WeBS annual indices
WeBS data are used for a wide variety of nature conservation purposes, at international, national and local levels. The UK is a signatory to a number of international conservation conventions; WeBS data are used to help fulfil associated requirements and provide robust evidence to support SSSI, SPA and Ramsar site designations. Data from WeBS are also provided to Wetlands International, which co-ordinates and reports on waterbird monitoring at an international scale.
Each year, outputs take the form of tables listing the principal sites of importance for each species, and annual indices which are used to derive each species’ long-term population trends. However, because the same sites are not necessarily covered by WeBS on every month in every year, relative changes in waterbird numbers cannot be determined simply by comparing the total number of birds counted each year. This issue is addressed by using indexing techniques that have been developed to track relative changes in numbers from incomplete data. For occasions when a particular site has not been visited, an expected count for each species is imputed based on the pattern of counts across months, years and other sites. This means that a complete set of counts are available for all years and all months for a sample of sites. Only sites that have a good overall level of coverage are used (at least 50% of possible visits undertaken) and the underlying assumption is that the pattern of change in numbers across these sites (the index) is representative of the pattern of change in numbers at the country level. Annual index values are expressed relative to the most recent year, which takes an arbitrary value of 100. The ‘Underhill index’ was specifically developed for waterbird populations, e.g. Underhill & Prŷs-Jones (1994).
Underhill, L.G. & Prŷs-Jones, R. (1994) Index numbers for waterbird populations. I. Review and methodology. Journal of Applied Ecology, 31: 463-480.
Annual WeBS Indices (241KB excel file)
The spreadsheet contains one sheet per species (named with their two-letter codes - note the top sheet sets out the species codes). For each species, index values are shown for UK (i.e. E, S, W, NI), GB (i.e. E, S, W), England, Scotland, Wales and Northern Ireland. For some species, not all of these regions are relevant (and hence are not always shown). Note also that not all regional index values start at the same year, for reasons of data availability.
Not all species are included in the indexing process. Terns are excluded because counting of these species is optional - although trends for gulls are now included in the annual report (from 2009/10 report onwards). Species are excluded for which are large proportion of the total population occur on habitats relatively poorly monitored by WeBS (e.g. Snipe), as are species that occur at sites only sporadically and/or in small numbers (e.g. Bean Goose, Smew).
The periods for which indices are calculated have been revised slightly in the light of recent analyses. Data for wildfowl continue to be presented for the period 1966/67 to present. Data from 1974/75 onwards have been used for waders because a high proportion of counts before this winter were imputed. For species added later to the scheme (i.e. Great Crested Grebe and Coot in 1982/83, Little Grebe in 1985/86, Cormorant in 1986/87 and gulls, terns, divers, rare grebes and other species from 1993/94), data from the first two years following their inclusion have been omitted from any indices as initial take-up by counters appears to have been incomplete. This resulted in apparent sharp increases in numbers during this time. For similar reasons, the first two years of data have been excluded from indices for Northern Ireland.