Seabird 2000
Great Cormorant Phalacrocorax
carbo
Historically, the British and Irish Great Cormorants have been
regarded as primarily coastal birds, but during the last 40 years
there has been a gradual shift of wintering quarters inland, to the
extent that almost every lowland lake and river has its compliment
of Great Cormorants. More recently in England, the number of Great
Cormorants nesting inland in trees has increased from just 151
pairs at one colony in 1986 to 1,334 pairs at 35 colonies in
1999-2002. This growth of inland breeders has been fuelled by the
immigration of the sub-species
P. c. sinensis from
continental Europe.
P. c. carbo nests predominantly on the
coast and constitutes most of the British and Irish population,
which make up 13% and 10% respectively of the world population that
is restricted to the northern Atlantic coasts.
Great Cormorants build large conspicuous nests and the count
unit during seabird 2000 was the apparently occupied nest
(AON).Coastal colonies are located on stacks, rocky islets, cliffs
or rocky promontories and are usually easy to locate. Many colonies
persist in the same place for long periods, but others come and go
or suddenly shift location – the presence of a colony in one year
is no guarantee that there will be one there the following year.
This introduces elementary uncertainty where counts from a number
of years have to be combined, as was the case with Operation
Seafarer (1969-70), SCR Census (1985-88) and Seabird 2000
(1999-2002). To reduce this problem, an effort was made during
Seabird 2000 to reduce the number of years over which counts were
obtained within as large an area as possible – 65% of admin areas
were surveyed in a single year and a further 21% in two successive
years. On a larger scale, all inland colonies in England and all
colonies in north mainland Scotland (east Sutherland to Lochaber)
and in the Firth of Forth were counted in single years. Conversely,
counts in Wales were spread over four years (1999-2002).Unlike many
other seabirds, the timing of breeding by different pairs of Great
Cormorants within the same colony is not always synchronous. This
means that there is no guarantee that a single count of the nests
will reflect precisely the true number of breeding attempts.
Seabird 2000, like previous censuses conducted a single count at an
optimum time (1 May-25 June), so population estimates are
comparable but means that the absolute size of the breeding
population is probably underestimated.