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Abstract from BTO Research Report No 239:

Atkinson, P.W., Austin, G.E., Burton, N.H.K., Musgrove, A.J., Pollitt, M & Rehfisch, M.M. (2002)

WeBS Alerts 1998/99: (2000) Changes in numbers of wintering waterbirds in the UK at National, Country and Protection Area (SPA) scales.

Executive summary

1. The Wetland Bird Survey (WeBS) is a nation-wide scheme that aims to monitor non-breeding waterbirds. It is a joint scheme of the British Trust for Ornithology (BTO), The Wildfowl and Wetlands Trust (WWT), the Royal Society for the Protection of Birds (RSPB), and the Joint Nature Conservation Committee (JNCC - on behalf of the Countryside Council for Wales, Natural England, the Environment and Heritage Service in Northern Ireland and Scottish Natural Heritage). WeBS is an amalgamation of two previous long running monitoring schemes, the Birds of Estuaries Enquiry (BoEE) and the National Waterfowl Counts (NWC).

2. The principal aims of WeBS are to identify important sites and to determine changes in the numbers and distribution of divers, grebes, Cormorant, herons, wildfowl, rails, waders, gulls, terns and Kingfisher in the United Kingdom. Core Counts are made at around 2,000 wetland sites of all habitats although estuaries and large still waters predominate. Volunteers carry out monthly co-ordinated counts, principally from September to March with fewer observations in the summer months. Approximately 250,000 records are collected annually.

3. One of the aims of WeBS is to monitor population change and highlight large changes in populations. This requirement led to the development of an ‘Alert’ system for waterbirds (Atkinson & Rehfisch 2000; Underhill 2000) through which population change can be assessed across a range of spatial scales and for a variety of species. The aim of the system is to take data from the monthly waterbird counts and remove the year-to-year variation in the counts by smoothing the data to reveal the underlying trend in that species’ population. The smoothed index is then used to calculate population changes over 5, 10 and 25 year periods and also over the entire time period that the species was counted for. Species which have undergone major population changes are flagged by issuing an Alert if the population has changed (either increased or decreased) by more than 25% (Medium Alert) and a higher level of Alert (High Alert) if the population has changed by over 50%. This method allows interpretation of the annual indices in terms of short-, medium- and long-term change in the population. For this first implementation, two different methods were used and evaluated. The first is based around a running mean of Underhill indices and in the second, smoothing is performed using General Additive Models (GAMs).

4. This report constitutes the first implementation of the Alert system to WeBS data. Population change was calculated for thirty-seven species of waterbird at UK and country (England, Northern Ireland, Scotland and Wales) scales. For those populations which reached national, or international, importance change was also calculated at 70 Special Protected Areas (SPAs) or proposed Special Protected Areas (pSPAs). Forty-eight of these sites were in England, 6 in Northern Ireland, 14 in Scotland and 2 in Wales. In total, data were available for 623 out of the 967 possible species/site combinations.

5. At the UK scale, most species were stable or increasing over all time periods. Four species raised an Alert. Medium Alerts were raised for Mallard over 10 years, European White-fronted Goose over 10 and all years, Knot over all years and Turnstone over 10 years. A similar situation was found for England but more Alerts were raised in other countries. In Northern Ireland 9 Medium and 3 High Alerts were raised, in Scotland 5 Medium and 5 High Alerts and in Wales 10 Medium and 7 High Alerts were raised.

6. Site Alerts were performed for 623 species/site combinations. Coverage varied between countries. Coverage in Wales was high at 95% but this was mostly due to the small number SPAs. Coverage in England and Northern Ireland was broadly similar at 70 and 68% but low in Scotland at 39%. The low coverage in Scotland was due to large numbers of WeBS species which are not currently covered by the WeBS Alert System.

7. An index of change across all species was developed for each site to identify sites where there are general declines in waterbirds. Over 25 years, most SPAs have shown a general increase in nationally or internationally important waterbird populations. Lindisfarne was the notable in that it has shown consistent declines in waterbirds. Over 10 and five year periods the following SPAs were identified as showing short-term declines in waterbirds: the Blackwater Estuary, Chichester and Langstone Harbours, Medway Estuary, Severn Estuary, the Wash, Belfast Lough, Loughs Neagh & Beg, the Inner Moray Firth and the Burry Inlet.

8. Further development work is recommended. SPA boundaries should be matched as closely as possible to WeBS boundaries as currently some WeBS sites incorporate more than one SPA and vice versa.

9. There is also a need to refine the analysis of population change of all species on each site to identify sites where general declines are occurring across all species. At present the change index is crude and each species has equal weighting in determining (a) the change index and (b) average inter-annual population change for all species. Weighting the inter-annual change by the total flyway population size for that species may be a more effective way of determining where large or widespread declines are taking place.

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