Twelve Years After Rat Eradication, Manx Shearwater Breeding Sites Reach Record High in Isles of Scilly

Twelve years after the launch of a major conservation initiative to eliminate invasive brown rats from the islands of St Agnes and Gugh in the Isles of Scilly, monitoring data from 2025 has revealed a record number of Manx shearwater breeding sites. According to the latest figures, the islands recorded 225 apparently occupied breeding sites for the species, marking a dramatic increase from the 22 pairs registered before the intervention began.

The brown rats that historically threatened the local seabird populations were originally introduced to the islands following 18th-century shipwrecks. Over the generations, these invasive predators established themselves and became a severe hazard to birds that nest in burrows, specifically targeting species such as Manx shearwaters and European storm petrels. Because these seabirds raise their young in underground burrows, their nests are exceptionally vulnerable to predation, creating a major barrier to population recovery.

To address the crisis, the Isles of Scilly Seabird Recovery Project was formed. The collaborative effort brought together local residents, conservationists, and wildlife specialists who partnered with Wildlife Management International Ltd to map out rat nests and activity levels ahead of the eradication campaign. The intensive baiting operation commenced in November 2013. Bait stations were distributed across both islands and checked daily by operational teams tracking consumption rates.

By the end of November 2013, the final evidence of rats on the islands was recorded, though ongoing surveillance was maintained to ensure no animals survived or managed to return. St Agnes and Gugh successfully remained rat-free throughout the winter of 2013-14 and were officially granted rat-free status in early 2017 following subsequent verification checks. Biosecurity measures have remained rigorous, including a visit by a detection dog in February 2025.

The Isles of Scilly Wildlife Trust noted that the removal strategy focused on complete eradication rather than population reduction, an approach that has been applied across more than 700 islands globally. The local ecosystem supports roughly 17,000 individual seabirds across 13 breeding species. The United Kingdom holds the majority of the global population for both European storm petrels and Manx shearwaters, heightening the ecological importance of the restoration effort.

Signs of avian recovery appeared almost immediately after the predators were removed. During the first two years following the operation, conservationists documented successful breeding and recorded more than 40 chicks from both Manx shearwaters and European storm petrels for the first time in living memory. More than a decade after the initial eradication, the population of Manx shearwater pairs has climbed to nearly 200, culminating in the 225 apparently occupied breeding sites documented in the 2025 survey.

The 2025 monitoring data also captured shifts among other seabird populations residing on St Agnes and Gugh. The lesser black-backed gull colony experienced a slight increase, reaching 491 pairs, which represents its highest level since 2014. Meanwhile, the kittiwake colony recorded 32 apparently occupied nests—an increase from the prior year—although observers noted that overall breeding success was lower. Monitoring teams additionally documented instances of cat predation affecting storm petrels.

Biosecurity teams also navigated minor challenges, such as a potential mouse presence detected during the winter of 2024 after probable signs were found in chicken feed. This prompted a four-month monitoring period, though no further evidence was discovered, leading officials to conclude the animal likely died or perished before reaching St Agnes.

Building on the success achieved at St Agnes and Gugh, conservation organizations are currently formulating plans to expand rat eradication efforts to neighboring islands. A new project is under development targeting St Martin's, Tresco, Bryher, and roughly 21 surrounding uninhabited islands. Because these landmasses are geographically interconnected, planners emphasize that any future operation would need to be executed simultaneously across the entire region. Since 2025, project organizers have hosted open days and co-design workshops to actively involve local island communities in the planning stages.