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Rathlin Census Counted 136,100 Common Murres After an HPAI-related Decline in 2023

Dense breeding colony of Common Murres on a coastal cliff
Conservation · Population & status

A full breeding-seabird census on Rathlin Island in Northern Ireland counted 136,100 common murres (Uria aalge) in 2025. Rathlin holds the largest colony of the species in Britain and Ireland, where the bird is generally known as the guillemot.

The 2025 total was higher than the 113,504 individuals counted in 2023, after an outbreak of highly pathogenic avian influenza (HPAI) in the colony during 2022. The 2021 census had counted 149,510 birds. The report describes the 2025 number as broadly similar to 2021 and interprets the sequence as a decline associated with HPAI followed by recovery.

The huge colony remains difficult to count precisely

The authors stress that common murres are exceptionally difficult to count on Rathlin because of the size and density of the colony. Some densely occupied sections cannot be seen adequately from land or sea, and aerial photographs have been used since 2023 to count birds in areas that were previously partly hidden. The resulting totals have a large and unquantified margin of error, on the scale of thousands of birds, so changes between censuses must be interpreted cautiously.

Differences in how hidden birds are detected also affect comparisons among individual cliff sections. Drone verification in 2025 showed that at least one section had probably been overestimated in earlier censuses, illustrating why the island-wide totals are best treated as monitoring estimates rather than exact population sizes.

Sources and image credit

  1. Mackie, K., Watson, H., Else, R., McFaul, L., & Upton, A. (2026). Rathlin Island breeding seabird census 2025. In A. J. Upton & H. El Haddad (Eds.), Northern Ireland Seabird Report 2025 (pp. 78–85). British Trust for Ornithology.Primary monitoring report published 26 March 2026. The 2025 Rathlin census counted 136,100 guillemots/common murres, compared with 113,504 in 2023 after the colony's HPAI outbreak and 149,510 in 2021. The authors describe 2025 as broadly similar to 2021 but warn that the dense, partly hidden colony produces a large unquantified counting margin and that aerial imagery since 2023 has changed detection in some sections.

Image: Casey Helton / Wikimedia Commons — common murre breeding colony at Cannon Beach, Oregon, 2 May 2026. Representative image for this story; not presented as an animal or scene from the reported study or event. · CC BY-SA 4.0 · Source

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