Sandwich Tern Sterna sandvicensis
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).
Sandwich terns exhibit the most erratic population trends and
distribution of any seabird breeding in the UK. The population
fluctuates dramatically among years owing to large variations in
the proportion of mature birds attempting to breed and distribution
varies owing to mass movements between colonies. The species
is distributed widely but patchily around the coasts of the British
Isles, broadly reflecting the availability of favoured nesting
habitat: low-lying offshore islands, islets in bays or brackish
lagoons, spits or remote mainland dunes. Despite frequent changes
in the sites used, the broad distribution in the UK has
changed little over the last 30 years. Sandwich terns are among the
most strongly gregarious of all seabirds, with the population
confined to a small number of relatively large colonies in
which birds nest at extremely high densities.
Tern populations in NW Europe were bought to the brink of
extirpation at the end of the 19th century by egg
collection for food and hunting of adults for the millinery trade,
but recovered in response to protective legislation in the early
20th century. Sandwich terns in the
UK increased from the 1920s to the mid 1980s, with protection
from increasing recreational disturbance on beaches as well as from
persecution probably facilitating this recovery. Annual counts of
the main colonies demonstrated that there was a sustained increase
between the first two national surveys, but that the population
fluctuated erratically around this trend.
Conservation status
Sandwich 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
| UK Population |
% Biogeographic Population |
% World Population |
| 12,500 AON* |
16.9 (ssp.
sandvicensis) |
9.6 |
*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)
Coverage of Sandwich tern colonies was comprehensive during all
three of the national surveys, and so long-term trends will be real
rather than artifacts of survey coverage. However, the size of the
breeding population fluctuates erratically from year to year so
trends based on comparison of two widely spaced surveys must
therefore be viewed with caution, since one of them may have
coincided with a year of temporarily depressed population size.
Because whole colonies may move site within a year or two in
response to changing conditions such movements have the potential
to produce severe bias in national population estimates that rely
on summing counts from colonies surveyed in different years.
To minimise this, all Sandwich tern colonies in the UK
(except for one) were surveyed in 2000. During the SCR
census, counts of colonies within regions were often taken from
different years so, if colonies moved, some pairs may have been
double counted and others omitted.
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(2000-2001)
|
| UK Population estimate (AON*) |
11,068 |
14,766 |
12,490 |
| % change since previous census |
N/a |
+33 |
-15 |
*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 Sandwich tern found in different regions, and
a map showing where colonies are found and how large they are is
provided in the Seabird 2000 Sandwich
tern results page (PDF, 1.0 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
Breeding abundance
Figure 1: Trend in UK abundance index (solid
line) of Sandwich tern 1986-2010 with 95% confidence limits (dotted
lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
Between censuses in 1969-70 and 1985-88, the
UK population increased from 10,500 pairs to 14,800, probably in
part a result of increased legal protection, helping to reduce
disturbance from recreation. The breeding abundance of Sandwich
terns in the UK peaked in 1987, declining thereafter until the
mid-90s when the index reached its lowest point. Since then an
increase is evident up until 2002 after which a decline
occurred. The spike in the index in 2009 was due to an
influx of Sandwich terns - apparently from
continental Europe - nesting at Minsmere (Suffolk) where
only one pair nested in 2008 and none were present in 2010.
Productivity
Figure 2: Trend in UK productivity (no. of
chicks fledged per pair) of Sandwich tern 1986-2010. Based on
SMP data; view the methods
of analysis (PDF 158 kb).
A marked decline in productivity is
obvious since 2000 when Sandwich terns fledged a record number of
chicks. In the 14 years prior to 2000 it could be argued that
productivity shows no clear trend, although in 1991 and between
1997-1999 it was particularly low. Few chicks fledged in these
years due to bad weather and predation and disturbance by a variety
of mammals and gulls; food shortage was implicated at only one
colony. Predation on eggs and chicks by red fox is probably the
most prevalent factor determining productivity, and abandonment of
a predated colony often results1. Red fox populations are
thought to have increased during the past few decades due to less
intensive management by gamekeepers. Nature reserve managers use
electric fences to exclude foxes, which are not always successful.
As Sandwich terns nest on low-lying ground close to the tide edge,
their nests are vulnerable to tidal inundation; predictions of
increased storminess and sea-level change under climate
change scenarios may lead to increased prevalence of such
events.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
2,465 |
2,286 |
1,068 |
| % change since previous
census |
N/a |
-7 |
-53 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of Sandwich tern in Scotland, 1986-2010 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
The Scottish population of Sandwich terns changed little between
the national censuses in 1969/70 and 1985/88 but a large decline
then occurred as numbers fell 53% by the Seabird 2000 census.
The majority of the Scottish population nests at the Sands of
Forvie, a well monitored colony, which held almost half of
Scotland's Sandwich terns during Seabird 2000. The
trend between 1987 and 2000 is largely governed
by the decline observed at Sands of Forvie, caused by
several years of predation by red foxes. However, other large
colonies have become extinct in this period, usually by
the mid to late 1990s (eg. Loch of Strathbeg, Inchmickery,
McDermotts), while at others (e.g. Long Craig, Isle of
May) nesting has only been sporadic. Gulls extirpated the
colony on Inchmickery.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of Sandwich
tern in Scotland, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
The productivity of Scottish Sandwich
terns is generally low, with average values usually below 0.50
chicks fledged per pair and in many years below 0.30.
Exceptions were in 2000 and 2001 when productivity almost
reached 0.70. Predation, by gulls and red foxes,
has often lowered productivity but in 2005 poor
productivity was attributed to a shortage of sandeels during chick
rearing.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
7,392 |
9,844 |
9,018 |
| % change since previous
census |
N/a |
+33 |
-8 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Trend in abundance index (solid
line) of Sandwich tern in England, 1986-2010 with 95% confidence
limits (dotted lines). Based on SMP data; view the methods
of analysis (PDF 158 kb).
In contrast to Scotland, Sandwich tern numbers in England have
generally fared better with an increase between Operation Seafarer
and the the Seabird Colony Register. Numbers have since
been fairly stable, although since 2002 there is some evidence
of a slight decline. The index spikes in 2009, with the
recolonisation of Minsmere (possibly by birds abandoning
a colony on the continent), which had been effectively
abandoned itself in 1977 and was deserted again by
2010.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of Sandwich
tern in England, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
Annual average productivity at English colonies is usually
higher than that recorded in Scotland but is very variable due to
the influences of predation and tidal inundation. However, since
peak productivity was recorded in 2000 there has
been a downward trend culminating in very low chick
production in 2009 and 2010.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
0 |
450 |
450 |
| % change since previous
census |
N/a |
N/a |
0 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of Sandwich tern at
Cemlyn, 1986-2010.
Sandwich terns are confined to just one location in Wales, at
Cemlyn Lagoon on Angelsey. No birds were recorded there during
Operation Seafarer although an influx of 50 pairs mid season
in 1970 were possibly displaced from elsewhere. Numbers had risen
to 450 pairs during the Seabird Colony Register and, aside from a
peak of short duration from 1987-1989, remained fairly
stable until Seabird 2000. Numbers increased
substantially from 2001 onward with the colony now
regularly holding over 1,100 pairs (although only 409
pairs nested in 2008). The count in 2010 was the highest yet
recorded.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of Sandwich
tern at Cemlyn, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
Sandwich terns nesting at Cemlyn have
been relatively successful compared with other British
colonies. Over one chick per pair has been fledged in several years
since 1996 which may also explain the increased numbers nesting at
the site since 2002 as young from successful breeding
seasons themselves return to nest. In 2007 and 2008, there was
almost complete failure with very few young fledged due to
predation and desertion of nests. Productivity in 2009 and
2010 was also lower than normal.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
1,211 |
2,186 |
1,954 |
| % change since previous
census |
N/a |
+80 |
-11 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of Sandwich tern at
loughs Strangford, Carlingford and Larne, 1986-2010.
National census data show that numbers of Sandwich terns in
Northern Ireland between Operation Seafarer and the Seabird Colony
Register increased by 80%, then fell by 11% up to Seabird 2000.
This later decline between 1987 and 2000 (years when the majority
of tern colonies were counted in the SCR and Seabird 2000
respectively) is broadly backed up by SMP data, presented
above. The overall picture is confused due to fluctuating
numbers at the largest colony, Strangford Lough, which was
abandoned in 1986 and 1988, but held over 2,000 pairs in 1987.
Extra detail supplied by colonies sampled for the SMP
suggest numbers declined after the SCR until the early
1990s but then increased steadily until 2005, although a
decline is now underway again.
Productivity
Few systematic data on the productivity
of Sandwich terns 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*) |
1,005 |
1,281 |
1,762 |
| % change since previous
census |
N/a |
+27 |
+38 |
* AON = Apparently Occupied Nests
Breeding abundance
Figure 1: Abundance of Sandwich tern at
Lady's Island Lake, 1986-2010.
Operation Seafarer recorded 1,005 pairs of Sandwich tern
in the Republic of Ireland. This number has increased during
each subsequent census with 1,762 pairs counted during
Seabird 2000. The largest colony of the eight known is at
Lady's Island Lake (Figure 1), which held approximately 60% of the
population during the last census. Numbers there have increased
substantially during the last decade with 1,838 pairs counted in
2010. With Inch Islet, the next largest colony, known to hold 315
pairs in 2007, the current national population surely
now exceeds 2,000 pairs.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of Sandwich
tern in the Republic of Ireland, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
Sandwich terns breeding in the Republic
of Ireland also show the typical fluctuating fortunes found in the
other countries of the British Isles. Particularly
low productivity was recorded in 1994, 1999 and 2008 but in
most years since 2000 Sandwich terns have been relatively
productive. No productivity data is available for 2009 or
2010.
Population estimates and change 1969-2002 (census data)
| |
Operation Seafarer
(1969-70)
|
Seabird Colony Register
(1985-88)
|
Seabird 2000
(1998-2002)
|
| Population estimate (AON*) |
2,216 |
3,467 |
3,716 |
| % change since previous
census |
N/a |
+56 |
+7 |
* AON = Apparently Occupied Nests
Breeding abundance
National census data show a 56% increase
in Sandwich tern numbers between Operation Seafarer and
the Seabird Colony Register. Seabird 2000 recorded similar numbers
to the SCR census. Over the decade since the
last census the three main colonies in Northern Ireland
and at Lady's Island Lake in Republic of Ireland have all
increased in number and in 2010 these colonies alone
held over 3,500 pairs. Taking smaller colonies and those not
counted recently (of which there are at least nine in
total) into account would suggest the all Ireland
population probably now lies between 4,000-4,500
pairs.
Productivity
Figure 2: Trend
in productivity (no. of chicks fledged per pair) of Sandwich
tern throughout Ireland, 1986-2010. Based on SMP data; view
the methods
of analysis (PDF 158 kb).
The trend in breeding performance shown
for the whole of Ireland closely matches that shown for the
Republic of Ireland where most of the data have been collected but
with slightly lowered average values. Particularly low levels
of productivity were recorded in 1994, 1999 and 2008 but in most
years since 2000 Sandwich terns have been relatively
productive. No productivity data is available for
2009 or 2010.
This species does not breed on the Isle of
Man.
This species does not breed on the Channel
Islands.
UK phenology, diet, survival rates
No data have been collected as part of the Seabird Monitoring
Programme.
References
1 Ratcliffe, N., Pickerell, G. And Brindley, E.
2000. Population trends of Little and Sandwich Terns Sterna
albifrons and S. sandvicensis in Britain and Ireland
from 1969 to 1998. Atlantic Seabirds 2: 211-26.
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 Sandwich tern appears courtesy
of Ian Rendall ©, is subject to international
copyright law and may not be reproduced in any form
whatsoever.