The Great Skua, or Bonxie, is famous for its aggressive
defence of territory against human intruders. The Great Skua has a
very restricted breeding range – confined to the northeast
Atlantic, the World population is only around 16,000 apparently
occupied territories, of which 60% are in Scotland, concentrated in
Shetland and Orkney. None breed elsewhere in Britain and it nested
for the first time in Ireland during Seabird 2000. However, its
population has been increasing since 1900, and it has progressively
extended its breeding range both northeast into the Barents Sea,
and south into the islands of west Scotland. Closely related
species breed in the Antarctic and sub-Antarctic, and show strong
adaptations to cold conditions and a predatory life-style. In
Scotland, Great Skuas nest on coastal moorland, often in loose
groups of scattered nests, but with some colonies numbering
thousands of pairs. When nesting at low density in small colonies,
most birds in the colony feed by killing birds. However, when
nesting in large colonies, the majority feed on fish, including
fishery discards, and only a small proportion specialize in killing
seabirds. Ringing has shown that Great Skuas from Shetland have
emigrated to form colonies in many other areas as far away as north
Russia, but the majority of chicks return to their natal colony to
try to establish a breeding territory.
Coverage of Great Skua breeding areas in the Seabird 2000
survey was good in most regions, however, small areas of possible
breeding habitat in parts of west and central mainland Shetland and
Caithness were not surveyed. According to Lloyd et al. (1991),
coverage of skuas in the SCR Census (1985-88) was complete.
However, surveys of Great Skuas in Orkney were actually conducted
in 1982 (Meek et al. 1985) and Lloyd et al. (1991) adjusted the
1982 count using observed trends (Furness 1986) to estimate
population size in 1986 to be comparable with the survey in
Shetland conducted in 1985-86 (Ewins et al. 1987). Counts from all
other areas used in the SCR Census were conducted during 1985-88.
Operation Seafarer (1969-70) did not attempt to find all inland
nesting skuas so will have underestimated numbers (Cramp et al.
1974). However, the proportion of Great Skuas nesting 'inland' is
small, and major colonies in Shetland such as Foula, Hermaness
(Unst), Fetlar and Noss were all surveyed completely in 1969. Small
numbers nesting inland on mainland Shetland would have been
overlooked. In addition, detailed counts of skuas were organised by
the RSPB and others in 1974-75 (Everett 1982) and in 1992 (Meek et
al. 1994, Sears et al. 1995), while many major skua colonies have
been censused regularly outwith these national surveys, so that
trends are very clearly documented. Not all colonies were counted
in the same year, but this should not cause problems, as Great
Skuas that have nested in one colony have never been recorded
moving to breed elsewhere in a subsequent year (Furness
1987).
By comparison with most other seabird species, Great Skuas are
relatively easy to census. Throughout the breeding season, but
especially during incubation and early chick-rearing (from early
May to late June), they show very high territory attendance, with
the territory hardly ever left unattended. Pairs that have lost
eggs or young chicks almost invariably remain on territory and
those that fail early (which is when most clutches are lost) will
lay a replacement clutch. Counts tend to be of 'Apparently Occupied
Territories' (AOT). An AOT can be a pair with a nest, a pair, or a
single bird, in apparently suitable nesting habitat and showing
signs of holding territory.