| |
Methods
WeBS Core Counts are made using so-called
‘look-see’ methodology (Bibby et al. 2000),
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 coast and estuaries. Numbers
of all waterbird species, as defined by Wetlands International
(Rose & Scott 1997), are recorded. In the UK, this
includes divers, grebes, cormorants, herons, Spoonbill,
swans, geese, ducks, rails, cranes, waders and Kingfisher.
Counts of gulls and terns are optional. In line with
the recommendations of Vinicombe et al. (1993), records
of all species recorded by WeBS, including escapes,
are collected to contribute to the proper assessment
of naturalised populations and escaped birds.
Most waterbirds are readily visible.
Secretive species, such as snipes, are generally under-recorded.
No allowance is made for these habits by the observer
and only birds seen or heard are recorded. The species
affected by such biases are well known and the problems
of interpretation are highlighted individually in the
Species Accounts. Most species and many subspecies 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 from 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.
Counts 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 by a single person in a reasonable
amount of time. Local Organisers ensure coordination
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. The dates used for individual sites may vary
due to differences in the tidal regime around the country.
Coordination within a site takes priority over national
synchronisation. Counts suspected to be gross underestimates
of the true number of non-secretive species present
are specifically noted, e.g. a large flock of roosting
waders only partially counted before being flushed by
a predator, or a distant flock of seaduck in heavy swell.
These counts may then be treated differently when calculating
site totals. Data are input by a professional data input
company. Data are keyed twice by different people 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.
Bibby, C.J., Burgess,
N.D., Hill, D.A. & Mustoe, S. 2000. Bird Census
Techniques. Second Edition. Academic Press, London.
Holmes, J.S. & Stroud,
D.A. 1995. Naturalised birds: feral, exotic, introduced
or alien? British Birds, 92: 2-11.
Rose, P.M. & Scott,
D.A. 1997. Waterfowl Population Estimates - Second Edition.
Wetlands International Publ. 44, Wageningen, The Netherlands.
Vinicombe, K., Marchant,
J. & Knox, A. 1993. Review of status and categorization
of feral birds on the British List. British Birds, 75:
1-11. |
Background 
