Arctic Tern Sterna paradisaea
Status; International importance;
Population
estimates; Distribution; Annual abundance/
productivity; Phenology/diet/survival
Description
The following was adapted from
original text by Norman Ratcliffe in
Seabird Populations of Britain and Ireland
(with permission from A&C Black,
London).
Arctic terns are the commonest tern breeding in the UK, but
their northerly distribution means they are less familiar to most
observers. The population is concentrated in the Northern
Isles, with 73% occurring there. In common with other tern species,
Arctic terns were probably reduced to low levels by hunting for the
millinery trade and egging, but have probably increased since the
1930s owing to legal protection. Increasing sandeel stocks in
waters around Shetland through the 1970s and early 1980s and so
improvement in food availability may have also contributed
to population growth. However, a collapse of the sandeel stock
around Shetland between 1984 and 1990 resulted in a reversal
of fortunes.
In western Scotland and the Western Isles, declines and
redistribution of the population have resulted from predation by
introduced American mink, Mustela
vison. Future population trends depend on the success of mink
eradication and control projects being implemented in these areas.
Many Arctic tern colonies at the southern range of the UK
population are increasing, probably in response to site management
for breeding terns.
Conservation status
Arctic tern is currently identified as a conservation priority
in the following:
Amber listed in Birds
of Conservation Concern 3 (2009 update)
EC Birds Directive -
listed in Annex 1
(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.
UK population estimates and change 1969-2002 (census
data)
All terns show a low degree of site faithfulness from one year
to the next, in response to predation or habitat change and
especially in areas where islands and other suitable habitat are
plentiful allowing often en masse movements between
different sites. This is such a problem that in order to gain
an accurate national estimate of tern numbers, a simultaneous
census was planned to cover all colonies in Britain within a single
year. Hence in 2000, the majority of British tern colonies were
surveyed including extensive surveys of Orkney and Shetland. The
main exception was in the Western Isles where most tern colonies in
Lewis and Harris were surveyed in 1999, while those in the south of
the Sound of Harris to Barra Head were surveyed in 2002. For
Northern Ireland it was decided to make the most of limited
resources and utilise results from the All-Ireland tern survey
conducted in 1995. More recent counts (with those from 2000 given
priority) were included for some colonies. Movements among these
regions are unlikely to have caused severe bias in trend
estimation. Thus, Seabird 2000 is likely to have included
counts from the vast majority of Arctic tern colonies, with
comprehensive coverage within their UK range. During
the SCR, coverage of the Northern Isles was poor but this was
overcome by inclusion of data from the 1980 survey of terns in
Orkney and Shetland. There is also debate concerning the
degree to which coverage, changes in methods and survey timing have
contributed to changes in status since Operation Seafarer, and
so long-term changes should be treated with caution.
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| UK Population estimate (AON*) |
51,411 |
76,886 |
53,380 |
| % change since previous census |
N/a |
+50 |
-31 |
*AON = Apparently Occupied Nests
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 Arctic tern found in different regions, and a
map showing where colonies are found and how large they are is
provided in the Seabird 2000 Arctic
tern results page (PDF, 1.1 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
Breeding abundance
Figure 1: Trend in UK abundance index (solid
line) of Arctic tern 1986-2010 with 95% confidence limits (dotted
lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
There is uncertainty, due to questions of compatibility of
methods between censuses, of the magnitude of changes in Arctic
tern population size between 1969/70 and 1985-88, though it
probably increased over this period. The Arctic tern abundance
index based on the SMP sample showed an apparent rapid
increase, followed by decrease, during 1986 to1990 (influenced
by changes at a few large colonies in the Northern Isles
during those years). Thereafter, the index declined to a low
in 2004, after which increases occurred. Declining abundance up to
and including 2004 was in part caused by period poor breeding
seasons in the Northern Isles, in turn attributable to sandeel
shortages linked to oceanographic changes (see below).
Declines in the Arctic tern population has also been caused (in
western Scotland) by predation by American mink, a non-native invasive species. A successful
mink eradication programme on the Western Isles and other
control1 is contributing
towards population increases seen in recent years.
Productivity
Figure 2: Trend in UK productivity (no. of
chicks fledged per pair) of Arctic tern 1986-2010.
Based on SMP data; view the methods
of analysis (PDF 158 kb).
The productivity of Arctic terns is
consistently amongst the lowest of any of the seabirds breeding in
the UK. Since 1986, the annual average productivity has only
risen above 0.40 chicks per pair once, in 2000, and in most
years it fell below 0.30. Very unproductive years were
recorded from 1988 to 1990 and in four of the last seven
seasons, associated with marked shortages in prey
(especially sandeels around the Northern Isles, where most Arctic
terns breed) the effects of which may be exacerbated by
poor weather hampering foraging and chilling eggs and chicks,
together with increased predation by gulls seeking alternative
food sources. Sandeel shortages in Shetland have probably been
caused by oceanographic changes that affected larval
sandeel transport and recruitment from spawning stock in Orkney
2,3.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
46,385 |
71,178 |
47,306 |
| % change since previous
census |
N/a |
+53 |
-34 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of Arctic tern in Scotland, 1986-2010 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
The abundance of Arctic terns in Scotland appears to
show a rapid increase between 1986 and 1988. Thereafter,
the index declined to a low in 2004, after which increases
occurred. This decline was in part caused by a period of poor
breeding seasons in the Northern Isles, which were driven
by sandeel shortages linked to oceanographic changes. In
western Scotland, declines have been caused by
American mink taking adults, eggs and chicks from near-shore
nesting islands. Successful mink eradication programmes on the
west coast are contributing towards population increases
seen recently. Over the longer term, Scotland's Arctic
tern population may not have changed much since Operation Seafarer
although there is some uncertainty over this due to
questions of compatibility of census methods between the national
surveys.
Productivity
Figure 2: Trend in productivity (no. of
chicks fledged per pair) of Arctic tern in Scotland,
1986-2010. Based on SMP data; view the methods
of analysis (PDF 158 kb).
Compared with their conspecific, the common tern, the
productivity of Arctic terns is consistently lower and indeed
amongst the lowest of any seabird breeding in Scotland. Annual
average productivity has risen above 0.40 chicks per pair
only twice in 25 years, and in most years
it is lower than 0.30. Very low productivity was
recorded between 1988 and 1990, 1998, 2001, 2004 and
2005, associated with marked food shortages (especially
sandeels around the Northern Isles, where most Arctic terns breed)
exacerbated by poor weather and increased predation by
gulls seeking alternative food sources. Sandeel shortages in
Shetland have probably been caused by oceanographic
changes that affected larval sandeel transport and recruitment
from spawning stock in Orkney 2,3.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
4,469 |
4,544 |
3,602 |
| % change since previous
census |
N/a |
+2 |
-21 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of Arctic tern in England, 1986-2010 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
There is uncertainty, due to questions of compatibility of
methods between censuses, of the magnitude of changes in Arctic
tern population size between 1969/70 and 1985-88. In England,
numbers appear to have been stable but had declined by one-fifth
Seabird 2000. The abundance index based on the SMP
sample broadly agrees with this but suggests an increase since
then, to 2006 at least after which numbers have fallen slightly
again although confidence intervals are very wide.
Productivity
Figure 2: Trend in productivity (no. of
chicks fledged per pair) of Arctic tern in England,
1986-2010. Based on SMP data; view the methods
of analysis (PDF 158 kb).
The productivity of Arctic terns in England has fluctuated
widely over the 25 years it has been recorded by the SMP. However,
it has not been as consistently low as that recorded at Scottish
colonies and, despite several poor years, 2004 is the only
year in common between the two countries when productivity was very
low.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
436 |
732 |
1,705 |
| % change since previous
census |
N/a |
+68 |
+133 |
* AON = Apparently Occupied Nests
Breeding abundance
There are only a few breeding sites for Arctic terns in Wales,
all situated on the island of Angelsey. Numbers have increased
during each national census, from 436 pairs in 1969/70 to 1,705
pairs in 1998-2002. This increase has continued and in
2010 over 3,500 pairs were found in two colonies so the
national population will have more than doubled over the last
decade. By far the largest colony is on The Skerries, which
held 2,912 pairs in 2010.
Productivity
Figure 2: Trend in productivity (no. of
chicks fledged per pair) of Arctic tern in Wales,
1986-2010. Based on SMP data; view the methods
of analysis (PDF 158 kb).
In Wales, the trend in productivity of Arctic terns was
upward during the period from the early to late 1990s after which
there was a few years of relatively consistent productivity.
However, since 2003 productivity has fluctuated widely possibly due
to food shortage in some years.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
121 |
432 |
767 |
| % change since previous
census |
N/a |
+257 |
+78 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of Arctic tern at
Strangford and Carlingford Loughs, 1986-2010.
During Operation Seafarer, only a few hundred Arctic terns
nested in Northern Ireland. Numbers increased in each subsequent
national census with Seabird 2000 recording 767 pairs. The national
population has increased since then as counts from two of the
more regularly monitored colonies, Strangford and Carlingford
Loughs, show (Figure 1). In addition the Copeland Islands, the
main concentration of nesting Arctic terns, have held between
800-1,000 pairs in each year since 2003. So in 2006
numbers approached 2,000 pairs nationally (there are few other
colonies) and even in 2010, when counts were lower
at Strangford and Carlingford, more than 1,400 pairs
nested at six colonies.
Productivity
Few systematic data on the productivity of Arctic terns at
colonies in Northern Ireland has 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*) |
848 |
1,856 |
2,735 |
| % change since previous
census |
N/a |
+119 |
+47 |
* AON = Apparently Occupied Nests
Breeding abundance
In the Republic of Ireland, Arctic tern numbers also increased
in each subsequent national census. Counts during Seabird 2000
were three times higher than that recorded by Operation Seafarer.
Few Arctic tern colonies are frequently monitored, so the
current status of the population is unknown. Counts at Rockabill
and Lady's Island Lake have been increasing over the last decade
but these colonies may not be representative of the national
trend.
Productivity
Relatively few data are available on productivity but
Arctic terns in the Republic of Ireland on average fledged
approximately 0.41 chicks per pair per year; 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*) |
969 |
2,288 |
3,502 |
| % change since previous
census |
N/a |
+136 |
+53 |
* AON = Apparently Occupied Nests
Breeding abundance
Seabird 2000 found that Arctic terns numbers throughout Ireland
had increased by 261% since Operation Seafarer. In the decade
since Seabird 2000, numbers in Northern Ireland have at least
doubled although it is unknown whether a similar increase has also
occurred in the Republic of Ireland due to a paucity of data.
Productivity
Figure 2: Trend in productivity (no. of
chicks fledged per pair) of Arctic tern throughout
Ireland, 1986-2010. Based on SMP data; view the methods
of analysis (PDF 158 kb).
Productivity data on Arctic terns collected from colonies
throughout Ireland certainly appears to have been declining
over the last decade with very few chicks fledged since 2007. Prior
to this data is lacking for several years so no clear trend is
evident.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
29 |
22 |
8 |
| % change since previous
census |
N/a |
-24 |
-64 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of Arctic
tern at The Ayres, 1986-2010.
The small population of Arctic tern nesting on the Isle of
Man are mostly found near The Ayres or Rue Point.
Twenty-nine pairs were recorded during Operation Seafarer and 22
pairs by the Seabird Colony Register. This had declined somewhat by
Seabird 2000 when only five pairs were recorded although numbers
had been lower during the early 1990s. Since Seabird
2000, greater numbers have nested but the colony still
holds fewer than half the number it once did.
Productivity
Very few systematic data on the productivity of the
Arctic tern have been collected on the Isle of Man as part of
the SMP.
This species does not breed on the Channel
Islands.
UK phenology, diet, survival rates
No data have been systematically collected as part of the
Seabird Monitoring Programme.
References
1 Craik, J.C.A. 1997. Long-term effects of North American Mink
Mustela vison on seabirds in western Scotland. Bird
Study 44; 303-9.
2 Wright, P. J. 1996. Is there a conflict between sandeel
fisheries and seabirds? A case study at Shetland. In: Greenstreet,
S. P. R., Tasker, M. L. (eds) Aquatic predators and their prey.
Fishing News Books, Oxford, pp. 154-165.
3 Wright, P.J. & M.C. Bailey. 1993. Biology of
sandeels in the vicinity of seabird colonies at Shetland.
Fisheries Research Report No. 15/93. SOAFD Marine Laboratory,
Aberdeen, Scotland.
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 Arctic tern appears courtesy of Ian
Rendall ©, is subject to international copyright law and may not be
reproduced in any form whatsoever.