Great Cormorant Phalacrocorax carbo
Status; International importance;
Population
estimates; Distribution; Annual abundance/
productivity; Phenology/diet/survival
Description
The following has been adapted from
original text by Robin M. Sellers in
Seabird Populations of Britain and Ireland (with
permission from A&C Black, London).
Historically, great cormorants have been regarded as primarily
coastal birds in Britain and Ireland, 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 some. In
England, the number 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 colonies 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 UK population, which accounts for
13% of the world population that is restricted to the northern
Atlantic coasts.
Conservation status
Great cormorant is currently identified as a conservation
priority in the following:
EC Birds Directive -
migratory species
Not listed in Birds
of Conservation Concern 3 (2009 update)
(further information on Conservation Designations for UK Taxa)
Amber listed in
Birds of Conservation Concern in Ireland 2 (2007
update)
International importance
*AON = Apparently Occupied
Nests
The UK population figure (rounded to the nearest hundred) was
derived from data in Mitchell, P.I., Newton, S.F.,
Ratcliffe, N. and
Dunn, T.E. (eds.) 2004. Seabird Populations of Britain
and Ireland. Poyser, London.
This was also the source of figures for the Biogeographic and World
populations.
Note: The UK population figure above includes data from both
inland and coastal colonies hence differs from that tabled
below.
UK population estimates and change 1969-2002 (census data)
Great cormorants build large conspicuous nests with coastal
colonies normally situated on stacks, rocky islets, cliffs or rocky
promontories. 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 uncertainty in
population size when counts from a number of years have to be
combined, as was the case during all three national censuses. To
limit this problem, an effort was made during Seabird 2000 to
reduce the number of years over which counts were obtained.
However, the timing of breeding by different pairs of great
cormorants within the same colony is not always synchronous
resulting in 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 within a given year (1 May-25
June), so population estimates are comparable although the
absolute size of the breeding population is probably
underestimated.
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| UK Population estimate (AON*) |
6,071 |
6,610 |
7,238 |
| % change since previous census |
N/a |
+9 |
+10 |
*AON = Apparently Occupied
Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
For census results for individual countries and Ireland, the
Channel Islands and the Isle of Man see under relevant
sections below.
Distribution/abundance
The Seabird
2000 census provides the most comprehensive recent
assessment of the distribution and abundance of breeding seabirds.
Numbers of great cormorant found in different regions,
and a map showing where colonies are found and how large they are,
is provided in the Seabird 2000 great
cormorant results page (PDF, 2.0 mb).
An
interactive map is available on the NBN Gateway, where you can
filter to display only the Seabird 2000 data. For more
recent, but less comprehensive, coverage view the distribution on
the NBN with all available contributing datasets
The locations sampled during the annual
Seabird Monitoring Programme provide some information on
distribution and are accessible via the Seabird Monitoring Programme
online database.
Annual abundance and productivity by geographical
area
With reference to the regional
accounts below please note the following.
Breeding abundance:
graphs of abundance index with 95% CLs are only shown for a region
where the trend produced has been deemed accurate (see
methods of analysis). Where a trend was thought to be
inaccurate, graphs of abundance at major colonies in a
region may be shown instead, particularly if such colonies
hold greater than 10% of the regional population,
are monitored frequently and may thus help illustrate regional
population fluctuations outwith national censuses.
Occasionally, too few data have been collected regionally to
produce either of these.
Productivity: graphs
of productivity are only shown if analysis of breeding
success data produced a significant result for regional and/or
year effects (again see
methods of analysis). If results were not significant,
then a regional mean productivity value is given.
However, on some occasions too few data are
available from which to provide a meaningful average.
Furthermore, for 11 species where the quality of monitoring data
available was considered high, population viability analysis
was undertaken at the UK level and the results of this are
also reported.
Breeding abundance
Figure 1: Trend in UK
abundance index (solid line) of great cormorant 1986-2011 with 95%
confidence limits (dotted lines). Based on SMP data; view
the methods
of analysis (PDF 158 kb).
The abundance index for cormorant (which
includes inland and coastal breeders) increased between 1986 and
1995 but has mostly been in decline since then, bar a short period
of increase from 2000-2004. The index in 2011 has fallen back
to levels recorded at the start of the SMP. In the longer term,
census results indicate that the coastal population increased
slightly between 1969/70 (6,100 pairs) and 1985-88 (6,600 pairs).
In the UK, inland breeding cormorants are largely confined to
England and increased markedly during the 1990s (helped in part by
immigration of birds from continental Europe of subspecies P.
c. sinensis) but have declined since 2002. At the last
census, in 1999-2002, inland breeders in the UK comprised about 18%
of the total breeding population (c.1,646 of 8,884 AON).
There is pronounced regional variation in the trends of abundance
in great cormorant; populations in northern Scotland have declined
particularly severely. Increases in abundance up to 1995
are likely to have been facilitated by increased legal protection
instigated under the Wildlife and Countryside Act
1981. Factors responsible for recent declines are
likely to include increased mortality from licensed and unlicensed
shooting1,2, as well as possible changes in
food availability.
Productivity
Figure 2: Trend in UK productivity (no. of
chicks fledged per pair) of great cormorant 1986-2011. Based
on SMP data; view the methods
of analysis (PDF 158 kb).
Average productivity has declined over
the monitored period and has been relatively low
since 2005; the causes of this are unknown.
Recent analysis of the SMP
dataset by Cook and Robinson3 found the
average breeding success in the great cormorant to be
1.89 between 1986 and 2008 and declined at a rate of
0.027 chicks per nest per year. This equates to a decline in
breeding success of 47% between 1986 and 2008. The quality of
the dataset meant a change greater than 5% over 25 years would
be detected with confidence. Although breeding success has
shown a significant decline over this time period, because the
number of nests monitored each year fluctuated widely from 48 in
1989 to 1,095 in 2002, this trend may not be representative of the
population as a whole. If this level of breeding success was
maintained, population viability analysis (using available life
history information such as population size, clutch size, age
at first breeding and survival rates of different age
classes) suggests a large population increase, greater than
200%, may be expected over the next two decades. For the
population to decline by 25% over 25 years, breeding success would
have to fall to 0.70 chicks per nest per year.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
3,438 |
2,986 |
3,626 |
| % change since previous
census |
N/a |
-13 |
+21 |
* AON = Apparently Occupied Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
Breeding Abundance
Between Operation Seafarer and the Seabird Colony Register the
great cormorant population declined by 13% although numbers had
subsequently recovered by Seabird 2000, increasing by 21% since the
previous census. The number of breeding pairs has probably declined
since Seabird 2000 although the SMP annual sample is not
thought to be representative of the national population (e.g. trend
indicates no change between SCR and Seabird 2000 in contrast to the
above table). However, 28 colonies counted in 2011 held 712 pairs,
38% fewer than the 1,143 pairs they held in
2000. Cormorant colonies can move location quite quickly
so apparent declines at those monitored regularly may be
due to birds moving to nest elsewhere. These new colonies may
not be detected during routine monitoring by the SMP but
will be detected by national censuses, when coverage is
complete.
Productivity
Great cormorants at Scottish colonies on
average fledged approximately 1.82 chicks per pair per
year between 1991 and 2011; there was no
statistically significant variation over time.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
1,057 |
1,220 |
1,315 |
| % change since previous
census |
N/a |
+13 |
+8 |
* AON = Apparently Occupied Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
Breeding Abundance
Figure 1: Trend in abundance index (solid line)
of great cormorant in England, 1986-2011 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
The abundance of great cormorants at colonies in England
peaked in 2003 but declined since, although there
has been greater fluctuation in the trend in recent years. National
census data also indicates that numbers at coastal colonies
have increased since 1969/70. The current population would
appear to be on a par with the 1,315 AON recorded during
Seabird 2000.
In the UK, inland breeding cormorants are largely confined to
England and increased markedly during the 1990s (helped in part by
immigration of birds from continental Europe of subspecies P.
c. sinensis), but have declined since 2002. At the last
census, in 1999-2002, the inland breeding population totalled
1,332 pairs - slightly more than recorded on the
coast.
Productivity
Great cormorants at colonies in England
on average fledged approximately 1.69 chicks per pair per
year between 1986 and 2011; there was no
statistically significant variation over time.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
1,468 |
1,668 |
1,634 |
| % change since previous
census |
N/a |
+14 |
-2 |
* AON = Apparently Occupied Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
Breeding Abundance
Figure 1: Trend in abundance index (solid
line) of great cormorant in Wales, 1986-2011 with 95% confidence
limits (dotted lines; drawing of upper limit restricted to preserve
detail in the abundance index). Based on SMP data; view
the methods
of analysis (PDF 158 kb).
National census data, together with the trend calculated from
colonies sampled by the SMP, indicate that great cormorant
abundance in Wales has remained fairly stable since 1969/70
although there has been some fluctuation. Abundance peaked in
1989 and 1994 with troughs in 1991, 1999 and 2002. Few inland
colonies exist in Wales.
Productivity
Figure 2: Trend in
productivity (no. of chicks fledged per pair) of great cormorant in
Wales, 1986-2011. Based on SMP data; view the methods
of analysis (PDF 158 kb).
Collection of productivity data at colonies in Wales has been
sporadic, especially in the first half of the recording period,
such that no clear trend is evident. However, recently there
appears to have been a decline in the number of chicks fledged each
year. The cause of this is unknown.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
108 |
736 |
663 |
| % change since previous
census |
N/a |
+581 |
-10 |
* AON = Apparently Occupied Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
Breeding Abundance
In Northern Ireland, where there are only a few great cormorant
colonies, numbers underwent a large increase between Operation
Seafarer and the Seabird Colony
Register, multiplying almost six-fold. This was followed
by a decline, with numbers during Seabird 2000 being 10% lower
than in 1985-88. Since then, the most frequently surveyed colony,
at Strangford Lough (the second largest colony in the
country), increased from 278 pairs in 2000 to 437 pairs in
2011 (with a peak of 490 pairs in 2005). It seems likely the
national population has increased too unless numbers on Sheep
Island (the largest colony in Seabird 2000, holding 344 pairs in
1999) have fallen drastically.
Productivity
No systematic data on the productivity of the great cormorant in
Northern Ireland have been collected as part of the SMP.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
1,842 |
3,981 |
4,073 |
| % change since previous
census |
N/a |
+116 |
+2 |
* AON = Apparently Occupied Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
Breeding Abundance
Great cormorant numbers in the Republic of Ireland more than
doubled between Operation Seafarer and the Seabird Colony Register
to 3,981 pairs. Seabird 2000 found numbers had been stable
since the SCR having increased by fewer than 100 pairs.
Data are received from few colonies each year but the three
largest colonies during Seabird 2000, Lambay, Ireland's Eye and St.
Patrick's Island, were all counted in 2009 and 2010 and held
totals of 1,706 and 1,385 pairs in these years respectively
(c.f. 1,539 pairs in 1999). The current status of the national
population is, however, largely unknown as cormorant colonies
may move between years and therefore increases at some
colonies may be offset by decreases elsewhere.
Productivity
Few systematic data on the productivity of the great cormorant
in Republic of Ireland have been collected as part of the
SMP.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
1,950 |
4,717 |
4,736 |
| % change since previous
census |
N/a |
+142 |
<+1 |
* AON = Apparently Occupied Nests
Note: Inland colonies were not counted during the two earlier
national censuses so to enable direct comparison the Seabird
2000 figure refers to the coastal population only.
Breeding Abundance
For the whole of Ireland combined, national censuses
show an increase between Operation Seafarer and the Seabird
Colony Register, but numbers between the SCR and Seabird 2000 were
similar. Since then, data from a few of the largest
colonies suggest numbers have been at least stable in the
interim. However, cormorant colonies may move between
years and therefore increases at some colonies may be offset
by decreases elsewhere, so without recent
comprehensive census data the current population
status is uncertain.
Productivity
Few systematic data on the productivity of the great cormorant
have been collected at colonies throughout Ireland as part of the
SMP.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
35 |
102 |
134 |
| % change since previous
census |
N/a |
+191 |
+40 |
* AON = Apparently Occupied Nests
Breeding Abundance
Great cormorant numbers on the Isle of Man almost trebled
between Operation Seafarer and the Seabird Colony Register to 102
pairs. By the time of Seabird 2000 a further increase had occurred
with numbers rising to 134 pairs. Colonies are few; all were
surveyed in 2008 and 2011 when totals of 232 and 206 pairs
were counted respectively. The 2011 data indicates an increase
of 54% since 1999 when all were counted for Seabird 2000.
Productivity
Few data have been collected on the productivity of great
cormorants on the Isle of Man as part of the SMP.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
62 |
113 |
115 |
| % change since previous
census |
N/a |
+82 |
+2 |
* AON = Apparently Occupied Nests
Breeding Abundance
National census data indicate numbers of great cormorants have
almost doubled since Operation Seafarer in 1969/70. However, this
increase occurred before the 1985-88 census as numbers recorded
during the Seabird Colony Register and Seabird 2000 were
virtually identical. No data have been collected since the
1998-2002 census.
Productivity
No systematic productivity data have been collected for
great cormorant in the Channel Islands as part of the SMP.
UK phenology, diet, survival rates
No data have been collected as part of the Seabird Monitoring
Programme.
References
1 Carss, D.N. 1994. Killing of piscivorous
birds at Scottish finfish farms, 1984-1987. Biol. Conserv.
68, 181-8.
2 C.V.Wernham, M.Armitage, S.J.Holloway,
B.Hughes, R.Hughes, M.Kershaw, J.R.Madden, J.H.Marchant, W.J.Peach
and M.M.Rehfisch, 1999. Population, Distribution. Movements and
Survival of Fish-eating Birds in Great Britain, Department of
the Environment, Transport and the Regions Report, London.
3 Cook, A.S.C.P. and Robinson, R.A. 2010. How
representative is the current monitoring of breeding success in the
UK? BTO Research Report No. 573, BTO, Thetford.
Partners
Data have been provided to the SMP by the generous contributions
of its partners, other organisations and volunteers throughout
Britain and Ireland. Partners to the SMP are: BirdWatch Ireland;
The British Trust for Ornithology; Centre for Ecology and
Hydrology; Countryside Council for Wales; Department of
Agriculture, Fisheries and Forestry (Isle of Man); Department of
Environment, Heritage and Local Government (Republic of Ireland);
States of Guernsey Government; JNCC; Manx Birdlife; Manx National
Heritage; The National Trust; National Trust for Scotland; Natural
England; Northern Ireland Environment Agency; The Royal Society for
the Protection of Birds; Scottish Natural Heritage; Seabird Group;
Shetland Oil Terminal Environmental Advisory Group; Scottish
Wildlife Trust. More about the SMP partners >>
Image of great cormorant appears courtesy of
Ian Rendall ©, is subject to international copyright law and may
not be reproduced in any form whatsoever.