The Little Tern is the smallest species of tern breeding in
Britain and Ireland. They nest exclusively on the coast in
well-camouflaged shallow scrapes on sand and shingle beaches, spits
or inshore islets. Little terns do not forage far from their
breeding site, which dictates a necessity for breeding close to
shallow, sheltered feeding areas where they can easily locate the
variety of small fish and invertebrates that make up their own and
their chicks' diet. Although colonies may be found around much of
the coastline, breeding is concentrated in the south and east of
England, where the species' preference for breeding on beaches also
favoured by people makes it vulnerable to disturbance.
Following a decline during the 19th century, the
Little Tern population in Britain increased in size during the
early 20th century, peaking in the 1930s. Thereafter,
numbers once again fell and Operation Seafarer (1969–70) estimated
the British and Irish population to be 1,900 pairs. Numbers then
peaked in the mid-1970s and then went into a long-term decline,
reaching an all-time low in 1998. The decline was most severe at
colonies along the coast of the English Channel, but was not
restricted to this region, indicating that the problem was more
widespread.
Survey coverage
Complete coverage of Little Tern colonies was achieved in
Britain and Ireland during Seabird 2000, as in Operation Seafarer
(1969-70) and the SCR Census (1985-88). Little Terns, like other
tern species, show a low degree of site fidelity from one year to
the next, in response to predation, disturbance or habitat change.
This is such a great problem that in order to gain an accurate
national estimate of Little Tern numbers, a simultaneous census was
planned to cover all colonies in Britain within a single year.
During Seabird 2000, 93% of the population in Britain were counted
in 2000. This was a marked improvement on the SCR Census when
counts were spread over four years, with 13% of the British
population counted in 1985, 21% in 1986, 63% in 1987 and the
remaining 3% in 1988. In the Republic of Ireland, Seabird 2000
surveys were conducted in 1999 in Louth and Dublin, in 2000 in Mayo
and Wexford and in 2001 in Wicklow. There are no known colonies of
Little Terns in Northern Ireland. Counts from all other admin areas
in Ireland were taken from the 1995 All Ireland Tern Survey (Hannon
et al. 1997) and these constituted 65% of colonies and 48%
of AONs in Ireland (1995-2001). Movements between admin areas in
successive years could have lead to an over or underestimate of the
population size in Ireland. For the SCR Census Lloyd et
al. (1991) used counts of Little Terns from the 1984 All
Ireland Tern Survey (Whilde 1985) together with more recent counts
from 1985-88 when they existed. This probably led to an
overestimate of the population, therefore, for the SCR Census
results in Table 1, only counts from the All Ireland Tern Survey in
1984 (which were less likely to be affected by movement of birds
between colonies) were used. However, any errors/biases in the
estimate for Ireland would have only a small effect on the accuracy
of the total population estimate for Britain and Ireland, since
only 10% of the population bred in here during Seabird 2000.
Census Methods
The prescribed count unit for Little Terns during Seabird 2000
was the apparently occupied nest (AON). Little Terns usually breed
at low densities and have inconspicuous nests and camouflaged eggs
and chicks. Thus, counts of fully formed nests containing eggs and
chicks made during systematic searches from within the colony
usually underestimate the number of AONs present. AONs are much
more conspicuous when an adult is in attendance, either incubating,
brooding or feeding chicks. Thus, counts of AONs are best made from
a good vantage point, far enough away from the colony as not to
cause any disturbance to attending adults. In Britain during
Seabird 2000, 81% of AONs were counted from a vantage point, 9%
were counted from within the colony and the remaining 10% were
estimated from counts of individuals attending the colony (no
information on methods was available from the All Ireland Tern
survey). During the SCR Census (1985-88), count methods were not
recorded, but 97% of AONs were counted directly, with only 3%
estimated from counts of individuals. Counts of individuals were
performed when there was no suitable vantage point and the colony
could either not be accessed or the level of disturbance caused by
doing so was deemed unjustifiable by the surveyor. During both
censuses, counts of individuals were divided by 1.5 to give an
estimate of AONs (Bullock & Gomersall 1981). The conversion of
counts of individuals to counts of AONs is prone to error resulting
from inclusion of attending non-breeders and the variation in
attendance of parents to the nest depending on time of day and
season. Furthermore, the correction factor was devised for Arctic
Terns in the Northern Isles rather than Little Terns.
The recommended count period for Seabird 2000 was from mid-May
to the end of June, coinciding with the peak nesting period. Pairs
that failed or deserted early in the season may have been missed
during surveys, and those that moved to other colonies may have
been counted twice. In Britain, during Seabird 2000, 40% of the
population were counted within the recommended dates, 28% were
outside and no date was recorded for 31% of counts, compared with
31%, 14% and 55% respectively during the SCR Census. In Ireland,
70% of the population were counted within the recommended dates
during Seabird 2000, but dates were not available for counts made
during the All Ireland Tern Survey in 1984 that were incorporated
into the SCR Census.
While the similarity in methods employed in both the SCR
Census and Seabird 2000 ensures a valid comparison of their
population estimates, the apparent population trend from a
comparison of two such widely spaced surveys may be misleading.
This is because the proportion of adult Little Terns choosing to
nest in any one year does fluctuate. Thus more accurate trends are
obtained from more frequently conducted counts (e.g. annually).
Annual monitoring of the size of Little Tern
Colonies has been conducted in Britain since 1969. The colonies
monitored currently hold about two-thirds of the British Little
Tern population.