Official statistic on wintering waterbird population trends

Geese and Swans

Having peaked in 2011/12, the trends for the Icelandic population of Greylag Goose suggest a continued apparent decline, with the latest index the lowest since 1965/66. However, this trend needs to be treated with caution due to the increase in the British/Irish population into areas traditionally used by Icelandic birds in winter, which is making an accurate estimate increasingly more difficult. Research into this is ongoing. The numbers of Pink-footed Goose, which also breeds in Iceland, have increased steadily since monitoring started in 1960/61. This increase appears to be slowing down, with a 10-year trend of +19%, down from +33% a year ago.

Both populations of the White-fronted Goose continue to decline, with the index for the Greenland population of the White-fronted Goose at its lowest since 1983/84. Having been affected by HPAI outbreaks in 2021/22 and 2022/23, Svalbard Barnacle Geese appear to have bounced back. The population index is in line with those recorded pre-HPAI outbreak, suggesting that HPAI is no longer depressing the population. Greenland Barnacle Goose was impacted by HPAI in 2022/23 and 2023/24, resulting in a sizeable drop in the population.

Trends for British Greylag Goose and the non-native Canada Goose continue to show increases, although the 10-year trends, +19% and +27% respectively, are slightly lower than the previous year. Another non-native species, Egyptian Goose, attained another record high index with a 10-year trend of +120%. The second consecutive year of lower indices for naturalised Barnacle Goose has reduced the 10-year trend to +9%.

Nearctic Light-bellied Brent Goose attained record high indices in England and Wales, and in Scotland the 10-year trend is still strongly positive at +164% despite a drop in the latest year’s index. The Northern Ireland population has fluctuated with a 10-year trend of -8%. Because the vast majority of the Nearctic population winters in Northern Ireland, the population overall is considered stable with a UK 10-year trend of -2%. The other two Brent Goose populations have contrasting trajectories. While the Svalbard Light-bellied Brent Goose 10-year trend is +13%, Dark-bellied Brent Goose continues to decline with a 10-year trend of -12%.

The index value for Bewick’s Swan is the lowest on record. Its wintering range is now almost entirely restricted to England with a 25-year trend of -96% and 10-year trend of -88%. In contrast, Whooper Swans continue to increase, particularly in England. Mute Swan is experiencing a slight population decline overall with UK 25-year and 10-year trends both at -6%. Regional differences are highlighted by the lowest index in England and in Wales since the late 1990s, while Northern Ireland recorded a positive 10-year trend.

Ducks

Dabbling ducks continue to exhibit variable trends. Shoveler continues to increase with a record high UK index (25-year trend of +82%, 10-year trend of +44%). This mirrors the trend and the record high index in England. In contrast, the populations in Scotland and Northern Ireland are fairly stable, whilst the index in Wales was the lowest since 1994/95 (10-year trend of -22%). Gadwall continued to increase (25-year trend of +51%) with a record high index in Scotland for the third consecutive year. Shelduck remains fairly stable overall (10-year trend of +9%), although a large drop in Northern Ireland this year has resulted in a record low index there. Teal and Wigeon numbers continue to be relatively stable, while Mallard continues to decline.

Diving ducks continued to show marked declines in both 25- and 10-year trends. Both Eider and Scaup have a record low index and UK 10-year trends of -20% and -57% respectively. Tufted Duck index is the lowest since 1985/86 with a UK 10-year trend of -22%. Goldeneye has the lowest index since 1969/70 and a UK 10-year trend of -20%. Pochard continues to decline across the UK (10-year trend of -33%). All of these three species recorded a record low index in Northern Ireland.

Both sawbill species, Goosander and Red-breasted Merganser, have declined with 25-year trends of -23% and -46% respectively. However, Goosander continues to increase in Wales, where the 25-year trend of +98% and 10-year trend of +46% are reflected in the record high index, while in England the 10-year trend is stable. Both the 10-year and 25-year trends in Scotland (-18%) indicate a decline, although this is largely disguised in the large between-year fluctuations that leave the interpretation open for a broadly stable trend. Red-breasted Merganser continues to decline across the UK with the lowest index in England since 1971/72 and in Wales since 1968/69.

Waders

Avocet, Black-tailed Godwit and Sanderling are the only common wintering wader species with clear long-term increases in the UK. Black-tailed Godwit 10-year trend is +22% and it attained its highest index on record for UK and in Wales. Avocet and sanderling continue to increase with 10-year trends of +35% and +25% respectively, although the index is somewhat down from the previous year’s record highs.

Curlew declines continue (25-year trend of -41% and 10-year trend of -24%) with a record low index in England and Northern Ireland. Bar-tailed Godwit, Lapwing and Golden Plover all continue to decline with 10-year trends of -22%, -12% and -17% respectively. Bar-tailed Godwit had its lowest index on record in Scotland and Northern Ireland.

A number of wader species in long-term decline have stabilised or increased in recent years, most notably Purple Sandpiper (overall 10-year trend +22%), and Dunlin which has an overall positive 10-year trend of +16% despite a record low in Northern Ireland. There is an overall stable 10-year trend also for Turnstone (+3%), Redshank (0%), Ringed Plover (+7%) and Grey Plover (-1%), even though the Redshank index in Northern Ireland and Grey Plover index in Scotland were record lows, and Ringed Plover had its lowest index in Scotland since 1981/82.

Oystercatcher trend is stable overall (10-year trend of -1%). Regional variations in Knot trends continue, including a strong short-term increase in Northern Ireland (10-year trend of +74%) and record low index in Scotland contrasting against a stable UK trend of +3%.

Other waterbirds

Little Egret population growth continues with a 10-year trend of +44% and record high index in Scotland. In contrast, Moorhen (10-year trend of -14%) and Coot (10-year trend of -18%) continue to decline.

Notable population changes

The four non-native species continue to increase in numbers and expand in range in the UK, and as a result show large population increases in both 25-year and 10-year trends. While the rate of increase in Canada Goose (+60% and +27%), naturalised Barnacle Goose (+178% and +9%) and Mandarin Duck (+129% and +30%) appears to be slightly lower than before, the rate of increase for Egyptian Goose shows signs of potentially accelerating (+577% and +120%).

Largest UK population changes for native species were:

• 25-year increases: Little Egret +573%, Avocet +174%, Whooper Swan +171%, Black-tailed Godwit +145%, British/Irish Greylag Goose +120%, Pink-footed Goose +96%.
• 10-year increases: Little Egret +44%, Shoveler +44%, Pintail +39%, Avocet +35%, Sanderling +25%, Whooper Swan +25%
• 25-year declines: Bewick’s Swan -96%, Scaup -73%, Pochard -72%, European White-fronted Goose -67%, Greenland White-fronted Goose -57%, Goldeneye -56%, Red-breasted Merganser -46%, Lapwing -41%, Ringed Plover -41%, Curlew -41%
• 10-year declines: Bewick’s Swan -88%, Scaup -57%, Icelandic Greylag Goose -44%, Pochard -33%, Curlew -24%

Climate/weather

Climate/weather impacts birds as it can affect food availability (e.g. frozen ground affects invertebrate availability), energy expenditure and survival rates. Climate/weather also influences migration routes and the timing of migration. Although species respond differently, the changing patterns of winter climate are contributing to changes in the timing and extent of migration by many species of waterbird, as birds move to milder areas to avoid freezing conditions. Analysis of this is best carried out within the context of longer-term species’ trends at the flyway level, and to this end, WeBS collaborates with other national monitoring schemes through the International Waterbird Census to provide such international contexts and encourage such analyses. The flyway population trends were published in 2021 in the 8th edition of the Conservation Status Report of the African-European Waterbird Agreement (AEWA). The 9th edition of the report published in late 2025 focuses on the pressures and drivers, with revised trends expected to be published in the 10th edition.

Breeding ground conditions

Most waterbirds that we see in the UK in the winter months are migratory, returning to the Arctic and Boreal zones (Greenland, Iceland, high Arctic Canada, Scandinavia, Russia) to breed in the summer. Population levels are influenced by breeding success which is influenced by conditions in their breeding grounds. Key factors influencing breeding ground conditions include climate/weather and predator-prey relations.

Direct human induced pressures

Other possible human-induced reasons for waterbird declines include changes in wetland management, deterioration in water quality, abstraction and drought causing falling water tables, and non-native predators such as Mink. A paper published in 2017 listed intensive agriculture, forestry, increases in generalist predator populations and climate change as possible factors negatively affecting the curlew during the breeding season (Franks et al. 2017), as can disturbance or loss of habitat that reduces the carrying capacity of the estuarine wintering habitat impact the over-wintering population (Woodward et al. 2021).

A potential site-specific pressure is coastal human disturbance; a recent study by Whittingham et al. (2019) found that offshore refuges appear to benefit Turnstone. Habitat loss in the form of loss of intertidal mudflats to land claim and various forms of erosion (Foster et al. 2013) and from impacts of the invasive plant species Spartina anglica (Borges et al. 2021), can also be factors. An example of site-specific pressure is development at busy estuaries, such as Southampton Water.

Conservation Interventions

The key UK sites – protected under various legislative measures – play a critical role in supporting waterbird populations under changing environmental conditions, and operate as a functional ecological network at national and international scales. The UK’s Special Protection Area network holds over one-third of the total UK over-wintering waterbird populations. Recent research by Gaget et al. (2021) suggests that conservation policy aimed at waterbirds and their habitats can help waterbird communities cope with climate warming.

Conservation action such as habitat management carried out on protected sites (as well as in other situations), has played a role in tempering population declines, or restoring or increasing some species population levels (Wauchope et al. 2022). For instance, the Pochard is notable in having less severe declines in SPAs, which may be due to site management in these areas (Caulfield et al. 2025).

Data collection and method overview

The wintering waterbird statistics are comprised of data from two survey components: WeBS core counts and GSMP goose counts.

WeBS core counts: Monthly counts (year-round, but with an emphasis on winter months) of all waterbirds are carried out on coastal and inland wetland sites. The WeBS waterbird counts are made using a so-called “look-see” method (Bibby et al. 2000) where the observer, familiar with the species involved, surveys the whole pre-defined area, and records the numbers of each species of waterbird present. The counts include all waterbird species, including non-natives and escapes, but recording of gulls is optional. Smaller sites are counted as one unit, while larger sites are divided into sectors with sector counts pooled to give the site totals. The same sites and sectors are revisited each survey. Counters are encouraged to do the WeBS counts on pre-determined weekends each month, although weather conditions and counter availability can result in different days being chosen. Synchronising the sector counts within a site is particularly important. Coastal sites are counted at high tide, with a small number at other states of the tide because of site-specific practical considerations.

GSMP goose counts: A number of native migratory goose populations are surveyed by the Goose and Swan Monitoring Programme. Co-ordinated counts are carried out, usually timed to coincide with each species’ peak populations on sites of known concentrations of birds. Goose count data are used to produce the trends for Icelandic Greylag Goose, Pink-footed Goose, Svalbard Barnacle Goose, Greenland Barnacle Goose, Taiga Bean Goose and Greenland White-fronted Goose.

Sample site selection

WeBS Core Counts: WeBS core counts are carried out on pre-determined coastal and inland sites across the UK. This site network is designed to cover the majority of sites where large aggregations of waterbirds are regularly found. Sites include all wetland habitats used by waterbirds – freshwater lakes, ponds, reservoirs, gravel pits, rivers, canals, freshwater marshes as well as open coast and estuaries. The latter habitat provides the majority of data for all the species that tend to congregate in key estuaries over the winter. Sites that are not being counted are advertised to WeBS counters on a vacant sites map, from which new and existing counters can select a site of their choice. The counters are encouraged to choose sites where obtaining data is particularly important, including on SSSIs and SPAs where wintering waterbirds are a feature.
GSMP goose counts: Since wintering geese tend to concentrate at specific locations, both on and off wetland sites, GSMP goose counts are carried out on sites with known concentrations of geese with the aim to arrive at as complete a count for the species or population as possible.

Data submission

The primary method for capturing waterbird data from both schemes is through an online capture system by surveyors themselves or by surveyor co-ordinators, although some data are submitted to BTO via paper forms or spreadsheets to be added to the dataset.

Data validation and verification

The online database is programmed to automatically carry out validation and verification processes of the inputted data. It attaches standardised species nomenclature to the data and carries out automated data integrity checks that ensure that all mandatory information is captured with valid date and time information and accurate geographic location information. Any particularly unusual counts are flagged, checked by the Local Organisers or National Organisers and confirmed with the counters if necessary.

Data analysis

WeBS core counts provide the information used in assessing population trends for most species wintering in the UK. Annual and monthly indices are produced for each species. There are occasions when counters miss one or two months during the season, and so an imputing process is used to derive these missing counts and to make the data comparable from one year to the next. Counts can also be marked incomplete by counters where the count is considered to be a minimum estimate due to visibility, disturbance or inaccessibility of some of the count area, meaning it is possible some birds were missed. At complex sites containing multiple count sectors, some of which may not have been visited, completeness for the entire site is calculated on a species-by-species basis using historical data of bird distribution within the site. The Underhill Index (Underhill and Prys-Jones 1994) was specifically developed to analyse the waterbird population counts to produce a time series of index values for each species or subpopulation. This method includes a calculation to estimate counts for missing and incomplete site-month combinations, based on counts in other months and all sites, hence making counts comparable across years.

Monthly indices reveal the patterns of seasonality for the species concerned. The annual index values are calculated from the average of counts carried out between November and March for waders (for which high numbers of passage birds need to be separated from wintering birds) and between September and March for other species. The index for the resident British/Irish Greylag Goose is based on data from April to September in order to separate them from the migratory Icelandic Greylag Geese in parts of their range where the two populations overlap. These are then fed into General Additive Models (GAMs) to produce smoothed indices and trends which are especially useful when assessing changes through time.

The 25-year and 10-year population trends are calculated by comparing the smoothed index value for the last but one year in the data series and the 25 and 10 years previous to that year. It is customary to truncate the final year when reporting smoothed trends, so whilst data from 2024/25 have been used in creating the smoothed index values, the trend period assessed and reported is until 2023/24.

Index values for 2020/21 are not available due to data limitations related to COVID-19 restrictions (Frost & Calbrade 2022).

More information on the survey protocols, analysis and results can be found on the scheme websites for Wetland Bird Survey and Goose and Swan Monitoring Programme.

Technical details of the WeBS method are set out in a separate document available on the BTO website.

Value of citizen science approach

Over 4,100 WeBS volunteer counters contributed to the survey in 2024/25, making around 44,700 count visits to over 5,900 sectors on over 3,400 sites (mostly estuaries and large still waters). GSMP surveys cover around 100 further sites. This ‘citizen science’ approach, with an estimated value of over £2.5 million per year (using a day rate of £150 a day for the majority of volunteers and £350 a day for ‘Local Organisers’), enables very broad simultaneous sampling coverage that would not be possible using professional surveyors only, and it provides an opportunity for volunteers to engage with nature in an active outdoor interest as they take part in a national conservation/monitoring project.