Capsule
UK breeding populations of Great Skua Stercorarius skua, Northern Gannet Morus bassanus, Sandwich Tern Thalasseus sandvicensis and Common Tern Sterna hirundo decreased substantially between the most recent national seabird census (2015–2021 Seabirds Count, hereafter SC) and 2023. The outbreak of high pathogenicity avian influenza (HPAI) in 2021–2022 is the likely primary driver of these declines.

Aims
To produce UK and regional estimates of breeding population change for seabird species of conservation concern since SC, following the 2021–2022 HPAI outbreak, and to determine the extent of colony-level declines during the outbreak in relation to background population trends.

Methods
Colony counts from 2023 for 12 seabird species were obtained from the Seabird Monitoring Programme, while pre-HPAI counts were collated from SC. We modelled 2023 counts using generalized additive models to predict counts for 2023 at sites that were not surveyed. We collated time-series of counts at individual colonies, which were modelled using a Bayesian state-space model for population counts to produce estimates of population size and growth rate, and probability of decline, for the periods 2015–2021 and 2021–2023.

Results
Populations of Great Skua, Northern Gannet, Sandwich Tern and Common Tern decreased between SC and 2023, while Roseate Tern Sterna dougallii numbers showed no change. Great Skua suffered the worst UK decline, decreasing by 73% to an estimated 2,909 apparently occupied territories in 2023. For all remaining species, estimates of 2023 UK breeding populations were highly uncertain, with heterogeneous colony-level population trends during 2015–2021 and 2021–2023, making it difficult to determine the impact of HPAI.

Conclusion
Breeding numbers of multiple seabird species of conservation concern suffered large declines following the 2021–2022 HPAI outbreak. Many UK seabird species then suffered further major losses in 2023. Ongoing monitoring effort is imperative to understand if and how these species recover.