Population trend Alerts for non-breeding waterbirds on Special Protection Areas (SPAs) of international importance

This page hosts the Official Statistic 'Population trend Alerts for non-breeding waterbirds on Special Protection Areas (SPAs) of international importance', published on 24 April 2025.

Official Statistics description

WeBS Alerts, using data from the BTO/JNCC/RSPB Wetland Bird Survey (WeBS), assess the performance of UK’s SPAs that contain one or more non-breeding waterbird ‘features’. A ‘feature’ is something for which an SPA has been designated, e.g. the population of a particular species or ‘waterbird assemblage’ using that site.

WeBS data from the suite of SPAs in the UK are analysed with the purpose to provide a review of the status of species of waterbirds on each of these sites. Where a species that is a designating feature of an SPA has undergone a major decline in numbers within that SPA, this is flagged by the issuing of an Alert. High (red) Alerts are issued where declines exceed 50%, and Medium (amber) Alerts are issued where declines are 25-50%. Change is assessed over the short term (5yr), medium term (10yr) and long term (25yr), as well as since the baseline data collection that was used for site designation. Trend analysis is also carried out for non-qualifying waterbird features found on sites where appropriate (e.g. where there are sufficient data), but equivalent declines recorded for these species are not officially issued with an ‘Alert status’.

WeBS Alerts are published as Official Statistics every few years to aid monitoring and management of SPAs by Statutory Conservation Agencies. The Alerts are intended to be advisory and to be used as a basis on which to direct research and subsequent conservation efforts if required.

WeBS is the principal scheme for monitoring the populations of the UK’s waterbird populations outside of their breeding season, providing an important indicator of their status and the health of wetlands. Data have been collected annually (since 1947 for wildfowl, 1969 for waders and the 1980s or 1990s for other species). UK and country trends produced from the survey data are published annually as Official Statistics.

The Wetland Bird Survey (WeBS) is a partnership jointly funded by the British Trust for Ornithology (BTO), Royal Society for the Protection of Birds (RSPB) and the Joint Nature Conservation Committee (JNCC), with fieldwork conducted by volunteers.

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Scope of the statistics

• The statistics are presented for UK SPAs where the population of at least one species of non-breeding waterbird is listed in the criteria for site designation and sufficient data exists for at least one of those species.
• The Statistics include trends of those waterbird species in each of UK SPAs where their non-breeding species/population is listed in the criteria for site designation.
• The population trends are calculated for all site/species combinations where sufficient survey data are available.
• The population trends are categorised as large decrease, moderate decrease, stable, moderate increase and large increase over 5, 10 and 25 years and since designation of the SPA.
• An Alert is issued for any designating species that have undergone a moderate or large decreases on an SPA in any of these time periods.

These headline waterbird trends for SPAs are published as Official Statistics. Further detail and site by site narratives can be accessed via WeBS Report Online.

The previous WeBS Alerts were published in 2019.

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Summary of results

Long term change was assessed for 506 site/species combinations on 87 SPAs for 46 species and overall waterbird assemblage. The lower number of assessments in the short and medium term is due to data availability for analysis. A total of 1,995 assessments were carried out across the four assessment periods, resulting in 404 high alerts and 365 medium alerts across these periods (Table 1). The percentage of Alerts (39%) triggered by the assessments is almost identical to the previous Alerts published in 2019 (40%), with an almost identical percentage of high Alerts (20% vs 19%) and a slight decrease in medium Alerts (18% vs 21%).

Table 1. Number of SPA-Species assessments made in short, medium and long term, and since the baseline assessment for an SPA was carried out.

SPA-Species Assessment	Short-term	Medium-term	Long-term	Since Baseline	Total
High Alert (>50% decrease)	46	69	147	142	404
Medium Alert (25–50% decrease)	87	89	106	83	365
Stable	292	250	151	148	841
Medium Increase (33–100% increase)	54	64	45	46	209
High Increase (>100% increase)	25	33	57	61	176
Total	504	505	506	480	1,995

 

The number of assessed waterbird species that are qualifying features on SPAs vary. While there are only one or two on over a third of the SPAs, ten or more feature species have been assessed on 16 of the SPAs. With 24, Firth of Forth has the largest number of assessed features. The number of Alerts triggered on an SPA is not proportionate to the number of waterbird features on the site.

The Alerts are displayed in Figure 1 as stacked histograms which show the Alert status for each of the 87 SPAs assessed during short-, medium- and long-term and since the SPA baseline period. Each bar represents an SPA identified by name and they are sized according to the number of designating waterbird species assessed, which ranges from one to 24. The bars are ordered by the number of Alerts triggered for that SPA, and show the Alerts status for each of the qualifying species in that SPA to help put the Alerts into context.

While no Alerts were issued in almost a quarter of the assessments, for five SPAs there were seven occurrences in the long-term and since baseline assessments where ten or more Alerts were triggered (Table 2).

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Interpretation of results

• These headline Alerts on sites need to be viewed in the context of wider population changes for the species. Declines of a species on a site could reflect wider population trends, site-based issues (i.e. local pressures), or distributional changes of a population within the UK or internationally, e.g. range shifts due to climate change. Site-based interpretation and contextual wider population trends are provided online to help with understanding any implications.
• Alerts are widely used both in a local context, e.g. to inform site management and casework, and in informing us of the wider picture. As an example of informing the wider picture, we know from sharing data with other European countries that waterbirds are increasingly wintering further east, so some of the declines (and hence Alerts), particularly at sites in the west, are likely to be caused by birds wintering at sites further east in the UK or in other countries. For example, there have been distributional shifts in wintering range for three diving duck species: Tufted Duck, Goldeneye and Goosander (Lehikoinen et al. 2013) and for Bewick’s Swan (Beekman et al. 2019) Alerts raised at some sites can be attributed to these broadscale distribution changes, most notably for Bewick’s Swan which has prompted long-term Alerts at all 16 SPAs where the species is a feature and which has disappeared completely from Northern Ireland. However, WeBS monitoring shows that SPA sites remain very important, especially as refuges in years with colder than average weather in eastern Europe.
• These headline Alerts are published to give an overview of the waterbird population trends on SPAs. Local knowledge and more localised WeBS data help give further context for site-specific interpretation of the Alerts, where appropriate. The Alerts assessments and associated narrative interpretations for individual sites are provided online. These highlight instances where local action may be most beneficial. In cases where the interpretation suggests that declines might be caused by site-specific issues, or where the data are difficult to interpret, local knowledge will usually be needed to identify the drivers of change and hence potential management actions. Where available, more localised data from individual WeBS sectors can also be used for this purpose.

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Background and Methods

The UK holds internationally important numbers of non-breeding waterbirds, and the government has agreed to protect these populations. The key mechanism for protection is the network of Special Protection Areas (SPAs), Sites of Special Scientific Interest (SSSI) and Areas of Special Scientific Interest (ASSI).

There are 286 SPAs in the UK, and almost half of these – 133 – have at least one out of 57 species or populations of non-breeding waterbirds, or the overall waterbird assemblage, as a designating feature.

Assessments were carried out on 87 of these SPAs. There were no assessments on 46 SPAs, largely because the designating features were only certain goose populations (33 SPAs) that are covered by Goose and Swan Monitoring Programme (GSMP) rather than WeBS, and for which Alerts assessments are currently not available. In addition, some SPAs do not have adequate WeBS coverage to generate the required survey data.

There were around 80 combinations of species and sites where the available data are unsuitable for assessment of trends either due to insufficient numbers or recorded too infrequently for meaningful interpretation using WeBS Alerts methodology. These include all sites for seven of the designating species which are either encountered too infrequently or are too cryptic to monitor reliably.

WeBS data from those SPAs where non-breeding waterbirds or waterbird assemblages are a designating feature in the UK are analysed periodically with the purpose of providing a review of their status on each of these sites. Records for waders from November to March, and for all other waterbirds from September to March, are included in the analysis. The change is calculated by smoothed trends fitted through site-species indices to iron out temporary fluctuations, for example those caused by the severity of a single winter. Change is assessed over the short term (5yr), medium term (10yr) and long term (25yr), as well as since the baseline data collection that was used for site designation. Note that this baseline data collection for most SPAs usually covered a five-year period. For the purposes of these analyses, the baseline winter used to calculate the change since designation is the middle winter of the five-year baseline/designation period.

The analyses use WeBS data up to and including the 2022/23 winter, with 2021/22 used as the reference winter when calculating the population trends for the 5,10 and 25 year periods and since designation. In line with standard WeBS methodology, the final winter is not used as a reference year due to higher sensitivity of the trend to individual index values in the first and last years.

Note that the same analytical process is carried out for SSSIs and ASSIs, although these are not published as Official Statistics.

More details of the methods, how the Alerts analysis was carried out, and how to interpret the WeBS Alerts are in: Austin, G., Frost, T. & Woodward, I. 2025. Guidance to interpretation of Wetland Bird Survey Alerts. Version 2.

The BTO website has more information on the Wetland Bird Survey. 

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

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Confidence in results, quality assurance and caveats

WeBS Alerts use data from the Wetland Bird Survey, which has run annually (in some form) for over 70 years. Skilled volunteers collect data following standardised, peer-reviewed methods. The submitted data undergo a combination of automated and manual validation and verification processes. Missing values are accounted for using the Underhill indexing method (Underhill & Prys-Jones 1994). The data are then used to produce annual index values for protected sites and for regions and countries within the UK. Changes over time are compared using smoothed trends produced with Generalised Additive Models – this reduces the risk that short-term fluctuations cause false Alerts or obscure long-term trends. The statistical approaches used to generate and compare indices are routinely used in peer-reviewed publications. WeBS Alerts are based on changes in population indices within SPAs.

There are certain groups of species, or aspects of site-specific usage by particular species, for which meaningful analyses cannot be undertaken, or robust interpretation cannot be made. For example, species found in small numbers on sites, such as the Bittern and the rare grebes, and species that are recorded in low numbers on sites due to their cryptic behaviour, e.g. moorhen and snipe. These have been excluded from the Alerts analysis to ensure results are meaningful.

It should be noted that Alerts need to be subject to careful interpretation, and are not always a cause for concern. For example, if a population partially moved to a neighbouring site due to new habitat creation in the area, then the original site may be issued with an alert even though its quality had not changed. Alerts are intended to be advisory and to be used as a basis on which to direct research and subsequent conservation efforts if required.

Further details on site selection, survey methods, and data analysis and limitations are available in the Alerts Guidance document published by BTO. While this document discusses interpretation on both SPAs and SSSIs/ASSIs, only reference in connection with SPAs is relevant for these Official Statistics.

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Drivers of change

Overall changes in bird populations are reported annually in WeBS Official Statistics on wintering waterbird trends, and these widespread changes will be reflected by the WeBS Alerts on individual sites (Mendez et al. 2018). The overarching drivers of change reported in the main WeBS results publication are thus also applicable here, although there is greater variation in causes of change when considering trends at a site level.

Widespread drivers include conditions on breeding grounds in the Arctic and Boreal zones for many migratory species and within the UK for species with resident or partially resident populations; predator-prey interactions; changes in food availability, survival, international distribution and numbers related to winter climate change and weather conditions; direct and indirect effects of water quality and pollution; and regulations throughout the flyway on harvesting and direct management.

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 (under Habitats Regulations 2017 (as amended) and equivalent in Northern Ireland and Scotland) holds over a third of the total UK over-wintering waterbird populations.

Drivers of change of waterbird populations on individual protected sites may be local as well as national in scale. Detailed information for each SPA and each waterbird species recorded on the SPAs is published under the WeBS Alerts tab on WeBS Report Online, which allows users to view population changes on individual SPAs in the context of larger scale population change, and provides some interpretive text for features on SPA sites as to whether the data suggests it is likely there are local site issues impacting the site.

A study of one species (Curlew) found that broadscale factors and breeding success explained recent winter trends better than local factors at a population-level, but that did not mean there would not be an impact of numbers of individual estuaries if disturbance or loss of habitat occurs to such an extent that the carrying capacity of estuaries at which Curlews winter is reduced (Woodward et al. 2022). The importance of having a resilient network of protected sites has been shown for Oystercatcher (Bowgen et al. 2022).

The WeBS Alerts tab on WeBS Report Online also allows users to view changes on the suite of SPAs for which the species if a feature, and the trend in the proportion of the population on the SPA suite. In some cases, this shows an increasing proportion of birds on SPAs, such as Pochard, a declining species. This could be due to factors such as management of protected sites for the species on which it is a feature, or the most suitable habitat being on those sites. The trend of the proportion on the suite may be downwards even if the trend on the suite is upwards, such as in the case of Avocet, a species that has been colonising sites outside its SPA suite as the total population increases.

Local drivers of change in wintering waterbird numbers at a site may include:

• Conservation action, such as habitat management, carried out on protected sites for waterbirds has played a role in tempering population declines, or in restoring or increasing some species population levels (Wauchope et al. 2022).
• Some species exhibit strong wintering site fidelity by individuals, which is a factor contributing to overall observed patterns of reduced numbers of individuals at sites, rather than reduction in site occupancy, in declining populations (Mendez et al. 2016).
• Habitat loss in form of loss of intertidal mudflats to land claim and other forms of erosion (Foster et al, 2013) and from impacts of the invasive plant species Spartina anglica (Borges et al. 2021), can also be a factor.
• There is some evidence of potential impacts of disturbance from human activities and recreation. For example, Whittingham et al. (2020) analysed Turnstone numbers at 19 sites in or ecologically linked to the Northumbria Coast SPA and found that they declined on mainland sites compared to less disturbed offshore refuges. However, they did not find a correlation in the degree of decline with data on recreation levels on the mainland sites.
• Shellfish stocks and their management can impact on numbers of individuals on the site. For example, Oystercatcher is a feature on the Burry Inlet SPA, but not the neighbouring Carmarthen Bay SPA. When the cockle population crashed on the Burry Inlet SPA birds moved to the neighbouring site, and numbers increased again on the Burry Inlet when cockle numbers recovered (Bowgen et al. 2022).
• An increase in kleptoparasitism by Carrion Crows and Herring Gulls is a plausible cause of declines in Oystercatcher on the Exe estuary (Goss-Custard et al. 2024).

Detailed local studies may help reveal further site-specific factors in SPAs that have High or Medium Alerts where the wider context suggests site-specific pressures may be involved.

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Involvement and contacts

• These statistics were produced by the Wetland Bird Survey partnership, with BTO having primary responsibility.
• Quality Assurance was carried out by BTO, JNCC and RSPB, and by ornithologists in each of the Country Nature Conservation Bodies. Additional information on quality assurance and WeBS methods is provided in the Methods section.
• The data are published as a JNCC Official Statistic. If you have any queries, please contact us.

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Relation to other Official and National Statistics, and broader relevance

These statistics form part of a suite of statistics produced through partnership monitoring schemes as part of JNCC’s terrestrial evidence programme.

WeBS Alerts are based on the same WeBS data as the annually published Official Statistic: Statistics on Waterbirds in the UK. The annual Official Statistic assesses overall species trends, whereas the WeBS Alerts assesses trends of qualifying waterbird features on individual protected sites, namely SPAs.

The UK is an important wintering area for waterbirds due to its climate, large number of estuarine feeding grounds, and position on bird migration routes. As a result, a high proportion of the world population of several waterbird species or subspecies occur in the UK. WeBS Alerts provide information that can be used to understand the status and trends of these key populations, thereby contributing to international reporting commitments and to national-level management and policy.

WeBS Alerts focus on population trends in protected sites, and are therefore particularly important for assessing site condition and for informing site management. Further to the Official Statistics presented here, Site Accounts are produced to provide further site-specific interpretation for each species and includes comparisons of the site-level trends with regional and national trends, which can suggest whether population changes are likely to be due to local pressures (which may be addressed by site management) or broader-scale (e.g. climate). Alerts can also be used to target research to determine what pressures may be driving underlying population changes.

JNCC web page with full information on SPAs. 

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References

Atkinson, P.W., Austin, G.E., Rehfisch, M.M., Baker, H., Cranswick, P., Kershaw, M., Robinson, J., Langston, R.H.W., Stroud, D.A., Van Turnhout, C. & Maclean, I.M.D. (2006). Identifying declines in waterbirds: The effects of missing data, population variability and count period on the interpretation of long-term survey data. Biological Conservation 130: 549-559.

Austin, G., Frost, T. & Woodward, I. (2025): Guidance to interpretation of Wetland Bird Survey Alerts. Version 2

Banks, A.N. and Austin, G.E. (2004). Statistical comparisons of waterbird site trends with regional and national trends for incorporation within the WeBS Alerts system. BTO research report 359. BTO Thetford, UK.

Beekman J, Koffijberg K, Wahl J, Kowallik C, Hall C, Devos K, Clausen P, Hornman M, Laubek B, Luigujõe L, Wieloch M, Boland H, Švažas S, Nilsson L, Stipniece A, Keller V, Gaudard C, Degen A, Shimmings P, Larsen B, Portolou D, Langendoen T, Wood KA & Rees EC (2019). Long-term population trends and shifts in distribution of Bewick’s Swans Cygnus columbianus bewickii wintering in northwest Europe. Wildfowl Special Issue 5: 73–102.

Borges, F.O., Santos, C.P., Paula, J.R., Mateos-Naranjo, E., Redondo-Gomez, S., Adams, J.B., Caçador, I., Fonseca, V.F., Reis-Santos, P., Duarte, B. and Rosa, R. (2021). Invasion and extirpation potential of native and invasive Spartina species under climate change. Frontiers in Marine Science, 8, p.696333

Bowgen, K.M., Wright, L.J., Calbrade, N.A., Coker, D., Dodd, S.G., Hainsworth, I., Howells, R.J., Hughes, D.S., Jenks, P., Murphy, M.D., Sanderson, W.G., Taylor, R.C. & Burton, N.H.K. (2022). Resilient protected area network enables species adaptation that mitigates the impact of a crash in food supply. Marine Ecology Progress Series 681: 211-225. https://doi.org/10.3354/meps13922.

Foster, N.M., Hudson, M.D., Bray, S. & Nicholls, R.J. (2013). Intertidal mudflat and saltmarsh conservation and sustainable use in the UK: A review. Journal of Environmental Management, 126, 96-104

Goss-Custard J.D., Austin G.E., Frost T.M., Sitters H.P. & Stillman R.A. (2024). Decline in the numbers of Eurasian Oystercatchers Haematopus ostralegus on the Exe estuary Special Protection Area. Ardea 112: 267–283. https://doi.org/10.5253/arde.2023.a23.

Lehikoinen A, Jaatinen, K, Vahatalo, AV, Clausen P, Crowe O, Deceuninck B, Hearn R, Holt CA, Hornman M, Keller V, Nilsson L, Langendoen T, Tomankova I, Wahl J & Fox AD (2013). Rapid climate driven shifts in wintering distributions of three common waterbird species. Global Change Biology 19:2071–2081. https://doi.org/10.1111/gcb.12200

Méndez V, Gill JA, Alves JA, Burton NHK, Davies RG. (2018). Consequences of population change for local abundance and site occupancy of wintering waterbirds. Diversity and Distributions 24:24–35. 

Stroud, D.A., Chambers, D., Cook, S., Buxton, N., Fraser, B., Clement, P., Lewis, P., McLean, I., Baker, H. & Whitehead, S. (2001). The UK SPA network: its scope and content. JNCC, Peterborough, UK.

Wauchope, H.S., Jones, J.P.G., Geldmann, J.et al. (2022). Protected areas have a mixed impact on waterbirds, but management helps. Nature 605:103–107. 

Whittingham, M.J. et al. (2019). Offshore refuges support higher densities and show slower population declines of wintering Ruddy Turnstones Arenaria interpres. Bird Study 66(4):431–440. 

Woodward, I. D., Austin, G. E., Boersch-Supan, P. H., Thaxter, C. B., & Burton, N. H. K. (2021). Assessing drivers of winter abundance change in Eurasian Curlews Numenius arquata in England and Wales. Bird Study, 68(3):289–301. 

Underhill, L.G. & Prŷs-Jones, R. (1994). Index numbers for waterbird populations. I. Review and methodology. J. Appl. Ecol.,31, 463-480.

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