Common terns are not the most abundant UK tern species, but are
probably the most familiar because their breeding
range extends around much of the British Isles coastline plus
inland on lakes, reservoirs and gravel pits along the large river
valleys of SE and Central England, notably the Thames, Ouse, Humber
and Trent, and along rivers in SE Scotland. They are absent
from most of Wales and SW England, and are largely replaced in
the Northern and Western Isles by Arctic terns.
All tern populations in NW Europe were bought to the brink of
extirpation at the end of the 19th century by hunting of
adults for the millinery trade, but recovered in response to
protective legislation in the early 20th century.
However, over the last three decades the UK common tern population
has remained broadly stable.
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.
All terns breeding in the British Isles 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, adults may move en
masse between different sites. Hence, 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. Thus the majority of British tern colonies were surveyed
in 2000 including extensive surveys of Orkney and Shetland. The
main exception was in the Western Isles; most Lewis and
Harris tern colonies were surveyed in 1999, with those in
the Sound of Harris to Barra Head surveyed in 2002. In
Northern Ireland results from the All-Ireland tern survey
conducted in 1995 were utilised. During the SCR, counts were made
in different years within regions, and inter-colony movements may
have caused greater inaccuracies. Also, coverage of the
Northern Isles was poor. This was overcome by inclusion of data
from the 1980 survey of terns in Orkney and Shetland. Coverage
of inland sites was probably more extensive during Seabird 2000, so
assessment of changes in range and status inland should be
made with caution.
Breeding populations can also fluctuate among years owing to
variations in the proportion of mature birds attempting to
nest. However, comparison with annual counts from sites
throughout the UK indicated that counts during the SCR and
Seabird 2000 were not atypically low. As such, trends estimated
between the two surveys should be reasonably robust.
For census results for individual countries and Ireland, the
Channel Islands and the Isle of Man see under relevant
sections below.
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.
Breeding abundance
Figure 1: Trend in UK abundance index (solid
line) of common tern 1986-2010 with 95% confidence limits (dotted
lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
Overall, the abundance of common terns has
increased slightly since 1986, and in the longer term, between the
first census in 1969/70 and 1985-88, a 9% increase occurred.
However, trends at finer spatial scales have varied considerably
and these are likely to reflect varying pressures facing common
terns in different habitats across their wide geographic
range. Increased predation by non-native species such as American mink
Mustela vison1
and the native red fox Vulpes vulpes have caused declines
in some areas but conservation management to ameliorate these
problems is being undertaken. The species has also benefited
from habitat creation by man in the form of gravel pits, tern rafts
in reservoirs, islets in industrial lagoons and structures in
ports, and from maintenance of habitat on reserves by control of
vegetation succession and gull competition. Maintaining the
population is likely to depend on the continuation of such
management in perpetuity.
Productivity
Figure 2: Trend in UK productivity (no. of
chicks fledged per pair) of common tern 1986-2010. Based on
SMP data; view the methods
of analysis (PDF 158 kb).
Like most of the other tern species
the productivity of common terns has fluctuated somewhat over
the recording period and, although the species has never been as
successful as roseate tern, seldom has it been as unproductive
as Arctic tern. Years of low productivity appear to have come
in pairs e.g. 1990 and 1991, 1996 and 1997 and particularly in 2007
and 2008. There is rarely a single reason for years of poor
productivity, which are usually due to several factors such as
predation2, bad
weather and poor feeding conditions, though common terns have a
broader dieter than many tern species, so are less
affected by changes in prey availability. As common terns
often nest on low-lying ground close to the tide edge, their nests
are vulnerable to erosion and tidal inundation; predictions of
increased storminess and sea-level change under climate
change scenarios may lead to increased prevalence of such
events, though managed realignment of coastal defences may create
new opportunities for nesting.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
4,285 |
6,784 |
4,784 |
| % change since previous
census |
N/a |
+58 |
-29 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of common 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 common terns in Scotland in 2010 was slightly
below the 1986 baseline. However, over this period abundance
increased up to and immediately after the Seabird Colony Register,
before declining, with Seabird 2000 recording 30% fewer common
terns than the previous national census. As for many tern species,
maintaining population levels depends on management of
breeding sites: predator control, habitat creation, competition for
nesting sites and reducing disturbance are key factors at most
breeding localities.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of common
terns in Scotland, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
Although the productivity of common terns
in Scotland has fluctuated somewhat over the recording period there
are very few years when the species has nested successfully; in
most productivity is below 0.60 chicks fledged per
pair. Productivity was particularly low in 1996, 2002 and 2008
due to the additive effects of predation, bad weather and
poor feeding conditions.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
6,099 |
4,659 |
4,676 |
| % change since previous
census |
N/a |
-24 |
<+1 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of common tern in England, 1986-2010 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
Common tern numbers in England decreased by 24% between
Operation Seafarer and the Seabird Colony Register, in contrast to
populations in Scotland and Wales. Seabird 2000 recorded
approximately the same number as during the 1985-88 survey although
the abundance trend above shows an actual increase over this time
which continued, with some fluctuation, up to 2010. The
species has benefited from habitat creation by man in the form
of gravel pits, tern rafts in reservoirs, islets in industrial
lagoons and structures in ports, and from maintenance of habitat on
reserves by control of vegetation succession and gull
competition.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of common
terns in England, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
The productivity of common terns in
England has fluctuated somewhat over the recording period without
showing any clear trend. Productivity has seldom been
high and, although it has been poor some years, neither has it
been excessively 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*) |
292 |
514 |
674 |
| % change since previous
census |
N/a |
+76 |
+31 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of common tern in Wales, 1986-2010 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
From 2007 to 2009, the abundance of common terns in
Wales decreased steeply, falling by two-thirds and below
the 1986 baseline. This has happened after a period, roughly since
the early 1990s, of steady increase. Abundance in 2010 soars again
but with wide confidence intervals, so this is likely
an artefact of the particular colonies sampled. The
population, as shown by national census data, has never been large
but increased during each subsequent census after Operation
Seafarer.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of common
terns in Wales, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
Common terns at Welsh colonies are among the
most successful in the UK, regularly fledging more than
one chick per pair. The low productivity in 1987 would be
considered a relatively productive year in England and
Scotland. Near complete failure was recorded in 1990. No real trend
is visible over the recording period, with productivity fluctuating
widely.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
1,302 |
1,096 |
1,704 |
| % change since previous
census |
N/a |
-16 |
+55 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of common terns at four
colonies in Northern Ireland, 1986-2010.
The number of common terns breeding in Northern Ireland
increased by 55% between the Seabird Colony Register and Seabird
2000 to 1,704 pairs. In 2006, the four largest colonies in the
region held over 2,300 pairs (c.f. 1,570 in 2000) - obviously there
had been an overall increase in the national population - but
numbers at these colonies have now declined steeply to
1,267 pairs in 2010. At Strangford Lough, the largest colony,
numbers in 2009 were close to double that normally recorded,
but had halved in 2010. It would seem that the national
population is again close to that recorded during Seabird 2000.
Productivity
Productivity data for common terns in Northern
Ireland showed no statistically significant variation over time. An
average of 0.35 chicks were fledged per pair 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*) |
2,804 |
1,574 |
2,485 |
| % change since previous
census |
N/a |
-44 |
+58 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of common terns on
Rockabill, 1986-2010.
Between Operation Seafarer and the Seabird Colony Register
common terns in the Republic of Ireland declined by 44%, and
although the population had increased again by 1998-2002, Seabird
2000 still recorded fewer pairs than during the first census.
However, increases have been recorded at the main colonies over the
last decade so the national population will now be higher than
that recorded during Seabird 2000. At the largest
colony, Rockabill (shown above), numbers increased almost
exponentially since 1986, possibly due to immigration,
and 1,940 pairs were recorded there in 2010. This is just one
of some 30 colonies in the country so It is likely that the current
national population lies close to the figure recorded by
Operation Seafarer.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of common
terns in the Republic of Ireland, 1986-2010. Based on SMP
data; view the methods
of analysis (PDF 158 kb).
Productivity in the Republic of Ireland was exceptionally high
in the early 1990s, but since 1998 has been more typical of
levels recorded in other countries in the British Isles.
Productivity in 2007, 2008 and 2010 was atypically 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*) |
4,106 |
2,670 |
4,189 |
| % change since previous
census |
N/a |
-35 |
+57 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of common tern throughout Ireland, 1986-2010 with
95% confidence limits (dotted lines). Based on SMP data; view
the methods
of analysis (PDF 158 kb).
The trend in abundance for common terns throughout Ireland has
been increasing since 1986 albeit with some fluctuation in recent
years. The national population during Seabird 2000 was similar
to that recorded by Operation Seafarer 30 years previously,
although numbers decreased in between. The upward trend evident
above continues beyond the period of Seabird 2000 and suggests
common terns are now more abundant than at any time since recording
began. Rockabill, the main colony in the Republic of
Ireland, has increased substantially during the last 25
years but at Strangford Lough, the main colony in Northern
Ireland, numbers increased until 2005 but have declined since
(excluding 2009) and are now on a par with 1986
levels.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of common
terns in throughout Ireland, 1986-2010. Based on SMP data;
view the methods
of analysis (PDF 158 kb).
The trend in productivity of common terns throughout Ireland is
rather similar to that shown for the Republic of Ireland,
where much of the data have been collected. Productivity
was exceptionally high in 1990 and very low in 2007,
2008 and 2010. If the very successful year of 1990 is
excluded no clear trend is visible; the number of chicks
fledged per pair has been relatively stable albeit with some
fluctuation.
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 |
7 |
N/a |
| % change since previous
census |
N/a |
+600 |
N/a |
* AON = Apparently Occupied Nests
Breeding abundance
Only a few pairs of common terns have bred on the Isle of Man.
One pair was found during Operation Seafarer which had risen to
seven pairs during the Seabird Colony Register. No pairs were
recorded during Seabird 2000 and none have been reported in
the decade since.
Productivity
No systematic data on productivity 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*) |
107 |
227 |
174 |
| % change since previous
census |
N/a |
+112 |
-23 |
* AON = Apparently Occupied Nests
Breeding abundance
Common terns are the only species of tern to breed in the
Channel Islands. During Seabird 2000, 174 pairs were recorded which
represented a decline of 23% since the Seabird Colony Register.
Prior to this numbers had doubled from 107 pairs in 1969/70. No
data has been collected during the last decade so the current
status of the population is unknown.
Productivity
Little systematic data on productivity have been collected
for common tern in the Channel Islands as part of the SMP.
No data have been collected as part of the Seabird Monitoring
Programme.
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 Craik, J.C.A. 1995. Effects of North American Mink Mustela
vison on the breeding success of terns and smaller gulls in
west Scotland. Seabird 17; 3-11.