WeBS Alerts methods - Species Coverage

The aim of WeBS Alerts is to raise awareness of decreases in numbers of waterbirds that give particular concern to conservationists.

For Special Protection Areas, the suite of species considered has been derived from Stroud et al. (2001). However, as the data suitable for the Alerts process may be restricted to the winter months species that are cited as qualifying in a SPA due to important passage numbers may not feature in the Alerts. Often, in particular for larger sites, it may be possible to assess the trends of additional species for which the site is not considered important. However, these species / site combinations are not considered as part of the standard WeBS Alerts process.

In the case of SSSIs, sites have often been designated for their "general waterbird interest" without specific information regarding the species composition being readily available. Consequently, species for SSSI assessment were selected on the basis that they were either listed specifically within the valid citation or are likely to be a component of an undefined assemblage, such as 'wintering waterfowl' listed in the citation. Their listing by WeBS-Alerts does not necessarily mean that they are legaly notified interests. The species included may be revised in future in response to consultation.

Wildfowl data have been collected from the majority of English, Scottish and Welsh sites since the winter of 1966/67, with wader data available from 1969/70. Earlier, less nationally complete counts date back to the winter of 1947/48 although these data have not been computerised. The survey was extended to include Coot and Great Crested Grebe from 1983/84, Little Grebe from 1985/86 and Cormorant from 1986/87. Wader numbers have been recorded from many sites in Northern Ireland since 1970/71, with numbers of other waterbirds recorded since 1986/87.

WeBS is primarily a survey of non-breeding waterbirds. Consequently, the WeBS Alerts System is concerned with highlighting changes in the abundance of waterbirds outside of the breeding season. With regard to national status, the type of habitat in which some of these species (e.g. Golden Plover, Curlew and Redshank) breed is not well monitored by current WeBS surveys. WeBS data are, therefore, generally not suitable for determining changes in the numbers of breeding waterbirds. For species that do occur on sites monitored by WeBS during the breeding season the WeBS methodology is generally unsuitable for monitoring breeding numbers. WeBS data are, therefore, not suitable for determining changes in the numbers of breeding waterbirds. Furthermore, the way in which numbers recorded by WeBS Core Counts relate to numbers using a site during passage has not been sufficiently researched. Consequently, until this issue is better understood the WeBS Alerts system does not aim to monitor changes in numbers of waterbirds during passage periods.

Thus, throughout the Alerts report the term "winter" is used when referring to the period from which data are used to assess the Alerts status of each species. This period differs between species. WeBS annual indices for wildfowl and other non-wader species are calculated using count data collected between September and March inclusive, and for waders using count data collected between November to March inclusive. These are the months for which the numbers of a given species are generally at their most stable. The same periods are adopted for the assessment of Alert status. A consequence of adopting the standard WeBS indexing months is that, in addition to the breeding season, the passage period is also excluded.

Of the species or distinct populations recorded the WeBS Core Counts scheme, 42 are encountered frequently enough for inclusion in the Alerts System. This list is reassessed with each release of the WeBS Alerts. The inclusion or exclusion of a species from WeBS Alerts reporting does not rest upon an assumption regarding the proportion of the numbers of a given species occurring nationally or on a designated site but rather that the changes in numbers recorded by WeBS are, to the best of knowledge, representative of changes in numbers of that country or site as a whole.

Data covering some species of geese have not yet been incorporated into the WeBS Alerts System. The numbers of these are not well monitored by monthly WeBS Core Counts but rather are preferentially censused using annual co-ordinated goose counts organised by the Wildfowl and Wetlands trust. These include Bean Goose, Pink-footed Goose, Greenland White-fronted Goose, Icelandic Greylag Goose and Barnacle Goose. It is intended that the Alert status of these species will also be assessed from the next release of WeBS Alerts.

For some species, in particular winter swans (Bewick's Swan & Whooper Swan), seaduck (Eider, Common Scoter & Red-breasted Merganser), grassland plovers (Lapwing, Golden Plover) and species occurring in large numbers on non-estuarine coasts (Ringed Plover, Purple Sandpiper, Sanderling & Turnstone), a substantial proportion of wintering birds occur away from those sites monitored by the WeBS Core Count scheme or use these sites at certain times of day that make them unlike to be encountered by WeBS counters. Consequently, although these species are included in the Alerts system this incomplete coverage needs to be borne in mind when interpreting results.  In the case of the winter swans, roost counts supplied by the Wildfowl and Wetlands Trust are used wherever possible rather than WeBS day-time counts.

With regard to the broad scale alerts, the proportion of a species monitored by the WeBS Core Count scheme may be quantified and biases understood by comparison to other surveys such as NEWS and WWT Annual Swan Reports. Thus it is known that WeBS Core Counts monitors between one quarter and one half of over-wintering Ringed Plover, Purple Sandpiper, Sanderling and Turnstone and that the trends reported will be biased towards changes occurring on estuaries. Similarly, trends reported for seaduck and grassland plovers will be biased towards changes occurring within estuaries although in these species the proportion of overall numbers monitored by WeBS Core Counts is less well understood. This is of less concern when assessing site level alerts except in cases of seaduck on sites where the designated boundaries extend out to sea beyond a distance that can be observed from the shore. Even so, if it can be shown that the numbers on that part of the site being monitored relate to numbers on the overall site then the Alerts will be valid. In the case of winter swans, although the sites on which they occur are generally well monitored by WeBS Core counts they are mainly used as roost sites by the birds and therefore changes in the birds daily routine with weather or local feeding opportunities may have considerable influence on the reported trends both at the national and site level.

When considering national status, one can expect a near total coverage of strictly estuarine species over the course of a winter, as over 90% of British estuaries, including all major sites, are counted each month between September and March. Similarly, species that occur principally on larger inland waterbodies are well monitored by WeBS Core Counts although the proportion of the numbers not monitored is largely unquantified. For these species the trends reported can be considered representative of the overall trends. For more widespread species, such as Mallard, a large proportion of birds occur at small inland sites and habitats not well monitored by WeBS Core Counts. The selection of such sites follows no formal sampling pattern and therefore it is unclear as to whether these wetlands are a representative sample of the country as a whole. Again this is of less concern when considering site Alerts other than when making comparisons with regional or national trends.

In some cases it may be feasible to assess national trends for a species where numbers on individual sites are insufficient to monitor site level trends (e.g. Little Egret). Conversely, in some cases where it is inappropriate to assess national trends for a species because WeBS only monitors a small proportion of sites on which that species occurs it may be feasible to assess trends on those sites that are well monitored by WeBS (e.g. Velvet Scoter on the North Norfolk Coast). There remains other waterbird species (e.g. Slavonian Grebe, Bittern) which, although recorded by WeBS, are either encountered too infrequently or in numbers too small from which to assess either national or site trends.


 

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.