Seabird Group Seabird Group

Predator exclusion fencing improves productivity at a mixed colony of Herring Gulls Larus argentatus, Lesser Black-backed Gulls L. fuscus and Great Black-backed Gulls L. marinus

Sarah A. Dalrymple*

* RSPB, The Lodge, Sandy, Bedfordshire SG19 2DL.

Full paper


A large multi-species gull colony at South Walney in Cumbria, northwest England, has suffered declines over the past 20 years, and from 2016 to 2020 no gull chicks fledged despite up to 4,000 pairs of adults attempting to breed each year. The primary cause of nest failure was predation. In an attempt to reverse this decline, a predator exclusion fence was erected around the remnant gull colony in March 2021, and population and productivity surveys were carried out over the 2021 and 2022 breeding seasons. In 2021, 53, 27 and 40 chicks fledged from 263 Herring Gull, 186 Lesser Black-backed and 38 Great Black-backed Gull nests, respectively, resulting in reproductive success rates of 0.20, 0.15, and 1.05 fledglings per nest. Following the fence erection, in 2022 numbers of nesting birds increased by 151% overall to 575 Herring Gull nests, 553 Lesser Black-backed Gull nests and 28 Great Black-backed Gull nests, with reproductive success rates of 0.4, 0.61 and 1.21 respectively.


Herring Gulls Larus argentatus, Lesser Black-backed Gulls L. fuscus, and Great Black-backed Gulls L. marinus are familiar birds of the coast but are in decline across much of Britain and Ireland.

The Herring Gull population within the United Kingdom was estimated as 130,230 Apparently Occupied Nests (AON) in the Seabird 2000 census (Mitchell et al. 2004), a decline of 13% from the 149,197 AON found in the Seabird Colony Register (1985–88) (Lloyd et al. 1991), which was itself a decline of 48% from the 285,929 AON found in Operation Seafarer (1979 –70) (Cramp et al. 1974).

Lesser Black-backed Gull populations within the United Kingdom showed an increase of 40% in breeding numbers, from 62,321 AON in the Seabird Colony Register (Lloyd et al. 1991) to 87,413 AON in Seabird 2000 (Mitchell et al. 2004). Since 2000, however, there has been evidence of significant declines at natural colonies for this species, including a 98% decline at Orford Ness in southern England between 2001 and 2013, a 46% decline at Skomer, in Wales, between 2000 and 2018, and a 56% decline at neighbouring Skokholm in the same time period, and, at South Walney, a decline of 91% from 19,487 AON in 2000 to 1,981 AON in 2018 (JNCC 2021).

Great Black-backed Gulls showed a small decline in breeding numbers, from 17,415 AON in the Seabird Colony Register (Lloyd et al. 1991) to 16,735 AON in Seabird 2000 (Mitchell et al. 2004). Most birds recorded nested in Scotland; of the 16,735 AON in Seabird 2000, 1,466 AON were in England (Mitchell et al. 2004), with over 50% of the English population nesting on the Isles of Scilly (JNCC 2021).

There are many potential causes of poor breeding success and population declines at gull colonies, including reduction in food sources through closure of landfills (Pons 1992), changes to fisheries discards (Oro et al. 2004), cannibalism (Brown 1967; Camphuysen & Gronert 2010), human disturbance (Robert & Ralph 1975), and predation (Southern et al. 1985; Ellis et al. 2007). Native predators, such as Red Fox Vulpes vulpes (hereafter Fox), can have a serious impact where their populations have increased due to human activities. For example, predator numbers can increase due to additional food source in the form of increased waste or introduced game birds (Delcourt et al. 2022), thereby further exacerbating population declines amongst their prey species. The productivity of Lesser Black-backed and Herring Gull has therefore been shown to decline as Fox sightings increased (Davis et al. 2018).

Predator exclusion fencing has been shown to reduce predation of adult birds, nests, chicks and eggs by mammals in many studies, summarised by literature reviews and meta-analyses (Côté & Sutherland, 1997; Smith et al. 2011; Laidlaw 2021). For example, nest success of Piping Plovers Charadrius relodus was 106% higher when mammalian predators were excluded using 1.1 m tall fencing (Ivan & Murphy, 2005). Furthermore, Minsky (1980) found that excluding Foxes from a colony of Least Terns Sterna albifrons with an electric fence led to an increase in productivity compared to those nests outside the exclusion area, as did Spear et al. (2007), who showed that the daily survival rate of Least Terns increased when an electric fence was erected at nesting sites. A 180 cm high fence with a foot apron and overhang created an effective barrier to feral Cats Felis catus and Foxes, while an electric wire offset further improved the fence efficacy (Moseby & Read, 2006). Robley et al. (2007) also found a fence height of 1.8 m to be most effective to exclude Foxes and feral Cats; they did not, however, find an electric wire addition to the fence to be necessary.

Removal or exclusion of predators is, therefore, an intervention achievable by conservation land managers to improve the breeding success of ground-nesting bird species.

South Walney Nature Reserve comprises the southern end of Walney Island, off the coast of Barrow- in-Furness in Cumbria. It has been managed by Cumbria Wildlife Trust since 1963, and forms part of the South Walney and Piel Flats Site of Special Scientific Interest (SSSI). A colony of Herring Gulls and Lesser Black-backed Gulls inhabit the reserve, as well as smaller numbers of Great Black-backed Gulls, and this gull colony forms a feature of the SSSI and the Morecambe Bay & Duddon Estuary Special Protection Area Special Protection Area.

The colony has experienced several years of low or zero productivity and declines in nesting pairs from 22,750 Herring Gull AON and 18,615 Lesser Black-backed Gull AON in 1974, to 3,218 Herring Gull and 9,489 Lesser Black-backed Gull AON in 2005, down to just 489 AON of Herring Gull and 420 of Lesser Black-backed Gull AON in 2020.

Great Black-backed Gull numbers have also fallen, from a peak of 120 AON in 1998 to 49 AON in 2020 (Cumbria Wildlife Trust, unpublished data). With predation believed to be the main cause of this decline, a permanent predator fence was installed in the winter of 2020/21. This paper assesses the impact on gull productivity in the two years following the installation of this fence.


This work was carried out in 2021 and 2022 as part of Cumbria Wildlife Trust’s annual monitoring of the South Walney Gull Colony. Funds for the permanent predator exclusion fence was provided by several funders including British Birds Charitable Trust, Tesco Community Grants, FCC Communities, Natural England and the LIFE Programme of the European Union as part of the project LIFE on the Edge: improving the condition and long-term resilience of key coastal SPAs in S, E and NW England (LIFE19 NAT/UK/000964). I extend thanks to Tim Frayling and Bart Donato who provided invaluable help with the fieldwork and gull ringing along with immeasurable amounts of guidance and advice. Additional support was provided by Matt Wood, Ian Hartley, Daniel Piec, Leigh Lock, Chantal Macleod- Nolan, Jake Taylor-Bruce, Paul Waterhouse, Lol Carlyle, and Guy Brooke and staff at Spirit Energy. Without the assistance, support and advice of all of those listed, this work could not have been completed.


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