-breeding individuals (Stewart 1938; Nelson 2002; Murray et al. 2015).

About 8 km from Sule Stack lies Sule Skerry (59°5’N 4°24’W), a low-lying island with a large colony of Atlantic Puffins Fratercula arctica and a variety of other seabirds (Figure 1; Blackburn et al. 2007). There is no history of Gannets nesting on Sule Skerry. A lighthouse constructed here in 1893–95 had resident lighthouse keepers until December 1982 when the light was automated. Tomison (1904) was a keeper there from 1896 to 1903 and the only Gannets he recorded on the island were sick or injured birds. None of the last keepers to leave the island recollected Gannets landing on the island and Stark (1967) who was on the island in July 1967 did not mention the species. Members of the Sule Skerry Ringing Group first visited Sule Skerry in 1975 and have returned to the island during 23 summers up to 2018. In the early 1990s, small numbers of Gannets were regularly ashore on the western side of the island but there was no evidence of nesting. In July 2002, about 50 Gannets were present on a flat area of bare rock without any nesting seabirds but close to a colony of Common Guillemots Uria aalge on the upper slopes of a south-facing and sheltered geo (narrow gully). These Gannets left after a few days but in July 2003 there were eight nests with eggs, five nests with chicks and two empty nests in this area (Blackburn & Budworth 2004).

This note (1) presents a count of Gannets breeding on Sule Skerry made in 2018, (2) documents changes in numbers and colony extent between the colony’s foundation in 2003 and 2018 and (3) uses ringing and retrapping data from Sule Stack and Sule Skerry to look for direct evidence of immigration fuelling the rapid increase in Gannet numbers on Sule Skerry.

Acknowledgements

We thank the many members of the Sule Skerry Ringing Group who helped with fieldwork and the Seabird Group for financial support, Stuart Murray and John Love for supplying their pictures of Sule Stack and Sule Skerry respectively, Bob Anderson of MV Halton and his various crew members for safely ferrying us to and from the island and Sarah Wanless and Steve Votier for substantially improving the manuscript.

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