WeBS Alerts methods - Site Coverage

The WeBS Alerts system assesses trends in waterbird numbers both at the broad scale (country and region) and on individual protected sites (SPAs and SSSIs/ASSIs) in the UK where waterbirds are a designated feature and sufficient data are available. Currently, a number of SPAs and many SSSIs/ASSIs cannot be assessed because count data for all cited species were unavailable or largely incomplete. Additionally, a few SPAs and many SSSIs/ASSIs cannot be assessed until ongoing work on establishing coincidence between WeBS count sector boundaries and designated site boundaries has been completed. The areas surveyed during WeBS ‘Core Counts’ at WeBS sites are matched as accurately as possible to designated site boundaries. Where WeBS count-sectors and SPA boundaries do not coincide, the optimum match is assessed.

In many cases, there may be practical reasons for discrepancy. For example, most WeBS counts of estuaries are made at high tide because birds can be more reliably counted as they arrive or when they settle at roost.  Some statutory site boundaries only encompass the intertidal habitat and by definition this will be submerged over the counting period, particularly as WeBS visits target spring high tides.  Also many protected site boundaries will exclude habitat in the adjoining areas where such roosts form and waders in particular may congregate on habitats such as golf courses, industrial sites, agricultural land and even helipads outside of the SPA boundaries.  When birds are distributed across large expanses of intertidal flats it can be extremely difficult to accurately assess numbers. Additionally, there are some designated sites for which WeBS data are available but for which the overlap between designated site boundaries and WeBS count sections is particularly low. Most larger WeBS sites are monitored by subdividing the overall site into manageable count areas. These count areas have evolved through time, generally through further subdivision. However, it is generally not possible to fine tune the best match to reflect recent structural changes because it is essential that the geographic extent of the data being assessed remains constant.

Finally the WeBS Alerts System reports on the overall trends across the SPA suite of sites for each species with the suites for Northern Ireland and Great Britain treated separately. The latter is necessary because of for many species the appropriate suite of months used to assess trends differs between Northern Ireland and Great Britain.


WeBS Alerts citation

This report should be cited as: Cook, A.S.C.P., Barimore, C., Holt, C.A., Read, W.J. & Austin, G.E. (2013). Wetland Bird Survey Alerts 2009/2010: Changes in numbers of wintering waterbirds in the Constituent Countries of the United Kingdom, Special Protection Areas (SPAs) and Sites of Special Scientific Interest (SSSIs). BTO Research Report 641. BTO, Thetford. http://www.bto.org/webs-alerts

WeBS is a partnership between the British Trust for Ornithology, the Royal Society for the Protection of Birds and the Joint Nature Conservation Committee (the last on behalf of the statutory nature conservation bodies: Natural England, Natural Resources Wales and Scottish Natural Heritage and the Department of Agriculture, Environment and Rural Affairs, Northern Ireland) in association with the Wildfowl and Wetlands Trust. Email: webs [at] bto.org.