Common Terns are not the most abundant tern species in Britain
and Ireland, but are probably the most familiar owing to their
breeding range being among the widest of seabird breeding in
Britain and Ireland. Colonies occur around most of the British and
Irish coasts and inland on lakes, reservoirs and gravel pits.
Common Terns are absent from most of mainland Wales and SW England,
and are comparatively scarce in the Northern and Western Isles
where they are replaced by Arctic Terns. Small colonies occur
inland along the large river valleys of SE and Central England,
notably the Thames, Ouse, Humber and Trent and their tributaries.
Inland colonies are also scattered along rivers in SE Scotland and
on islets in the freshwater loughs of Ireland.
Tern populations in NW Europe were bought to the brink of
extirpation at the end of the nineteenth century by hunting of
adults for the millinery trade, but recovered in response to
protective legislation in the early twentieth century. Over the
last three decades the population has remained broadly stable,
although there have been marked variations in trends among regions.
Declines in W Scotland and England are likely to be due to
increased predation by American Mink Mustela vison and Red
Fox Vulpes vulpes respectively, and conservation
management to ameliorate these problems is being exercised. Common
Terns have 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 Common Tern population at current levels is likely
to depend on the continuation of such management in
perpetuity.
Census Methods
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 and
often move en masse between different colonies. This is such a
great 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, co-ordinated by RSPB and SNH. 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. In Ireland
it was decided by the Seabird 2000 Steering Group to make the most
of limited resources and utilise results from the All-Ireland tern
survey conducted in 1995 (Hannon et al. 1997). More recent
counts (with those from 2000 given priority) were included for some
colonies, including all those along the entire east coast.
Movements among these regions are unlikely to have caused severe
bias in trend estimation. During the SCR, counts were made in
different years within regions, and inter-colony movements may have
caused greater inaccuracies.
Tern breeding populations can fluctuate among years owing to
variations in the proportion of mature birds that attempt to nest.
As such, counts that compare status in two years need to be viewed
with caution since one could coincide with a period during which
the proportion of birds attempting to breed was depressed. This
problem is likely to be less for Common than for Sandwich or Arctic
Terns since their generalist diet and broad habitat selection means
they are less prone to environmental perturbation than these
species. Annual counts of Common Terns are also available from
sites throughout England, Wales, eastern Ireland and some sites in
SW and SE Scotland, and these indicate that the counts during the
SCR and Seabird 2000 were not atypically low. As such, the
long-term trends estimated for Common Terns between the two surveys
will be reasonably robust.
Common Terns are most often counted in units of Apparently
Occupied Nests (AONs), using either counts of nests along transects
through the colony or counts of apparently incubating adults.
Direct counts of nests along transects are generally the most
accurate method of counting Common Tern colonies, especially where
long vegetation or uneven ground prevents counts of incubating
adults from a vantage point. Flush counts of adult birds were used
to count 9% of pairs during Seabird 2000. These were mainly in the
Northern Isles and Western Isles where Common Terns nest within
larger colonies of Arctic Terns. During the SCR Census, the
proportion of colonies censused using flush counts and the
distribution of these was broadly similar, and so there will be
little bias in estimation of regional trends owing to changes in
the methods used. Flush counts are less accurate than counts of
AONs owing to variations in adult attendance. The counts of birds
were divided by 1.5 in order to convert them into AONs. This
correction factor was estimated for Artic Terns in the Northern
Isles during 1980 and so will only be approximate for Common Terns
during Seabird 2000.
Surveys need to be timed to coincide with the peak of
incubation activity when the maximum number of nests and incubating
adults are present for AON counts and when adult attendance for
flush counts is most stable. Surveys prior to this date will omit
pairs that have not yet laid, while those afterwards will omit
nests that have failed or whose chicks have hatched and dispersed.
Common Tern laying phenology is relatively synchronous and between
80 and 90% of pairs will be incubating during mid June provided
that flooding or predation don't cause large-scale nest losses.
During Seabird 2000, 79% of pairs were counted within the
recommended time-period, and so the majority of counts should be
accurate.
Identification can present problems with surveys of Common
Terns since adults look similar to Arctic Terns, and their nests
and contents cannot be separated reliably. In mixed colonies of
Common and Arctic Terns, counts of incubating adults are
preferable, but at some colonies vegetation cover or lack of
suitable vantage points make this method impractical. In such
cases, a count of the total number of nests needs to be made,
followed by an estimate of the proportion of adults of each species
circling over the colony. The number of breeding pairs of Common
Terns is calculated as the product of the proportion of adult
Common Terns and the number of nests of both species.
Species-specific differences in adult colony attendance rates may
lead to biases in such estimates. Alternatively, observers will
sometimes combine counts of the two species and report them as
"Commic" Terns rather than attempting to determine the numbers of
each. This makes estimation of changes in numbers of each species
difficult. However, Commic Terns only represent 1.6% of total
Common Tern pairs counted during Seabird 2000 and so identification
problems will not produce a large bias in estimation of status.
However, during the SCR the percentage of Commic Terns was higher
at 12%, and so trends will be upwardly biased owing to a larger
proportion of birds being identified during Seabird 2000. The
majority of Commic Terns records were from Western Scotland, the
Northern Isles and West Ireland.
Survey Coverage
Coverage of coastal Common Tern colonies in Britain and
Ireland was likely to have been broadly comprehensive during
Seabird 2000 with the exception of western Ireland. This was
rectified by inclusion of data from the 1995 All Ireland Tern
Survey (Hannon et al. 1997). During the SCR, coverage of
the Northern Isles and west coast of Ireland was poor. This was
overcome by inclusion of data from the 1980 survey of terns in
Orkney and Shetland (Bullock & Gomersall 1980) and the 1983-84
All Ireland Tern Survey (Whilde 1985). The variation in time-period
over which trends are calculated among regions will produce some
bias in the overall national population trend.
An inventory of inland colonies was made using county bird
reports, and co-ordinators were asked to arrange for each of these,
and any other colonies that they knew of in their county, to be
surveyed. Some small colonies that were previously unoccupied or
unreported may therefore have been missed. Coverage of inland sites
was probably less extensive during the SCR than in Seabird 2000, so
assessment of changes in range and status inland needs to be made
with caution.