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Britain’s Wintering Black-headed Gull Estimate Fell About 49% in Two Decades

Black-headed gulls in winter plumage on water
Conservation · Population & status

Great Britain’s wintering black-headed gull (Chroicocephalus ridibundus) population was estimated at about 1.1 million birds in the 2023/24–2024/25 Winter Gull Survey. The comparable estimate from 2003/04–2005/06 was about 2.16 million, making the new full-population point estimate 48.9% lower.

The survey was the first comprehensive British winter gull assessment in roughly two decades. Because the two survey periods did not cover exactly the same set of places, the report also used a separate analysis restricted to sectors that had been surveyed in both periods.

Roost surveys were used to estimate birds beyond the largest known sites

Observers counted gulls gathering to roost, usually around dusk. Major known inland and coastal roosts were covered directly, while additional inland and coastal areas were sampled with a stratified design. These sampled areas were then used to estimate birds beyond the best-known large roosts.

The figure of 1,101,791 black-headed gulls is therefore a population estimate rather than a direct count of exactly that many birds. The report shows substantial uncertainty around the precise total, so the result is normally reported as about 1.1 million.

Both comparisons showed a major decline

Comparing the two full population estimates gives a decline of 48.9%. The separate like-for-like analysis of sectors surveyed in both periods estimated an overall decline of about one third across Great Britain.

These figures answer slightly different questions. The full estimates use all available information from each survey period, including sites that were not covered both times. The paired-sector analysis asks how numbers changed within the same set of repeatedly surveyed areas. Both therefore point to a substantial long-term decline, but they should not be treated as interchangeable percentages.

England and Scotland showed the clearest country-level declines

In the paired-sector analysis, both England and Scotland showed clear overall declines of roughly one third. Wales did not show a clear overall change in the same analysis, even though its full population point estimate was lower than in the earlier survey.

This distinction matters because national point estimates can shift when survey coverage or the geographical distribution of birds changes between periods. Repeatedly surveyed sectors provide the more direct comparison of change in the same places.

No single cause explains the decline

The report does not attribute the long-term decline to one cause. Highly pathogenic avian influenza, including severe mortality in 2023, may have contributed to recent losses, but monitoring indicates that black-headed gull numbers were already declining before the major H5N1 outbreaks.

The authors also discuss changes in breeding habitat, disturbance, predation, food availability and broader changes elsewhere in the migratory population as possible influences. The Winter Gull Survey itself cannot determine how much each factor contributed.

The smaller population estimate also lowers the number of birds needed for a winter roost to hold 1% of the Great Britain population. With about 1.1 million birds estimated nationally, that threshold is now roughly 11,000 black-headed gulls.

Sources and image credit

  1. Caulfield, E. B., Clarke, J. A., Burton, N. H. K., Boersch-Supan, P. H., Frost, T. M., Balmer, D. E., & Woodward, I. D. (2026). Winter gulls in the United Kingdom: Results from the 2023/24–2024/25 Winter Gull (Roost) Survey (BTO Research Report 807). British Trust for Ornithology.Primary research report with Great Britain and country estimates, paired trend analyses, methods and discussion of possible drivers.
  2. British Trust for Ornithology. (2026, May 18). Gulls all at sea as UK populations take a dive.Official BTO news release announcing the new Winter Gull Survey results.

Image: Jan Ove Gjershaug / Norwegian Institute for Nature Research / Artsdatabanken — black-headed gulls (Chroicocephalus ridibundus) in winter plumage; representative species image; cropped to 16:9 · CC BY 4.0 — https://creativecommons.org/licenses/by/4.0/ · Source

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