Seabird 2000
Black-legged Kittiwake Risa
tridactyla
The following was adapted from original
text by Martin Heubeck in
Seabird Populations of Britain and Ireland (with
permission from A&C Black, London).
As well as being the most numerous species of
gull in the world, the Black-legged Kittiwake is the most oceanic
in its habits and most adapted to nesting on vertical rocky
sea-cliffs. In Britain and Ireland, the largest and most numerous
colonies are found along the North Sea coasts of Britain, around
Orkney and Shetland, and off NW Scotland. Colony size varies from
less than ten pairs to tens of thousands, but the locations of
colonies tend to be traditional over many decades. Although most
colonies are on sheer cliffs, in a few instances man-made
structures such as buildings, bridges, sea walls and even offshore
oil installations have been utilised. During the breeding season
Black-legged Kittiwakes feed mainly on small pelagic shoaling fish;
around the British Isles. These consist of energy-rich species such
as sandeels, sprats and young herring. However, Kittiwakes will
also scavenge for offal and discards around fishing boats, which
can be an important food source in years when their preferred prey
species are less abundant. Outside the breeding season the species
is essentially oceanic, and it is probable that populations from
many different breeding localities mix together in the N. Atlantic
and North Sea during winter, with some birds from British and Irish
colonies (especially first-winter and immatures) spending time off
the eastern seaboard of North America.
Census Methods
Coverage of known Black-legged Kittiwake
colonies in Britain and Ireland was complete during both the SCR
Census (1985-88) and Seabird 2000, with the exceptions of a colony
in Galway (Ireland) not counted during the former and one in Sligo
(Ireland) not counted during the latter survey. Since Black-legged
Kittiwakes are highly colonial at traditional sites on sea cliffs,
build conspicuous nests on narrow ledges and have a relatively
synchronised breeding season, they would appear to be an easy
species to census. The count unit is the apparently occupied nest
(AON), defined as a well-built nest capable of holding eggs or
young at which at least one adult is present. At some colonies,
most nests are visible from cliff-top vantage points and can be
counted easily using binoculars or a telescope, with larger
colonies being divided into sections using physical features of the
cliff to aid counting. However some sections of colonies are
invisible from land (e.g. offshore stacks, inaccessible islands,
around cave entrances, or on long linear stretches of cliff) and
therefore are best counted from the sea, although the instability
of a boat as a viewing platform can create problems at larger
colonies.
The recommended count period is during the late
incubation period, in Britain and Ireland from late May to
mid-June, although in some years nest building can be delayed by
two to three weeks due to a scarcity of food during spring. In such
years, a high proportion of pairs (up to 40%) may begin nest
building, but not complete a structure qualifying as an AON or
progess to laying. However, since no such events were documented
during Seabird 2000 census counts this is unlikely to have caused a
serious bias. Counts late in the season, when breeding failures may
have resulted in the partial or complete disintegration of some
nests, are likely to under-estimate breeding numbers. During
Seabird 2000, approximately 5% of the British and Irish total of
AONs were counted outwith the recommended count period; 60% of
these (12,624 AON) were along the cliffs of Caithness during the
first week of July 1999. Overall, however, it is unlikely that such
"out of season" counts had much influence on gross estimates of
population change.