In Britain the Arctic Skua is confined to breeding in north
and west Scotland, at the southern extremity of its circumpolar,
high latitude breeding range. Most Scottish Arctic Skuas nest in
moorland colonies close to aggregations of auks, kittiwakes and
Arctic Terns from which they obtain food by piracy. In a few
places, such as the moors of Caithness, Arctic Skuas nest further
inland in rather scattered breeding territories, where feeding on
berries, insects and small birds may be more important. Unlike
their larger cousin the Great Skua, Arctic Skuas do not normally
scavenge behind fishing boats or feed as members in multi-species
flocks of seabirds on surface shoals of fish. Arctic Skuas are
simply too small to compete in such situations. Although numbers of
Arctic Skuas nesting in Scotland increased in the 1970s and 1980s,
most of their breeding sites have been established for many decades
or centuries. Few new colonies have been formed and the breeding
range has remained remarkably static.
The count unit for Arctic Skuas is the Apparently Occupied
Territory (AOT). This is very similar to the number of breeding
pairs, but also includes the small numbers of birds that are
holding territory but not necessarily breeding. Counting Arctic
Skuas is made difficult by the scattered nature of territories in
many areas – in Mainland Shetland and Caithness, single pairs may
be scattered, often considerable distances from the coast and might
be easily overlooked. Coverage of areas known to hold breeding
Arctic Skuas was thorough during Seabird 2000 and was also thought
to be good for the SCR Census (1985-88) but rather incomplete
during Operation Seafarer (1969-70) when the focus of effort on
coastal surveys missed many Arctic Skuas, especially those breeding
more than 1km inland that may have included up to 405 pairs of
Arctic Skuas in Shetland. Surveys of Arctic Skuas during the SCR
Census were conducted in Orkney in 1982 (Meek et al. 1985) and the
total was increased by 15% (Lloyd et al. 1991) to make it
comparable with the results of the survey of Arctic Skuas conducted
in Shetland in 1985-86 (Ewins et al. 1987). The SCR Census
also included counts from Caithness conducted in 1979-80 (Reed
et al. 1983), whereas all other areas were surveyed during
the main census period of 1985-88. Although Arctic Skuas usually
breed every year once they have recruited into a colony, and
usually return to exactly the same territory year after year, in
seasons when food supply is particularly poor, birds may fail to
lay. This appears to have been the case during Seabird 2000 surveys
of Arctic Skuas in Shetland during 2000 and 2001 and may have
resulted in an underestimate of the number that would normally be
breeding under more favourable conditions. Breeding success of
Arctic Skuas in Shetland was also poor in 2000 and 2001, and birds
that lost eggs early on may have shown low attendance in the
territory, so that some breeding AOTs may also have been missed.
The Shetland counts in 2002 were also affected by poor weather
conditions, with a considerable area surveyed in fog. This may also
have contributed to an underestimate of numbers in parts of
Shetland. Apparently, counting conditions and breeding success
during Seabird 2000 were better in other regions and should have
had little impact on count accuracy.