WeBS Core Counts

WeBS Core Counts are made using so-called "look-see" methodology (Bibby et al. 1992), whereby the observer, familiar with the species involved, surveys the whole of a predefined area.

Counts are made at all wetland habitats, including lakes, lochs/loughs, ponds, reservoirs, gravel pits, rivers, freshwater marshes, canals, sections of open cost and estuaries.

Most waterfowl are readily visible. Secretive species, such as snipes, are generally unterrecorded. No allowance is made for these habits by the observer and only birds seen or heard are recorded. The species affected by such lases are well known and the problems of interpretation are highlighted individually in the Species Accounts.

Most species and may su-species are readily identifiable during the counts. Categories may be used, e.g. unidentified scoter species, where it is not possible to be confident of identification, e.g. under poor light conditions.

Species present in relatively small numbers or dispersed widely may be counted singly. The number of birds in large flocks is generally estimated by mentally dividing the birds into groups, which may vary form five to 1,000 depending on the size of the flock, and counting the number of groups. Notebooks and tally counters may be used to aid counts.

Conunts are made once per month, ideally on predetermined 'priority dates'. This enables counts across the whole country to be synchronised, thus reducing the likelihood of birds being double-counted or missed. Such synchronisation is imperative at large sites which are divided into sectors, each of which can be practicably counted bu a single person in a reasonable amount of time. Local Organisers ensure co-ordination in these cases due to the high possibility of local movements affecting count totals.

The priority dates are pre-selected with a view to optimising tidal conditions for counters covering coastal sites at high tide on a Sunday (see Coverage). The dates used for individual sites may vary due to differences in the tidal regime around the country. Co-ordination within a site takes priority over national synchronisation.

The accuracy of each count is recorded. Counts suspected to be gross underestimates of the true number of non-secretive species present are specifically noted, e.g. large flock of roosting waders only partially counted before being flushed by a predator, or a distant flock of sea-duck in heavy swell. These counts may then be traeted differently when calculating site totals (see Analysis).

Observers do not receive official training but most are experienced omithologists and/or counters. Data are input by a professional data input company. Data are keyed twice by different peoble and discrepancies identified by computer for correction. Any particularly unusual counts are checked by the National Organisers and are confirmed with the counters if necessary.

Goose roost censuses

Since many 'gray geese' spend daylight hours in agricultural landscapes, most are missed during counts at wetland by WeBS. Theses species are usually best censused as they fly to or from their roost sites at dawn or dusk since these are generally discrete wetlands and birds often follow traditional flight lines approaching or leaving the site. Even in half-light, birds can generally be counted with relative ease against the sky, although they may not be specifically identifiable at mixed species roosts.

İn some areas, where roost sites are poorly known or difficult to access, counts are made during daytime of birds in fields.

As with WeBS Core Counts, the accuracy of the counts is noted.

Sea-ducks

Consequently, the best counts of most divers, grebes and sea-duck at open coast and many estuarine sites are made simply when conditions allow; only rarely will such conditions occur by chance during WeBS counts. Synchronisation between different sites may be difficult or impossible to achieve, and thus co-ordination of most counts to date has occurred at a regional or site level, e.g. within the Moray Firth, within North Cardigan Bay.

Non-estuarine coastal waterfowl survey

Open coast habitats are relatively poorly covered by WeBS and, consequently, the non-estuarine coastal waterfowl survey (NEWS) was undertaken in December 1997 and January 1998. This concentrated on those waders for which a large proportion of the population occur on the coast away from estuaries, notably Ringed Plover, Sanderling, Purple Sandpiper and Turnstone, thus repeating the Winter Shorebird Count of 1984.

Methods were broadly similar to WeBS, except that counts were usually made on the ebbing tide or at low water and observers walked along the intertidal habitat to ensure that birds amongst boulders and weed in particular were no overlooked.

İrish Wetland Bird Survey

Productivity monitoring

Changes in numbers of waterfowl counted in the UK between years are likely to result from a number of factors, including coverage and weather; particularly for European and Russianbreeding species which may winter further east or west within Europe according to the changes in population size will result from differences in recruitment and mortality between years.

For several species of swans and geese, young of the year can be readily identified in the field and a measure of productivity can be obtained by recording the number of young birds in sampled flocks, expressed as a percentage of the total number of birds aged. Experienced fieldworkers, by observing the behaviour of and relationship between individuals in a flock, can record brood sizes as the number of young birds associating with two adults.

İn fulfilment of the WeBS objectives, results are presented in a number of different sections. An outline of the analyses undertaken for each is given here; further detail is provided in Appendix 3. A number of limitations of the data or these analytical techniques necessitate caution when interpreting the results presented in this report (see İnterpretation of Waterfowl Counts).

National totals

Population estimates are revised once every three years, in keeping with internationally agreed timetables (Rose & Stroud 1994). UK waterfowl population will next be revised in 2000, altrough a number have been revised recently (Appendix 2) for inclusion in the third edition of international Waterfowl Population Estimates for presentation to the Seventh Meeting of the Contacting Parties to the Ramsar Convention, in Costa Rica in May 1999.

The count nearest the priority date or, alternatively, the count co-ordinated with nearby sites if there is considered to be significant interchage, is chosen for use in this report if several accurate counts are available for the same month. A count from any date is used if it is the only one available.

Data from other national surveys are used instead of WeBS counts where the census total provides a better estimate of the total numbers, e.g. the national census of Pink-footed and Greylag geese in October and November. Totals from different censuses are not combined to produce national totals due to lack of synchronisation (birds counted at roost by one method may be effectively double-counted during the WeBS count at a different site in that month), with the exception of a few goose populations where the risk of duble-counting is minimal (see Appendix 2). Consequently, counts from site or regional-based surveys of sea-ducks, for example, are not included in national totals. Data from NEWS are not included in national totals.

For some scarcer species, including many escaped or introduced species, an estimate of the total number recorded by WeBS throughout the country has been provided using summed site maxima, calculated by summing the highest count at each site, irrespective of the month in which it occurred. For some species, this is likely to result in double-counting where birds move between sites.

Annual indices

Because the same WeBS sites are not necessarily covered each year, changes in waterfowl population sizes cannot be determined simply by comparing the total number of birds counted in each year. Consequently, indexing techniques have been developed which allow between-year comparisons of numbers, even if the true population size is unknown.

The 'Underhill index' (Underhill 1989) was specifically developed for waterbird populations and is used in this report for most species. A full explanation of this indexing process is given in Prys-Jones et all. (1994), Underhill & Prys-Jones (1994) and Kirby et al. (1995), with additional information on its use in this report in Appendix 3.

Monthly indices

The abundance of different wildfowl species varies during the winter due to a number factors, most notably the timing of their movements along the flyway, whilst severe weather, particularly on the continent, may also affect numbers in the UK. However, due to differences in sites coverage between months, such patterns cannot be reliably detected using count totals. Consequently, an index is calculated for each month to reflect changes in relative abundance during the seaseon.

The index uses only counts from sites covered in all seven months (September to March). Totals calculated for each month from theses sites only can then be compared directly (expressed as a percentage of the maximum numbers), thus revealing patterns of seasonality for the species considered. These are presented as graphs in the species accounts, giving both the value for the 1997-98 winter, and the average value from the five preceding winters, 1992-93 to 1996-97. Non-migratory, scarce and irregularly counted species are omitted and only WeBS Core Counts have been used in the index.

Broad differences in the monthly values between species reflect their status in the UK. Resident species or those with large UK breeding populations, e.g. some grebes and Mallard, are present in large numbers early in the winter. Declines through the winter result in part from mortality of first year birds, but also birds returning to remote or small breeding sites that are not covered by WeBS. The majority of UK wildfowl either occur solely as winter visitors, or have small breeding populations that are swelled by winter immigrants, with peak abundance generally occurring in mild winter.

The vast majority of the wintering populations of many wader species are found on estuaries, and since coverage of this habitat is relatively complete and mor or less constant throughout winter, meaningful comparisons of total monthly counts can be made for many species. Consequently, monthly indices are not calculated for waders. As counting of gulls and terns is optional, indices are not calculated for these species either.

Site importance

Tables in the Species Accounts rank the principal sites for each species according to average seasonal maxima for the last five seasons in line with recommendations of the Ramsar Convention (see Appendix 2 and Presentation and notation).

The count nearest the priority date or, alternatively, the count co-ordinated with nearby sites if there is considered to be significant interchange, is chosen for use in this report if several accurate counts are available for the same month. A count from any date is used if it is the only one available.

Data from other sources, often involving different methods, e.g. goose roost censuses, are used where these provide better, i.e. larger, counts for individual sites. NEWS data have only been presented for selected species (Ringed Pover, Sanderling, Purple Sandpiper, Bar-tailed Godwit and Turnstone) and only for sites previously noted as being of national importance.

Counts at any site are considered to be incomplete whenever significant under-recording is thought to have occurred, due to part of the site not being counted or adverse counting conditions. When counts from individual count sectors are summed to give an overall species count for complex sites, these counts might have been done under very different conditions, particularly at very large sites and consequently may have quite different qualities assigned to accuracy of the count. Additionally a variable amount of the overall site may have been uncounted.

The importance of the contribution of each count sector to the site total is based on is average contribution to the total at the time of yaer in question and on recent years (to allow for seasonal and long term trends). Further, consideration is given to the fact that a count sector which normally holds a significant proportion of a sites total for species A may hold only a small proportion of the site total for species B. Consequently, if such a count sector is not completely counted, the site total will now be treated as complete for species B but incomplete for species A. These species-by-species qualities are assigned to waders, gulls, terns and herons.