Abstract from BTO Research Report No. 380:

N.H.K. Burton, A.N. Banks, J.R. Calladine, G.E. Austin, M.J.S. Armitage & S.J. Holloway (2005)

Indexing Winter Gull Populations in England, Wales, Scotland and Northern Ireland: an analysis of data from the 1953 to 2004 Winter Gull Roost Surveys ISBN: 1-906204-37-2

Executive Summary

1. Since a first survey of gulls using inland roosts in England in January 1953, gulls have been counted at winter roost sites in England, and later in the rest of the UK, every 10 years. The latest survey covers the winters of 2003/04 to 2005/06 and aims to provide population estimates for the five main species that winter in the country: Black-headed Gull Larus ridibundus, Common Gull L. canus, Lesser Black-backed Gull L. fuscus, Herring Gull L. argentatus and Great Black-backed Gull L. marinus.
   
2. In this report, we compare results obtained from roost sites surveyed in January 2004 with data from the same sites from earlier surveys in order to evaluate how wintering populations of these species have changed over the last five decades.
   
3. As previous surveys did not provide complete population estimates (for the whole of the UK or for individual countries) it is difficult to assess the change in gull populations by simply comparing the summed totals from these surveys. Instead, we used generalized linear models, relating numbers of gulls counted to site and year factors, to create indices of the numbers of gulls of each species in each country.

   Limitations of the models caused by the accuracy of the counts themselves, difficulties in species identification, differences between inland and coastal sites and the lack of ‘nil’ counts in early datasets are considered.

   Models for England considered data from 1953 to 2004; those for Wales, data from 1973; and those for Scotland and Northern Ireland, data from 1983. The models for Northern Ireland were perhaps less representative as only a small number of sites had been surveyed in more than one year.

4. Indices for Black-headed Gull (the most numerous species of gull wintering in the UK) showed that the numbers of this species wintering in England rose by over 200% between 1953 and 1983, but that they have declined by 27% since 1993. Recent declines were also recorded in Wales and Scotland. No trend was apparent for Northern Ireland.
   
5. Common Gulls have risen by over 300% in number in England in winter since the first survey in 1953, though were similar in 1993 and 2004. In Wales, numbers peaked in 1993 and showed a sharp decline between 1993 and 2004. There were no significant differences between years in Scotland. In Northern Ireland, the species’ index showed a marked decline between 1983 and 1993, but a subsequent increase.
   
6. The Lesser Black-backed Gull was formerly an uncommon wintering species in the UK, but has increased hugely in numbers over the last 50 years (by an estimated 30,000% in England). The indices show a continued increase in England and Scotland, but a decline in Wales since 1993. No Lesser Black-backed Gulls were reported in Northern Ireland by the surveys prior to 2004.
   
7. Wintering numbers of Herring Gull rose in England between 1953 and 1973, but fell by 45% over the following decade. Numbers have shown negligible change since then. In Wales, indices show a continued decline since 1973, while in Scotland numbers declined after 1993 following a slight increase over the previous decade. There were insufficient data to ascertain a representative trend for Northern Ireland.
   
8. Wintering numbers of Great Black-backed Gull also rose (by over 300%) in England between 1953 and 1993, but declined by 39% between 1993 and 2004. There were no significant differences in numbers between years in Wales, Scotland or Northern Ireland.
   
9. The changes seen in wintering gull populations in the UK to a great extent reflect those recently recorded by the Seabird 2000 survey of breeding seabird populations in Britain and Ireland (Mitchell et al. 2004). They also reflect changes in the sizes of the other populations of gulls that winter in the UK and changes in species’ wintering distributions.
   
   The increases in gull populations seen through much of the 20th century are thought to be primarily the result of a reduction in human persecution (e.g. from hunters and egg-collection) and an increase in food availability from fishery waste, refuse tips, sewage outfalls and some forms of agriculture (notably outdoor pig farms).

Increases in wintering numbers over this period may also reflect the increased availability of inland roost sites (in the form of gravel pits and water reservoirs) which will have aided the inland spread of species. The large increase in Lesser Black-backed Gull numbers reflects a northward shift in this species’ wintering distribution over the last century.

Recent population declines have been attributed to reductions in fish catches and thus in the amount of discarded waste, decreased use and improved management of refuse tips and improved sewage treatment. Inland wintering Black-headed Gulls might also have been affected by agricultural intensification. The declines in Herring Gull populations since the 1970s are thought to have been in large part due to outbreaks of botulism.

Back to Research Reports 345 onwards